Network Working Group U. Blumenthal
Request for Comments: 3414 B. Wijnen
STD: 62 Lucent Technologies
Obsoletes: 2574 December 2002
Category: Standards Track
 User-based Security Model (USM) for version 3 of the
 Simple Network Management Protocol (SNMPv3)
Status of this Memo
 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements. Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
 Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
 This document describes the User-based Security Model (USM) for
 Simple Network Management Protocol (SNMP) version 3 for use in the
 SNMP architecture. It defines the Elements of Procedure for
 providing SNMP message level security. This document also includes a
 Management Information Base (MIB) for remotely monitoring/managing
 the configuration parameters for this Security Model. This document
 obsoletes RFC 2574.
Table of Contents
 1. Introduction.......................................... 4
 1.1. Threats............................................... 4
 1.2. Goals and Constraints................................. 6
 1.3. Security Services..................................... 6
 1.4. Module Organization................................... 7
 1.4.1. Timeliness Module..................................... 8
 1.4.2. Authentication Protocol............................... 8
 1.4.3. Privacy Protocol...................................... 8
 1.5. Protection against Message Replay, Delay
 and Redirection....................................... 9
 1.5.1. Authoritative SNMP engine............................. 9
 1.5.2. Mechanisms............................................ 9
 1.6. Abstract Service Interfaces........................... 11
 1.6.1. User-based Security Model Primitives
 for Authentication.................................... 11
 1.6.2. User-based Security Model Primitives
 for Privacy........................................... 12
 2. Elements of the Model................................. 12
 2.1. User-based Security Model Users....................... 12
 2.2. Replay Protection..................................... 13
 2.2.1. msgAuthoritativeEngineID.............................. 14
 2.2.2. msgAuthoritativeEngineBoots and
 msgAuthoritativeEngineTime............................ 14
 2.2.3. Time Window........................................... 15
 2.3. Time Synchronization.................................. 15
 2.4. SNMP Messages Using this Security Model............... 16
 2.5. Services provided by the User-based Security Model.... 17
 2.5.1. Services for Generating an Outgoing SNMP Message...... 17
 2.5.2. Services for Processing an Incoming SNMP Message...... 20
 2.6. Key Localization Algorithm............................ 22
 3. Elements of Procedure................................. 22
 3.1. Generating an Outgoing SNMP Message................... 22
 3.2. Processing an Incoming SNMP Message................... 26
 4. Discovery............................................. 31
 5. Definitions........................................... 32
 6. HMAC-MD5-96 Authentication Protocol................... 51
 6.1. Mechanisms............................................ 51
 6.1.1. Digest Authentication Mechanism....................... 51
 6.2. Elements of the Digest Authentication Protocol........ 52
 6.2.1. Users................................................. 52
 6.2.2. msgAuthoritativeEngineID.............................. 53
 6.2.3. SNMP Messages Using this Authentication Protocol...... 53
 6.2.4. Services provided by the HMAC-MD5-96
 Authentication Module................................. 53
 6.2.4.1. Services for Generating an Outgoing SNMP Message...... 53
 6.2.4.2. Services for Processing an Incoming SNMP Message...... 54
 6.3. Elements of Procedure................................. 55
 6.3.1. Processing an Outgoing Message........................ 55
 6.3.2. Processing an Incoming Message........................ 56
 7. HMAC-SHA-96 Authentication Protocol................... 57
 7.1. Mechanisms............................................ 57
 7.1.1. Digest Authentication Mechanism....................... 57
 7.2. Elements of the HMAC-SHA-96 Authentication Protocol... 58
 7.2.1. Users................................................. 58
 7.2.2. msgAuthoritativeEngineID.............................. 58
 7.2.3. SNMP Messages Using this Authentication Protocol...... 59
 7.2.4. Services provided by the HMAC-SHA-96
 Authentication Module................................. 59
 7.2.4.1. Services for Generating an Outgoing SNMP Message...... 59
 7.2.4.2. Services for Processing an Incoming SNMP Message...... 60
 7.3. Elements of Procedure................................. 61
 7.3.1. Processing an Outgoing Message........................ 61
 7.3.2. Processing an Incoming Message........................ 61
 8. CBC-DES Symmetric Encryption Protocol................. 63
 8.1. Mechanisms............................................ 63
 8.1.1. Symmetric Encryption Protocol......................... 63
 8.1.1.1. DES key and Initialization Vector..................... 64
 8.1.1.2. Data Encryption....................................... 65
 8.1.1.3. Data Decryption....................................... 65
 8.2. Elements of the DES Privacy Protocol.................. 65
 8.2.1. Users................................................. 65
 8.2.2. msgAuthoritativeEngineID.............................. 66
 8.2.3. SNMP Messages Using this Privacy Protocol............. 66
 8.2.4. Services provided by the DES Privacy Module........... 66
 8.2.4.1. Services for Encrypting Outgoing Data................. 66
 8.2.4.2. Services for Decrypting Incoming Data................. 67
 8.3. Elements of Procedure................................. 68
 8.3.1. Processing an Outgoing Message........................ 68
 8.3.2. Processing an Incoming Message........................ 69
 9. Intellectual Property................................. 69
 10. Acknowledgements...................................... 70
 11. Security Considerations............................... 71
 11.1. Recommended Practices................................. 71
 11.2. Defining Users........................................ 73
 11.3. Conformance........................................... 74
 11.4. Use of Reports........................................ 75
 11.5. Access to the SNMP-USER-BASED-SM-MIB.................. 75
 12. References............................................ 75
 A.1. SNMP engine Installation Parameters................... 78
 A.2. Password to Key Algorithm............................. 80
 A.2.1. Password to Key Sample Code for MD5................... 81
 A.2.2. Password to Key Sample Code for SHA................... 82
 A.3. Password to Key Sample Results........................ 83
 A.3.1. Password to Key Sample Results using MD5.............. 83
 A.3.2. Password to Key Sample Results using SHA.............. 83
 A.4. Sample encoding of msgSecurityParameters.............. 83
 A.5. Sample keyChange Results.............................. 84
 A.5.1. Sample keyChange Results using MD5.................... 84
 A.5.2. Sample keyChange Results using SHA.................... 85
 B. Change Log............................................ 86
 Editors' Addresses.................................... 87
 Full Copyright Statement.............................. 88
1. Introduction
 The Architecture for describing Internet Management Frameworks
 [RFC3411] describes that an SNMP engine is composed of:
 1) a Dispatcher,
 2) a Message Processing Subsystem,
 3) a Security Subsystem, and
 4) an Access Control Subsystem.
 Applications make use of the services of these subsystems.
 It is important to understand the SNMP architecture and the
 terminology of the architecture to understand where the Security
 Model described in this document fits into the architecture and
 interacts with other subsystems within the architecture. The reader
 is expected to have read and understood the description of the SNMP
 architecture, as defined in [RFC3411].
 This memo describes the User-based Security Model as it is used
 within the SNMP Architecture. The main idea is that we use the
 traditional concept of a user (identified by a userName) with which
 to associate security information.
 This memo describes the use of HMAC-MD5-96 and HMAC-SHA-96 as the
 authentication protocols and the use of CBC-DES as the privacy
 protocol. The User-based Security Model however allows for other
 such protocols to be used instead of or concurrent with these
 protocols. Therefore, the description of HMAC-MD5-96, HMAC-SHA-96
 and CBC-DES are in separate sections to reflect their self-contained
 nature and to indicate that they can be replaced or supplemented in
 the future.
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in [RFC2119].
1.1. Threats
 Several of the classical threats to network protocols are applicable
 to the network management problem and therefore would be applicable
 to any SNMP Security Model. Other threats are not applicable to the
 network management problem. This section discusses principal
 threats, secondary threats, and threats which are of lesser
 importance.
 The principal threats against which this SNMP Security Model should
 provide protection are:
 - Modification of Information The modification threat is the danger
 that some unauthorized entity may alter in-transit SNMP messages
 generated on behalf of an authorized principal in such a way as to
 effect unauthorized management operations, including falsifying the
 value of an object.
 - Masquerade The masquerade threat is the danger that management
 operations not authorized for some user may be attempted by
 assuming the identity of another user that has the appropriate
 authorizations.
 Two secondary threats are also identified. The Security Model
 defined in this memo provides limited protection against:
 - Disclosure The disclosure threat is the danger of eavesdropping on
 the exchanges between managed agents and a management station.
 Protecting against this threat may be required as a matter of local
 policy.
 - Message Stream Modification The SNMP protocol is typically based
 upon a connection-less transport service which may operate over any
 sub-network service. The re-ordering, delay or replay of messages
 can and does occur through the natural operation of many such sub-
 network services. The message stream modification threat is the
 danger that messages may be maliciously re-ordered, delayed or
 replayed to an extent which is greater than can occur through the
 natural operation of a sub-network service, in order to effect
 unauthorized management operations.
 There are at least two threats that an SNMP Security Model need not
 protect against. The security protocols defined in this memo do not
 provide protection against:
 - Denial of Service This SNMP Security Model does not attempt to
 address the broad range of attacks by which service on behalf of
 authorized users is denied. Indeed, such denial-of-service attacks
 are in many cases indistinguishable from the type of network
 failures with which any viable network management protocol must
 cope as a matter of course.
 - Traffic Analysis This SNMP Security Model does not attempt to
 address traffic analysis attacks. Indeed, many traffic patterns
 are predictable - devices may be managed on a regular basis by a
 relatively small number of management applications - and therefore
 there is no significant advantage afforded by protecting against
 traffic analysis.
1.2. Goals and Constraints
 Based on the foregoing account of threats in the SNMP network
 management environment, the goals of this SNMP Security Model are as
 follows.
 1) Provide for verification that each received SNMP message has not
 been modified during its transmission through the network.
 2) Provide for verification of the identity of the user on whose
 behalf a received SNMP message claims to have been generated.
 3) Provide for detection of received SNMP messages, which request or
 contain management information, whose time of generation was not
 recent.
 4) Provide, when necessary, that the contents of each received SNMP
 message are protected from disclosure.
 In addition to the principal goal of supporting secure network
 management, the design of this SNMP Security Model is also influenced
 by the following constraints:
 1) When the requirements of effective management in times of network
 stress are inconsistent with those of security, the design of USM
 has given preference to the former.
 2) Neither the security protocol nor its underlying security
 mechanisms should depend upon the ready availability of other
 network services (e.g., Network Time Protocol (NTP) or key
 management protocols).
 3) A security mechanism should entail no changes to the basic SNMP
 network management philosophy.
1.3. Security Services
 The security services necessary to support the goals of this SNMP
 Security Model are as follows:
 - Data Integrity is the provision of the property that data has not
 been altered or destroyed in an unauthorized manner, nor have data
 sequences been altered to an extent greater than can occur non-
 maliciously.
 - Data Origin Authentication is the provision of the property that
 the claimed identity of the user on whose behalf received data was
 originated is corroborated.
 - Data Confidentiality is the provision of the property that
 information is not made available or disclosed to unauthorized
 individuals, entities, or processes.
 - Message timeliness and limited replay protection is the provision
 of the property that a message whose generation time is outside of
 a specified time window is not accepted. Note that message
 reordering is not dealt with and can occur in normal conditions
 too.
 For the protocols specified in this memo, it is not possible to
 assure the specific originator of a received SNMP message; rather, it
 is the user on whose behalf the message was originated that is
 authenticated.
 For these protocols, it not possible to obtain data integrity without
 data origin authentication, nor is it possible to obtain data origin
 authentication without data integrity. Further, there is no
 provision for data confidentiality without both data integrity and
 data origin authentication.
 The security protocols used in this memo are considered acceptably
 secure at the time of writing. However, the procedures allow for new
 authentication and privacy methods to be specified at a future time
 if the need arises.
1.4. Module Organization
 The security protocols defined in this memo are split in three
 different modules and each has its specific responsibilities such
 that together they realize the goals and security services described
 above:
 - The authentication module MUST provide for:
 - Data Integrity,
 - Data Origin Authentication,
 - The timeliness module MUST provide for:
 - Protection against message delay or replay (to an extent greater
 than can occur through normal operation).
 - The privacy module MUST provide for
 - Protection against disclosure of the message payload.
 The timeliness module is fixed for the User-based Security Model
 while there is provision for multiple authentication and/or privacy
 modules, each of which implements a specific authentication or
 privacy protocol respectively.
1.4.1. Timeliness Module
 Section 3 (Elements of Procedure) uses the timeliness values in an
 SNMP message to do timeliness checking. The timeliness check is only
 performed if authentication is applied to the message. Since the
 complete message is checked for integrity, we can assume that the
 timeliness values in a message that passes the authentication module
 are trustworthy.
1.4.2. Authentication Protocol
 Section 6 describes the HMAC-MD5-96 authentication protocol which is
 the first authentication protocol that MUST be supported with the
 User-based Security Model. Section 7 describes the HMAC-SHA-96
 authentication protocol which is another authentication protocol that
 SHOULD be supported with the User-based Security Model. In the
 future additional or replacement authentication protocols may be
 defined as new needs arise.
 The User-based Security Model prescribes that, if authentication is
 used, then the complete message is checked for integrity in the
 authentication module.
 For a message to be authenticated, it needs to pass authentication
 check by the authentication module and the timeliness check which is
 a fixed part of this User-based Security model.
1.4.3. Privacy Protocol
 Section 8 describes the CBC-DES Symmetric Encryption Protocol which
 is the first privacy protocol to be used with the User-based Security
 Model. In the future additional or replacement privacy protocols may
 be defined as new needs arise.
 The User-based Security Model prescribes that the scopedPDU is
 protected from disclosure when a message is sent with privacy.
 The User-based Security Model also prescribes that a message needs to
 be authenticated if privacy is in use.
1.5. Protection against Message Replay, Delay and Redirection
1.5.1. Authoritative SNMP Engine
 In order to protect against message replay, delay and redirection,
 one of the SNMP engines involved in each communication is designated
 to be the authoritative SNMP engine. When an SNMP message contains a
 payload which expects a response (those messages that contain a
 Confirmed Class PDU [RFC3411]), then the receiver of such messages is
 authoritative. When an SNMP message contains a payload which does
 not expect a response (those messages that contain an Unconfirmed
 Class PDU [RFC3411]), then the sender of such a message is
 authoritative.
1.5.2. Mechanisms
 The following mechanisms are used:
 1) To protect against the threat of message delay or replay (to an
 extent greater than can occur through normal operation), a set of
 timeliness indicators (for the authoritative SNMP engine) are
 included in each message generated. An SNMP engine evaluates the
 timeliness indicators to determine if a received message is
 recent. An SNMP engine may evaluate the timeliness indicators to
 ensure that a received message is at least as recent as the last
 message it received from the same source. A non-authoritative
 SNMP engine uses received authentic messages to advance its notion
 of the timeliness indicators at the remote authoritative source.
 An SNMP engine MUST also use a mechanism to match incoming
 Responses to outstanding Requests and it MUST drop any Responses
 that do not match an outstanding request. For example, a msgID
 can be inserted in every message to cater for this functionality.
 These mechanisms provide for the detection of authenticated
 messages whose time of generation was not recent.
 This protection against the threat of message delay or replay does
 not imply nor provide any protection against unauthorized deletion
 or suppression of messages. Also, an SNMP engine may not be able
 to detect message reordering if all the messages involved are sent
 within the Time Window interval. Other mechanisms defined
 independently of the security protocol can also be used to detect
 the re-ordering replay, deletion, or suppression of messages
 containing Set operations (e.g., the MIB variable snmpSetSerialNo
 [RFC3418]).
 2) Verification that a message sent to/from one authoritative SNMP
 engine cannot be replayed to/as-if-from another authoritative SNMP
 engine.
 Included in each message is an identifier unique to the
 authoritative SNMP engine associated with the sender or intended
 recipient of the message.
 A message containing an Unconfirmed Class PDU sent by an
 authoritative SNMP engine to one non-authoritative SNMP engine can
 potentially be replayed to another non-authoritative SNMP engine.
 The latter non-authoritative SNMP engine might (if it knows about
 the same userName with the same secrets at the authoritative SNMP
 engine) as a result update its notion of timeliness indicators of
 the authoritative SNMP engine, but that is not considered a
 threat. In this case, A Report or Response message will be
 discarded by the Message Processing Model, because there should
 not be an outstanding Request message. A Trap will possibly be
 accepted. Again, that is not considered a threat, because the
 communication was authenticated and timely. It is as if the
 authoritative SNMP engine was configured to start sending Traps to
 the second SNMP engine, which theoretically can happen without the
 knowledge of the second SNMP engine anyway. Anyway, the second
 SNMP engine may not expect to receive this Trap, but is allowed to
 see the management information contained in it.
 3) Detection of messages which were not recently generated.
 A set of time indicators are included in the message, indicating
 the time of generation. Messages without recent time indicators
 are not considered authentic. In addition, an SNMP engine MUST
 drop any Responses that do not match an outstanding request. This
 however is the responsibility of the Message Processing Model.
 This memo allows the same user to be defined on multiple SNMP
 engines. Each SNMP engine maintains a value, snmpEngineID, which
 uniquely identifies the SNMP engine. This value is included in each
 message sent to/from the SNMP engine that is authoritative (see
 section 1.5.1). On receipt of a message, an authoritative SNMP
 engine checks the value to ensure that it is the intended recipient,
 and a non-authoritative SNMP engine uses the value to ensure that the
 message is processed using the correct state information.
 Each SNMP engine maintains two values, snmpEngineBoots and
 snmpEngineTime, which taken together provide an indication of time at
 that SNMP engine. Both of these values are included in an
 authenticated message sent to/received from that SNMP engine. On
 receipt, the values are checked to ensure that the indicated
 timeliness value is within a Time Window of the current time. The
 Time Window represents an administrative upper bound on acceptable
 delivery delay for protocol messages.
 For an SNMP engine to generate a message which an authoritative SNMP
 engine will accept as authentic, and to verify that a message
 received from that authoritative SNMP engine is authentic, such an
 SNMP engine must first achieve timeliness synchronization with the
 authoritative SNMP engine. See section 2.3.
1.6. Abstract Service Interfaces
 Abstract service interfaces have been defined to describe the
 conceptual interfaces between the various subsystems within an SNMP
 entity. Similarly a set of abstract service interfaces have been
 defined within the User-based Security Model (USM) to describe the
 conceptual interfaces between the generic USM services and the
 self-contained authentication and privacy services.
 These abstract service interfaces are defined by a set of primitives
 that define the services provided and the abstract data elements that
 must be passed when the services are invoked. This section lists the
 primitives that have been defined for the User-based Security Model.
1.6.1. User-based Security Model Primitives for Authentication
 The User-based Security Model provides the following internal
 primitives to pass data back and forth between the Security Model
 itself and the authentication service:
 statusInformation =
 authenticateOutgoingMsg(
 IN authKey -- secret key for authentication
 IN wholeMsg -- unauthenticated complete message
 OUT authenticatedWholeMsg -- complete authenticated message
 )
 statusInformation =
 authenticateIncomingMsg(
 IN authKey -- secret key for authentication
 IN authParameters -- as received on the wire
 IN wholeMsg -- as received on the wire
 OUT authenticatedWholeMsg -- complete authenticated message
 )
1.6.2. User-based Security Model Primitives for Privacy
 The User-based Security Model provides the following internal
 primitives to pass data back and forth between the Security Model
 itself and the privacy service:
 statusInformation =
 encryptData(
 IN encryptKey -- secret key for encryption
 IN dataToEncrypt -- data to encrypt (scopedPDU)
 OUT encryptedData -- encrypted data (encryptedPDU)
 OUT privParameters -- filled in by service provider
 )
 statusInformation =
 decryptData(
 IN decryptKey -- secret key for decrypting
 IN privParameters -- as received on the wire
 IN encryptedData -- encrypted data (encryptedPDU)
 OUT decryptedData -- decrypted data (scopedPDU)
 )
2. Elements of the Model
 This section contains definitions required to realize the security
 model defined by this memo.
2.1. User-based Security Model Users
 Management operations using this Security Model make use of a defined
 set of user identities. For any user on whose behalf management
 operations are authorized at a particular SNMP engine, that SNMP
 engine must have knowledge of that user. An SNMP engine that wishes
 to communicate with another SNMP engine must also have knowledge of a
 user known to that engine, including knowledge of the applicable
 attributes of that user.
 A user and its attributes are defined as follows:
 userName
 A string representing the name of the user.
 securityName
 A human-readable string representing the user in a format that is
 Security Model independent. There is a one-to-one relationship
 between userName and securityName.
 authProtocol
 An indication of whether messages sent on behalf of this user can
 be authenticated, and if so, the type of authentication protocol
 which is used. Two such protocols are defined in this memo:
 - the HMAC-MD5-96 authentication protocol.
 - the HMAC-SHA-96 authentication protocol.
 authKey
 If messages sent on behalf of this user can be authenticated, the
 (private) authentication key for use with the authentication
 protocol. Note that a user's authentication key will normally be
 different at different authoritative SNMP engines. The authKey is
 not accessible via SNMP. The length requirements of the authKey
 are defined by the authProtocol in use.
 authKeyChange and authOwnKeyChange
 The only way to remotely update the authentication key. Does that
 in a secure manner, so that the update can be completed without
 the need to employ privacy protection.
 privProtocol
 An indication of whether messages sent on behalf of this user can
 be protected from disclosure, and if so, the type of privacy
 protocol which is used. One such protocol is defined in this
 memo: the CBC-DES Symmetric Encryption Protocol.
 privKey
 If messages sent on behalf of this user can be en/decrypted, the
 (private) privacy key for use with the privacy protocol. Note
 that a user's privacy key will normally be different at different
 authoritative SNMP engines. The privKey is not accessible via
 SNMP. The length requirements of the privKey are defined by the
 privProtocol in use.
 privKeyChange and privOwnKeyChange
 The only way to remotely update the encryption key. Does that in
 a secure manner, so that the update can be completed without the
 need to employ privacy protection.
2.2. Replay Protection
 Each SNMP engine maintains three objects:
 - snmpEngineID, which (at least within an administrative domain)
 uniquely and unambiguously identifies an SNMP engine.
 - snmpEngineBoots, which is a count of the number of times the SNMP
 engine has re-booted/re-initialized since snmpEngineID was last
 configured; and,
 - snmpEngineTime, which is the number of seconds since the
 snmpEngineBoots counter was last incremented.
 Each SNMP engine is always authoritative with respect to these
 objects in its own SNMP entity. It is the responsibility of a non-
 authoritative SNMP engine to synchronize with the authoritative SNMP
 engine, as appropriate.
 An authoritative SNMP engine is required to maintain the values of
 its snmpEngineID and snmpEngineBoots in non-volatile storage.
2.2.1. msgAuthoritativeEngineID
 The msgAuthoritativeEngineID value contained in an authenticated
 message is used to defeat attacks in which messages from one SNMP
 engine to another SNMP engine are replayed to a different SNMP
 engine. It represents the snmpEngineID at the authoritative SNMP
 engine involved in the exchange of the message.
 When an authoritative SNMP engine is first installed, it sets its
 local value of snmpEngineID according to a enterprise-specific
 algorithm (see the definition of the Textual Convention for
 SnmpEngineID in the SNMP Architecture document [RFC3411]).
2.2.2. msgAuthoritativeEngineBoots and msgAuthoritativeEngineTime
 The msgAuthoritativeEngineBoots and msgAuthoritativeEngineTime values
 contained in an authenticated message are used to defeat attacks in
 which messages are replayed when they are no longer valid. They
 represent the snmpEngineBoots and snmpEngineTime values at the
 authoritative SNMP engine involved in the exchange of the message.
 Through use of snmpEngineBoots and snmpEngineTime, there is no
 requirement for an SNMP engine to have a non-volatile clock which
 ticks (i.e., increases with the passage of time) even when the
 SNMP engine is powered off. Rather, each time an SNMP engine
 re-boots, it retrieves, increments, and then stores snmpEngineBoots
 in non-volatile storage, and resets snmpEngineTime to zero.
 When an SNMP engine is first installed, it sets its local values of
 snmpEngineBoots and snmpEngineTime to zero. If snmpEngineTime ever
 reaches its maximum value (2147483647), then snmpEngineBoots is
 incremented as if the SNMP engine has re-booted and snmpEngineTime is
 reset to zero and starts incrementing again.
 Each time an authoritative SNMP engine re-boots, any SNMP engines
 holding that authoritative SNMP engine's values of snmpEngineBoots
 and snmpEngineTime need to re-synchronize prior to sending correctly
 authenticated messages to that authoritative SNMP engine (see Section
 2.3 for (re-)synchronization procedures). Note, however, that the
 procedures do provide for a notification to be accepted as authentic
 by a receiving SNMP engine, when sent by an authoritative SNMP engine
 which has re-booted since the receiving SNMP engine last (re-
 )synchronized.
 If an authoritative SNMP engine is ever unable to determine its
 latest snmpEngineBoots value, then it must set its snmpEngineBoots
 value to 2147483647.
 Whenever the local value of snmpEngineBoots has the value 2147483647
 it latches at that value and an authenticated message always causes
 an notInTimeWindow authentication failure.
 In order to reset an SNMP engine whose snmpEngineBoots value has
 reached the value 2147483647, manual intervention is required. The
 engine must be physically visited and re-configured, either with a
 new snmpEngineID value, or with new secret values for the
 authentication and privacy protocols of all users known to that SNMP
 engine. Note that even if an SNMP engine re-boots once a second that
 it would still take approximately 68 years before the max value of
 2147483647 would be reached.
2.2.3. Time Window
 The Time Window is a value that specifies the window of time in which
 a message generated on behalf of any user is valid. This memo
 specifies that the same value of the Time Window, 150 seconds, is
 used for all users.
2.3. Time Synchronization
 Time synchronization, required by a non-authoritative SNMP engine
 in order to proceed with authentic communications, has occurred
 when the non-authoritative SNMP engine has obtained a local notion
 of the authoritative SNMP engine's values of snmpEngineBoots and
 snmpEngineTime from the authoritative SNMP engine. These values
 must be (and remain) within the authoritative SNMP engine's Time
 Window. So the local notion of the authoritative SNMP engine's
 values must be kept loosely synchronized with the values stored
 at the authoritative SNMP engine. In addition to keeping a local
 copy of snmpEngineBoots and snmpEngineTime from the authoritative
 SNMP engine, a non-authoritative SNMP engine must also keep one
 local variable, latestReceivedEngineTime. This value records the
 highest value of snmpEngineTime that was received by the
 non-authoritative SNMP engine from the authoritative SNMP engine
 and is used to eliminate the possibility of replaying messages
 that would prevent the non-authoritative SNMP engine's notion of
 the snmpEngineTime from advancing.
 A non-authoritative SNMP engine must keep local notions of these
 values (snmpEngineBoots, snmpEngineTime and latestReceivedEngineTime)
 for each authoritative SNMP engine with which it wishes to
 communicate. Since each authoritative SNMP engine is uniquely and
 unambiguously identified by its value of snmpEngineID, the
 non-authoritative SNMP engine may use this value as a key in order to
 cache its local notions of these values.
 Time synchronization occurs as part of the procedures of receiving an
 SNMP message (Section 3.2, step 7b). As such, no explicit time
 synchronization procedure is required by a non-authoritative SNMP
 engine. Note, that whenever the local value of snmpEngineID is
 changed (e.g., through discovery) or when secure communications are
 first established with an authoritative SNMP engine, the local values
 of snmpEngineBoots and latestReceivedEngineTime should be set to
 zero. This will cause the time synchronization to occur when the
 next authentic message is received.
2.4. SNMP Messages Using this Security Model
 The syntax of an SNMP message using this Security Model adheres to
 the message format defined in the version-specific Message Processing
 Model document (for example [RFC3412]).
 The field msgSecurityParameters in SNMPv3 messages has a data type of
 OCTET STRING. Its value is the BER serialization of the following
 ASN.1 sequence:
 USMSecurityParametersSyntax DEFINITIONS IMPLICIT TAGS ::= BEGIN
 UsmSecurityParameters ::=
 SEQUENCE {
 -- global User-based security parameters
 msgAuthoritativeEngineID OCTET STRING,
 msgAuthoritativeEngineBoots INTEGER (0..2147483647),
 msgAuthoritativeEngineTime INTEGER (0..2147483647),
 msgUserName OCTET STRING (SIZE(0..32)),
 -- authentication protocol specific parameters
 msgAuthenticationParameters OCTET STRING,
 -- privacy protocol specific parameters
 msgPrivacyParameters OCTET STRING
 }
 END
 The fields of this sequence are:
 - The msgAuthoritativeEngineID specifies the snmpEngineID of the
 authoritative SNMP engine involved in the exchange of the message.
 - The msgAuthoritativeEngineBoots specifies the snmpEngineBoots value
 at the authoritative SNMP engine involved in the exchange of the
 message.
 - The msgAuthoritativeEngineTime specifies the snmpEngineTime value
 at the authoritative SNMP engine involved in the exchange of the
 message.
 - The msgUserName specifies the user (principal) on whose behalf the
 message is being exchanged. Note that a zero-length userName will
 not match any user, but it can be used for snmpEngineID discovery.
 - The msgAuthenticationParameters are defined by the authentication
 protocol in use for the message, as defined by the
 usmUserAuthProtocol column in the user's entry in the usmUserTable.
 - The msgPrivacyParameters are defined by the privacy protocol in use
 for the message, as defined by the usmUserPrivProtocol column in
 the user's entry in the usmUserTable).
 See appendix A.4 for an example of the BER encoding of field
 msgSecurityParameters.
2.5. Services provided by the User-based Security Model
 This section describes the services provided by the User-based
 Security Model with their inputs and outputs.
 The services are described as primitives of an abstract service
 interface and the inputs and outputs are described as abstract data
 elements as they are passed in these abstract service primitives.
2.5.1. Services for Generating an Outgoing SNMP Message
 When the Message Processing (MP) Subsystem invokes the User-based
 Security module to secure an outgoing SNMP message, it must use the
 appropriate service as provided by the Security module. These two
 services are provided:
 1) A service to generate a Request message. The abstract service
 primitive is:
 statusInformation = -- success or errorIndication
 generateRequestMsg(
 IN messageProcessingModel -- typically, SNMP version
 IN globalData -- message header, admin data
 IN maxMessageSize -- of the sending SNMP entity
 IN securityModel -- for the outgoing message
 IN securityEngineID -- authoritative SNMP entity
 IN securityName -- on behalf of this principal
 IN securityLevel -- Level of Security requested
 IN scopedPDU -- message (plaintext) payload
 OUT securityParameters -- filled in by Security Module
 OUT wholeMsg -- complete generated message
 OUT wholeMsgLength -- length of generated message
 )
 2) A service to generate a Response message. The abstract service
 primitive is:
 statusInformation = -- success or errorIndication
 generateResponseMsg(
 IN messageProcessingModel -- typically, SNMP version
 IN globalData -- message header, admin data
 IN maxMessageSize -- of the sending SNMP entity
 IN securityModel -- for the outgoing message
 IN securityEngineID -- authoritative SNMP entity
 IN securityName -- on behalf of this principal
 IN securityLevel -- Level of Security requested
 IN scopedPDU -- message (plaintext) payload
 IN securityStateReference -- reference to security state
 -- information from original
 -- request
 OUT securityParameters -- filled in by Security Module
 OUT wholeMsg -- complete generated message
 OUT wholeMsgLength -- length of generated message
 )
 The abstract data elements passed as parameters in the abstract
 service primitives are as follows:
 statusInformation
 An indication of whether the encoding and securing of the message
 was successful. If not it is an indication of the problem.
 messageProcessingModel
 The SNMP version number for the message to be generated. This
 data is not used by the User-based Security module.
 globalData
 The message header (i.e., its administrative information). This
 data is not used by the User-based Security module.
 maxMessageSize
 The maximum message size as included in the message. This data is
 not used by the User-based Security module.
 securityParameters
 These are the security parameters. They will be filled in by the
 User-based Security module.
 securityModel
 The securityModel in use. Should be User-based Security Model.
 This data is not used by the User-based Security module.
 securityName
 Together with the snmpEngineID it identifies a row in the
 usmUserTablethat is to be used for securing the message. The
 securityName has a format that is independent of the Security
 Model. In case of a response this parameter is ignored and the
 value from the cache is used.
 securityLevel
 The Level of Security from which the User-based Security module
 determines if the message needs to be protected from disclosure
 and if the message needs to be authenticated.
 securityEngineID
 The snmpEngineID of the authoritative SNMP engine to which a
 dateRequest message is to be sent. In case of a response it is
 implied to be the processing SNMP engine's snmpEngineID and so if
 it is specified, then it is ignored.
 scopedPDU
 The message payload. The data is opaque as far as the User-based
 Security Model is concerned.
 securityStateReference
 A handle/reference to cachedSecurityData to be used when securing
 an outgoing Response message. This is the exact same
 handle/reference as it was generated by the User-based Security
 module when processing the incoming Request message to which this
 is the Response message.
 wholeMsg
 The fully encoded and secured message ready for sending on the
 wire.
 wholeMsgLength
 The length of the encoded and secured message (wholeMsg).
 Upon completion of the process, the User-based Security module
 returns statusInformation. If the process was successful, the
 completed message with privacy and authentication applied if such was
 requested by the specified securityLevel is returned. If the process
 was not successful, then an errorIndication is returned.
2.5.2. Services for Processing an Incoming SNMP Message
 When the Message Processing (MP) Subsystem invokes the User-based
 Security module to verify proper security of an incoming message, it
 must use the service provided for an incoming message. The abstract
 service primitive is:
 statusInformation = -- errorIndication or success
 -- error counter OID/value if error
 processIncomingMsg(
 IN messageProcessingModel -- typically, SNMP version
 IN maxMessageSize -- of the sending SNMP entity
 IN securityParameters -- for the received message
 IN securityModel -- for the received message
 IN securityLevel -- Level of Security
 IN wholeMsg -- as received on the wire
 IN wholeMsgLength -- length as received on the wire
 OUT securityEngineID -- authoritative SNMP entity
 OUT securityName -- identification of the principal
 OUT scopedPDU, -- message (plaintext) payload
 OUT maxSizeResponseScopedPDU -- maximum size of the Response PDU
 OUT securityStateReference -- reference to security state
 ) -- information, needed for response
 The abstract data elements passed as parameters in the abstract
 service primitives are as follows:
 statusInformation
 An indication of whether the process was successful or not. If
 not, then the statusInformation includes the OID and the value of
 the error counter that was incremented.
 messageProcessingModel
 The SNMP version number as received in the message. This data is
 not used by the User-based Security module.
 maxMessageSize
 The maximum message size as included in the message. The User-bas
 User-based Security module uses this value to calculate the
 maxSizeResponseScopedPDU.
 securityParameters
 These are the security parameters as received in the message.
 securityModel
 The securityModel in use. Should be the User-based Security
 Model. This data is not used by the User-based Security module.
 securityLevel
 The Level of Security from which the User-based Security module
 determines if the message needs to be protected from disclosure
 and if the message needs to be authenticated.
 wholeMsg
 The whole message as it was received.
 wholeMsgLength
 The length of the message as it was received (wholeMsg).
 securityEngineID
 The snmpEngineID that was extracted from the field
 msgAuthoritativeEngineID and that was used to lookup the secrets
 in the usmUserTable.
 securityName
 The security name representing the user on whose behalf the
 message was received. The securityName has a format that is
 independent of the Security Model.
 scopedPDU
 The message payload. The data is opaque as far as the User-based
 Security Model is concerned.
 maxSizeResponseScopedPDU
 The maximum size of a scopedPDU to be included in a possible
 Response message. The User-based Security module calculates this
 size based on the msgMaxSize (as received in the message) and the
 space required for the message header (including the
 securityParameters) for such a Response message.
 securityStateReference
 A handle/reference to cachedSecurityData to be used when securing
 an outgoing Response message. When the Message Processing
 Subsystem calls the User-based Security module to generate a
 response to this incoming message it must pass this
 handle/reference.
 Upon completion of the process, the User-based Security module
 returns statusInformation and, if the process was successful, the
 additional data elements for further processing of the message. If
 the process was not successful, then an errorIndication, possibly
 with a OID and value pair of an error counter that was incremented.
2.6. Key Localization Algorithm.
 A localized key is a secret key shared between a user U and one
 authoritative SNMP engine E. Even though a user may have only one
 password and therefore one key for the whole network, the actual
 secrets shared between the user and each authoritative SNMP engine
 will be different. This is achieved by key localization [Localized-
 key].
 First, if a user uses a password, then the user's password is
 converted into a key Ku using one of the two algorithms described in
 Appendices A.2.1 and A.2.2.
 To convert key Ku into a localized key Kul of user U at the
 authoritative SNMP engine E, one appends the snmpEngineID of the
 authoritative SNMP engine to the key Ku and then appends the key Ku
 to the result, thus enveloping the snmpEngineID within the two copies
 of user's key Ku. Then one runs a secure hash function (which one
 depends on the authentication protocol defined for this user U at
 authoritative SNMP engine E; this document defines two authentication
 protocols with their associated algorithms based on MD5 and SHA).
 The output of the hash-function is the localized key Kul for user U
 at the authoritative SNMP engine E.
3. Elements of Procedure
 This section describes the security related procedures followed by an
 SNMP engine when processing SNMP messages according to the User-based
 Security Model.
3.1. Generating an Outgoing SNMP Message
 This section describes the procedure followed by an SNMP engine
 whenever it generates a message containing a management operation
 (like a request, a response, a notification, or a report) on behalf
 of a user, with a particular securityLevel.
 1) a) If any securityStateReference is passed (Response or Report
 message), then information concerning the user is extracted
 from the cachedSecurityData. The cachedSecurityData can now be
 discarded. The securityEngineID is set to the local
 snmpEngineID. The securityLevel is set to the value specified
 by the calling module.
 Otherwise,
 b) based on the securityName, information concerning the user at
 the destination snmpEngineID, specified by the
 securityEngineID, is extracted from the Local Configuration
 Datastore (LCD, usmUserTable). If information about the user
 is absent from the LCD, then an error indication
 (unknownSecurityName) is returned to the calling module.
 2) If the securityLevel specifies that the message is to be protected
 from disclosure, but the user does not support both an
 authentication and a privacy protocol then the message cannot be
 sent. An error indication (unsupportedSecurityLevel) is returned
 to the calling module.
 3) If the securityLevel specifies that the message is to be
 authenticated, but the user does not support an authentication
 protocol, then the message cannot be sent. An error indication
 (unsupportedSecurityLevel) is returned to the calling module.
 4) a) If the securityLevel specifies that the message is to be
 protected from disclosure, then the octet sequence representing
 the serialized scopedPDU is encrypted according to the user's
 privacy protocol. To do so a call is made to the privacy
 module that implements the user's privacy protocol according to
 the abstract primitive:
 statusInformation = -- success or failure
 encryptData(
 IN encryptKey -- user's localized privKey
 IN dataToEncrypt -- serialized scopedPDU
 OUT encryptedData -- serialized encryptedPDU
 OUT privParameters -- serialized privacy parameters
 )
 statusInformation
 indicates if the encryption process was successful or not.
 encryptKey
 the user's localized private privKey is the secret key that
 can be used by the encryption algorithm.
 dataToEncrypt
 the serialized scopedPDU is the data to be encrypted.
 encryptedData
 the encryptedPDU represents the encrypted scopedPDU, encoded
 as an OCTET STRING.
 privParameters
 the privacy parameters, encoded as an OCTET STRING.
 If the privacy module returns failure, then the message cannot
 be sent and an error indication (encryptionError) is returned
 to the calling module.
 If the privacy module returns success, then the returned
 privParameters are put into the msgPrivacyParameters field of
 the securityParameters and the encryptedPDU serves as the
 payload of the message being prepared.
 Otherwise,
 b) If the securityLevel specifies that the message is not to be be
 protected from disclosure, then a zero-length OCTET STRING is
 encoded into the msgPrivacyParameters field of the
 securityParameters and the plaintext scopedPDU serves as the
 payload of the message being prepared.
 5) The securityEngineID is encoded as an OCTET STRING into the
 msgAuthoritativeEngineID field of the securityParameters. Note
 that an empty (zero length) securityEngineID is OK for a Request
 message, because that will cause the remote (authoritative) SNMP
 engine to return a Report PDU with the proper securityEngineID
 included in the msgAuthoritativeEngineID in the securityParameters
 of that returned Report PDU.
 6) a) If the securityLevel specifies that the message is to be
 authenticated, then the current values of snmpEngineBoots and
 snmpEngineTime corresponding to the securityEngineID from the
 LCD are used.
 Otherwise,
 b) If this is a Response or Report message, then the current value
 of snmpEngineBoots and snmpEngineTime corresponding to the
 local snmpEngineID from the LCD are used.
 Otherwise,
 c) If this is a Request message, then a zero value is used for
 both snmpEngineBoots and snmpEngineTime. This zero value gets
 used if snmpEngineID is empty.
 The values are encoded as INTEGER respectively into the
 msgAuthoritativeEngineBoots and msgAuthoritativeEngineTime
 fields of the securityParameters.
 7) The userName is encoded as an OCTET STRING into the msgUserName
 field of the securityParameters.
 8) a) If the securityLevel specifies that the message is to be
 authenticated, the message is authenticated according to the
 user's authentication protocol. To do so a call is made to the
 authentication module that implements the user's authentication
 protocol according to the abstract service primitive:
 statusInformation =
 authenticateOutgoingMsg(
 IN authKey -- the user's localized authKey
 IN wholeMsg -- unauthenticated message
 OUT authenticatedWholeMsg -- authenticated complete message
 )
 statusInformation
 indicates if authentication was successful or not.
 authKey
 the user's localized private authKey is the secret key that
 can be used by the authentication algorithm.
 wholeMsg
 the complete serialized message to be authenticated.
 authenticatedWholeMsg
 the same as the input given to the authenticateOutgoingMsg
 service, but with msgAuthenticationParameters properly
 filled in.
 If the authentication module returns failure, then the message
 cannot be sent and an error indication (authenticationFailure)
 is returned to the calling module.
 If the authentication module returns success, then the
 msgAuthenticationParameters field is put into the
 securityParameters and the authenticatedWholeMsg represents the
 serialization of the authenticated message being prepared.
 Otherwise,
 b) If the securityLevel specifies that the message is not to be
 authenticated then a zero-length OCTET STRING is encoded into
 the msgAuthenticationParameters field of the
 securityParameters. The wholeMsg is now serialized and then
 represents the unauthenticated message being prepared.
 9) The completed message with its length is returned to the calling
 module with the statusInformation set to success.
3.2. Processing an Incoming SNMP Message
 This section describes the procedure followed by an SNMP engine
 whenever it receives a message containing a management operation on
 behalf of a user, with a particular securityLevel.
 To simplify the elements of procedure, the release of state
 information is not always explicitly specified. As a general rule,
 if state information is available when a message gets discarded, the
 state information should also be released. Also, an error indication
 can return an OID and value for an incremented counter and optionally
 a value for securityLevel, and values for contextEngineID or
 contextName for the counter. In addition, the securityStateReference
 data is returned if any such information is available at the point
 where the error is detected.
 1) If the received securityParameters is not the serialization
 (according to the conventions of [RFC3417]) of an OCTET STRING
 formatted according to the UsmSecurityParameters defined in
 section 2.4, then the snmpInASNParseErrs counter [RFC3418] is
 incremented, and an error indication (parseError) is returned to
 the calling module. Note that we return without the OID and
 value of the incremented counter, because in this case there is
 not enough information to generate a Report PDU.
 2) The values of the security parameter fields are extracted from
 the securityParameters. The securityEngineID to be returned to
 the caller is the value of the msgAuthoritativeEngineID field.
 The cachedSecurityData is prepared and a securityStateReference
 is prepared to reference this data. Values to be cached are:
 msgUserName
 3) If the value of the msgAuthoritativeEngineID field in the
 securityParameters is unknown then:
 a) a non-authoritative SNMP engine that performs discovery may
 optionally create a new entry in its Local Configuration
 Datastore (LCD) and continue processing;
 or
 b) the usmStatsUnknownEngineIDs counter is incremented, and an
 error indication (unknownEngineID) together with the OID and
 value of the incremented counter is returned to the calling
 module.
 Note in the event that a zero-length, or other illegally sized
 msgAuthoritativeEngineID is received, b) should be chosen to
 facilitate engineID discovery. Otherwise the choice between a)
 and b) is an implementation issue.
 4) Information about the value of the msgUserName and
 msgAuthoritativeEngineID fields is extracted from the Local
 Configuration Datastore (LCD, usmUserTable). If no information
 is available for the user, then the usmStatsUnknownUserNames
 counter is incremented and an error indication
 (unknownSecurityName) together with the OID and value of the
 incremented counter is returned to the calling module.
 5) If the information about the user indicates that it does not
 support the securityLevel requested by the caller, then the
 usmStatsUnsupportedSecLevels counter is incremented and an error
 indication (unsupportedSecurityLevel) together with the OID and
 value of the incremented counter is returned to the calling
 module.
 6) If the securityLevel specifies that the message is to be
 authenticated, then the message is authenticated according to the
 user's authentication protocol. To do so a call is made to the
 authentication module that implements the user's authentication
 protocol according to the abstract service primitive:
 statusInformation = -- success or failure
 authenticateIncomingMsg(
 IN authKey -- the user's localized authKey
 IN authParameters -- as received on the wire
 IN wholeMsg -- as received on the wire
 OUT authenticatedWholeMsg -- checked for authentication
 )
 statusInformation
 indicates if authentication was successful or not.
 authKey
 the user's localized private authKey is the secret key that
 can be used by the authentication algorithm.
 wholeMsg
 the complete serialized message to be authenticated.
 authenticatedWholeMsg
 the same as the input given to the authenticateIncomingMsg
 service, but after authentication has been checked.
 If the authentication module returns failure, then the message
 cannot be trusted, so the usmStatsWrongDigests counter is
 incremented and an error indication (authenticationFailure)
 together with the OID and value of the incremented counter is
 returned to the calling module.
 If the authentication module returns success, then the message is
 authentic and can be trusted so processing continues.
 7) If the securityLevel indicates an authenticated message, then the
 local values of snmpEngineBoots, snmpEngineTime and
 latestReceivedEngineTime corresponding to the value of the
 msgAuthoritativeEngineID field are extracted from the Local
 Configuration Datastore.
 a) If the extracted value of msgAuthoritativeEngineID is the same
 as the value of snmpEngineID of the processing SNMP engine
 (meaning this is the authoritative SNMP engine), then if any
 of the following conditions is true, then the message is
 considered to be outside of the Time Window:
 - the local value of snmpEngineBoots is 2147483647;
 - the value of the msgAuthoritativeEngineBoots field differs
 from the local value of snmpEngineBoots; or,
 - the value of the msgAuthoritativeEngineTime field differs
 from the local notion of snmpEngineTime by more than +/- 150
 seconds.
 If the message is considered to be outside of the Time Window
 then the usmStatsNotInTimeWindows counter is incremented and
 an error indication (notInTimeWindow) together with the OID,
 the value of the incremented counter, and an indication that
 the error must be reported with a securityLevel of authNoPriv,
 is returned to the calling module
 b) If the extracted value of msgAuthoritativeEngineID is not the
 same as the value snmpEngineID of the processing SNMP engine
 (meaning this is not the authoritative SNMP engine), then:
 1) if at least one of the following conditions is true:
 - the extracted value of the msgAuthoritativeEngineBoots
 field is greater than the local notion of the value of
 snmpEngineBoots; or,
 - the extracted value of the msgAuthoritativeEngineBoots
 field is equal to the local notion of the value of
 snmpEngineBoots, and the extracted value of
 msgAuthoritativeEngineTime field is greater than the
 value of latestReceivedEngineTime,
 then the LCD entry corresponding to the extracted value of
 the msgAuthoritativeEngineID field is updated, by setting:
 - the local notion of the value of snmpEngineBoots to the
 value of the msgAuthoritativeEngineBoots field,
 - the local notion of the value of snmpEngineTime to the
 value of the msgAuthoritativeEngineTime field, and
 - the latestReceivedEngineTime to the value of the value of
 the msgAuthoritativeEngineTime field.
 2) if any of the following conditions is true, then the
 message is considered to be outside of the Time Window:
 - the local notion of the value of snmpEngineBoots is
 2147483647;
 - the value of the msgAuthoritativeEngineBoots field is
 less than the local notion of the value of
 snmpEngineBoots; or,
 - the value of the msgAuthoritativeEngineBoots field is
 equal to the local notion of the value of snmpEngineBoots
 and the value of the msgAuthoritativeEngineTime field is
 more than 150 seconds less than the local notion of the
 value of snmpEngineTime.
 If the message is considered to be outside of the Time
 Window then an error indication (notInTimeWindow) is
 returned to the calling module.
 Note that this means that a too old (possibly replayed)
 message has been detected and is deemed unauthentic.
 Note that this procedure allows for the value of
 msgAuthoritativeEngineBoots in the message to be greater
 than the local notion of the value of snmpEngineBoots to
 allow for received messages to be accepted as authentic
 when received from an authoritative SNMP engine that has
 re-booted since the receiving SNMP engine last
 (re-)synchronized.
 8) a) If the securityLevel indicates that the message was protected
 from disclosure, then the OCTET STRING representing the
 encryptedPDU is decrypted according to the user's privacy
 protocol to obtain an unencrypted serialized scopedPDU value.
 To do so a call is made to the privacy module that implements
 the user's privacy protocol according to the abstract
 primitive:
 statusInformation = -- success or failure
 decryptData(
 IN decryptKey -- the user's localized privKey
 IN privParameters -- as received on the wire
 IN encryptedData -- encryptedPDU as received
 OUT decryptedData -- serialized decrypted scopedPDU
 )
 statusInformation
 indicates if the decryption process was successful or not.
 decryptKey
 the user's localized private privKey is the secret key that
 can be used by the decryption algorithm.
 privParameters
 the msgPrivacyParameters, encoded as an OCTET STRING.
 encryptedData
 the encryptedPDU represents the encrypted scopedPDU,
 encoded as an OCTET STRING.
 decryptedData
 the serialized scopedPDU if decryption is successful.
 If the privacy module returns failure, then the message can
 not be processed, so the usmStatsDecryptionErrors counter is
 incremented and an error indication (decryptionError) together
 with the OID and value of the incremented counter is returned
 to the calling module.
 If the privacy module returns success, then the decrypted
 scopedPDU is the message payload to be returned to the calling
 module.
 Otherwise,
 b) The scopedPDU component is assumed to be in plain text and is
 the message payload to be returned to the calling module.
 9) The maxSizeResponseScopedPDU is calculated. This is the maximum
 size allowed for a scopedPDU for a possible Response message.
 Provision is made for a message header that allows the same
 securityLevel as the received Request.
 10) The securityName for the user is retrieved from the usmUserTable.
 11) The security data is cached as cachedSecurityData, so that a
 possible response to this message can and will use the same
 authentication and privacy secrets. Information to be
 saved/cached is as follows:
 msgUserName,
 usmUserAuthProtocol, usmUserAuthKey
 usmUserPrivProtocol, usmUserPrivKey
 12) The statusInformation is set to success and a return is made to
 the calling module passing back the OUT parameters as specified
 in the processIncomingMsg primitive.
4. Discovery
 The User-based Security Model requires that a discovery process
 obtains sufficient information about other SNMP engines in order to
 communicate with them. Discovery requires an non-authoritative SNMP
 engine to learn the authoritative SNMP engine's snmpEngineID value
 before communication may proceed. This may be accomplished by
 generating a Request message with a securityLevel of noAuthNoPriv, a
 msgUserName of zero-length, a msgAuthoritativeEngineID value of zero
 length, and the varBindList left empty. The response to this message
 will be a Report message containing the snmpEngineID of the
 authoritative SNMP engine as the value of the
 msgAuthoritativeEngineID field within the msgSecurityParameters
 field. It contains a Report PDU with the usmStatsUnknownEngineIDs
 counter in the varBindList.
 If authenticated communication is required, then the discovery
 process should also establish time synchronization with the
 authoritative SNMP engine. This may be accomplished by sending an
 authenticated Request message with the value of
 msgAuthoritativeEngineID set to the newly learned snmpEngineID and
 with the values of msgAuthoritativeEngineBoots and
 msgAuthoritativeEngineTime set to zero. For an authenticated Request
 message, a valid userName must be used in the msgUserName field. The
 response to this authenticated message will be a Report message
 containing the up to date values of the authoritative SNMP engine's
 snmpEngineBoots and snmpEngineTime as the value of the
 msgAuthoritativeEngineBoots and msgAuthoritativeEngineTime fields
 respectively. It also contains the usmStatsNotInTimeWindows counter
 in the varBindList of the Report PDU. The time synchronization then
 happens automatically as part of the procedures in section 3.2 step
 7b. See also section 2.3.
5. Definitions
SNMP-USER-BASED-SM-MIB DEFINITIONS ::= BEGIN
IMPORTS
 MODULE-IDENTITY, OBJECT-TYPE,
 OBJECT-IDENTITY,
 snmpModules, Counter32 FROM SNMPv2-SMI
 TEXTUAL-CONVENTION, TestAndIncr,
 RowStatus, RowPointer,
 StorageType, AutonomousType FROM SNMPv2-TC
 MODULE-COMPLIANCE, OBJECT-GROUP FROM SNMPv2-CONF
 SnmpAdminString, SnmpEngineID,
 snmpAuthProtocols, snmpPrivProtocols FROM SNMP-FRAMEWORK-MIB;
snmpUsmMIB MODULE-IDENTITY
 LAST-UPDATED "200210160000Z" -- 16 Oct 2002, midnight
 ORGANIZATION "SNMPv3 Working Group"
 CONTACT-INFO "WG-email: snmpv3@lists.tislabs.com
 Subscribe: majordomo@lists.tislabs.com
 In msg body: subscribe snmpv3
 Chair: Russ Mundy
 Network Associates Laboratories
 postal: 15204 Omega Drive, Suite 300
 Rockville, MD 20850-4601
 USA
 email: mundy@tislabs.com
 phone: +1 301-947-7107
 Co-Chair: David Harrington
 Enterasys Networks
 Postal: 35 Industrial Way
 P. O. Box 5004
 Rochester, New Hampshire 03866-5005
 USA
 EMail: dbh@enterasys.com
 Phone: +1 603-337-2614
 Co-editor Uri Blumenthal
 Lucent Technologies
 postal: 67 Whippany Rd.
 Whippany, NJ 07981
 USA
 email: uri@lucent.com
 phone: +1-973-386-2163
 Co-editor: Bert Wijnen
 Lucent Technologies
 postal: Schagen 33
 3461 GL Linschoten
 Netherlands
 email: bwijnen@lucent.com
 phone: +31-348-480-685
 "
 DESCRIPTION "The management information definitions for the
 SNMP User-based Security Model.
 Copyright (C) The Internet Society (2002). This
 version of this MIB module is part of RFC 3414;
 see the RFC itself for full legal notices.
 "
-- Revision history
 REVISION "200210160000Z" -- 16 Oct 2002, midnight
 DESCRIPTION "Changes in this revision:
 - Updated references and contact info.
 - Clarification to usmUserCloneFrom DESCRIPTION
 clause
 - Fixed 'command responder' into 'command generator'
 in last para of DESCRIPTION clause of
 usmUserTable.
 This revision published as RFC3414.
 "
 REVISION "199901200000Z" -- 20 Jan 1999, midnight
 DESCRIPTION "Clarifications, published as RFC2574"
 REVISION "199711200000Z" -- 20 Nov 1997, midnight
 DESCRIPTION "Initial version, published as RFC2274"
 ::= { snmpModules 15 }
-- Administrative assignments ****************************************
usmMIBObjects OBJECT IDENTIFIER ::= { snmpUsmMIB 1 }
usmMIBConformance OBJECT IDENTIFIER ::= { snmpUsmMIB 2 }
-- Identification of Authentication and Privacy Protocols ************
usmNoAuthProtocol OBJECT-IDENTITY
 STATUS current
 DESCRIPTION "No Authentication Protocol."
 ::= { snmpAuthProtocols 1 }
usmHMACMD5AuthProtocol OBJECT-IDENTITY
 STATUS current
 DESCRIPTION "The HMAC-MD5-96 Digest Authentication Protocol."
 REFERENCE "- H. Krawczyk, M. Bellare, R. Canetti HMAC:
 Keyed-Hashing for Message Authentication,
 RFC2104, Feb 1997.
 - Rivest, R., Message Digest Algorithm MD5, RFC1321.
 "
 ::= { snmpAuthProtocols 2 }
usmHMACSHAAuthProtocol OBJECT-IDENTITY
 STATUS current
 DESCRIPTION "The HMAC-SHA-96 Digest Authentication Protocol."
 REFERENCE "- H. Krawczyk, M. Bellare, R. Canetti, HMAC:
 Keyed-Hashing for Message Authentication,
 RFC2104, Feb 1997.
 - Secure Hash Algorithm. NIST FIPS 180-1.
 "
 ::= { snmpAuthProtocols 3 }
usmNoPrivProtocol OBJECT-IDENTITY
 STATUS current
 DESCRIPTION "No Privacy Protocol."
 ::= { snmpPrivProtocols 1 }
usmDESPrivProtocol OBJECT-IDENTITY
 STATUS current
 DESCRIPTION "The CBC-DES Symmetric Encryption Protocol."
 REFERENCE "- Data Encryption Standard, National Institute of
 Standards and Technology. Federal Information
 Processing Standard (FIPS) Publication 46-1.
 Supersedes FIPS Publication 46,
 (January, 1977; reaffirmed January, 1988).
 - Data Encryption Algorithm, American National
 Standards Institute. ANSI X3.92-1981,
 (December, 1980).
 - DES Modes of Operation, National Institute of
 Standards and Technology. Federal Information
 Processing Standard (FIPS) Publication 81,
 (December, 1980).
 - Data Encryption Algorithm - Modes of Operation,
 American National Standards Institute.
 ANSI X3.106-1983, (May 1983).
 "
 ::= { snmpPrivProtocols 2 }
-- Textual Conventions ***********************************************
KeyChange ::= TEXTUAL-CONVENTION
 STATUS current
 DESCRIPTION
 "Every definition of an object with this syntax must identify
 a protocol P, a secret key K, and a hash algorithm H
 that produces output of L octets.
 The object's value is a manager-generated, partially-random
 value which, when modified, causes the value of the secret
 key K, to be modified via a one-way function.
 The value of an instance of this object is the concatenation
 of two components: first a 'random' component and then a
 'delta' component.
 The lengths of the random and delta components
 are given by the corresponding value of the protocol P;
 if P requires K to be a fixed length, the length of both the
 random and delta components is that fixed length; if P
 allows the length of K to be variable up to a particular
 maximum length, the length of the random component is that
 maximum length and the length of the delta component is any
 length less than or equal to that maximum length.
 For example, usmHMACMD5AuthProtocol requires K to be a fixed
 length of 16 octets and L - of 16 octets.
 usmHMACSHAAuthProtocol requires K to be a fixed length of
 20 octets and L - of 20 octets. Other protocols may define
 other sizes, as deemed appropriate.
 When a requester wants to change the old key K to a new
 key keyNew on a remote entity, the 'random' component is
 obtained from either a true random generator, or from a
 pseudorandom generator, and the 'delta' component is
 computed as follows:
 - a temporary variable is initialized to the existing value
 of K;
 - if the length of the keyNew is greater than L octets,
 then:
 - the random component is appended to the value of the
 temporary variable, and the result is input to the
 the hash algorithm H to produce a digest value, and
 the temporary variable is set to this digest value;
 - the value of the temporary variable is XOR-ed with
 the first (next) L-octets (16 octets in case of MD5)
 of the keyNew to produce the first (next) L-octets
 (16 octets in case of MD5) of the 'delta' component.
 - the above two steps are repeated until the unused
 portion of the keyNew component is L octets or less,
 - the random component is appended to the value of the
 temporary variable, and the result is input to the
 hash algorithm H to produce a digest value;
 - this digest value, truncated if necessary to be the same
 length as the unused portion of the keyNew, is XOR-ed
 with the unused portion of the keyNew to produce the
 (final portion of the) 'delta' component.
 For example, using MD5 as the hash algorithm H:
 iterations = (lenOfDelta - 1)/16; /* integer division */
 temp = keyOld;
 for (i = 0; i < iterations; i++) {
 temp = MD5 (temp || random);
 delta[i*16 .. (i*16)+15] =
 temp XOR keyNew[i*16 .. (i*16)+15];
 }
 temp = MD5 (temp || random);
 delta[i*16 .. lenOfDelta-1] =
 temp XOR keyNew[i*16 .. lenOfDelta-1];
 The 'random' and 'delta' components are then concatenated as
 described above, and the resulting octet string is sent to
 the recipient as the new value of an instance of this object.
 At the receiver side, when an instance of this object is set
 to a new value, then a new value of K is computed as follows:
 - a temporary variable is initialized to the existing value
 of K;
 - if the length of the delta component is greater than L
 octets, then:
 - the random component is appended to the value of the
 temporary variable, and the result is input to the
 hash algorithm H to produce a digest value, and the
 temporary variable is set to this digest value;
 - the value of the temporary variable is XOR-ed with
 the first (next) L-octets (16 octets in case of MD5)
 of the delta component to produce the first (next)
 L-octets (16 octets in case of MD5) of the new value
 of K.
 - the above two steps are repeated until the unused
 portion of the delta component is L octets or less,
 - the random component is appended to the value of the
 temporary variable, and the result is input to the
 hash algorithm H to produce a digest value;
 - this digest value, truncated if necessary to be the same
 length as the unused portion of the delta component, is
 XOR-ed with the unused portion of the delta component to
 produce the (final portion of the) new value of K.
 For example, using MD5 as the hash algorithm H:
 iterations = (lenOfDelta - 1)/16; /* integer division */
 temp = keyOld;
 for (i = 0; i < iterations; i++) {
 temp = MD5 (temp || random);
 keyNew[i*16 .. (i*16)+15] =
 temp XOR delta[i*16 .. (i*16)+15];
 }
 temp = MD5 (temp || random);
 keyNew[i*16 .. lenOfDelta-1] =
 temp XOR delta[i*16 .. lenOfDelta-1];
 The value of an object with this syntax, whenever it is
 retrieved by the management protocol, is always the zero
 length string.
 Note that the keyOld and keyNew are the localized keys.
 Note that it is probably wise that when an SNMP entity sends
 a SetRequest to change a key, that it keeps a copy of the old
 key until it has confirmed that the key change actually
 succeeded.
 "
 SYNTAX OCTET STRING
-- Statistics for the User-based Security Model **********************
usmStats OBJECT IDENTIFIER ::= { usmMIBObjects 1 }
usmStatsUnsupportedSecLevels OBJECT-TYPE
 SYNTAX Counter32
 MAX-ACCESS read-only
 STATUS current
 DESCRIPTION "The total number of packets received by the SNMP
 engine which were dropped because they requested a
 securityLevel that was unknown to the SNMP engine
 or otherwise unavailable.
 "
 ::= { usmStats 1 }
usmStatsNotInTimeWindows OBJECT-TYPE
 SYNTAX Counter32
 MAX-ACCESS read-only
 STATUS current
 DESCRIPTION "The total number of packets received by the SNMP
 engine which were dropped because they appeared
 outside of the authoritative SNMP engine's window.
 "
 ::= { usmStats 2 }
usmStatsUnknownUserNames OBJECT-TYPE
 SYNTAX Counter32
 MAX-ACCESS read-only
 STATUS current
 DESCRIPTION "The total number of packets received by the SNMP
 engine which were dropped because they referenced a
 user that was not known to the SNMP engine.
 "
 ::= { usmStats 3 }
usmStatsUnknownEngineIDs OBJECT-TYPE
 SYNTAX Counter32
 MAX-ACCESS read-only
 STATUS current
 DESCRIPTION "The total number of packets received by the SNMP
 engine which were dropped because they referenced an
 snmpEngineID that was not known to the SNMP engine.
 "
 ::= { usmStats 4 }
usmStatsWrongDigests OBJECT-TYPE
 SYNTAX Counter32
 MAX-ACCESS read-only
 STATUS current
 DESCRIPTION "The total number of packets received by the SNMP
 engine which were dropped because they didn't
 contain the expected digest value.
 "
 ::= { usmStats 5 }
usmStatsDecryptionErrors OBJECT-TYPE
 SYNTAX Counter32
 MAX-ACCESS read-only
 STATUS current
 DESCRIPTION "The total number of packets received by the SNMP
 engine which were dropped because they could not be
 decrypted.
 "
 ::= { usmStats 6 }
-- The usmUser Group ************************************************
usmUser OBJECT IDENTIFIER ::= { usmMIBObjects 2 }
usmUserSpinLock OBJECT-TYPE
 SYNTAX TestAndIncr
 MAX-ACCESS read-write
 STATUS current
 DESCRIPTION "An advisory lock used to allow several cooperating
 Command Generator Applications to coordinate their
 use of facilities to alter secrets in the
 usmUserTable.
 "
 ::= { usmUser 1 }
-- The table of valid users for the User-based Security Model ********
usmUserTable OBJECT-TYPE
 SYNTAX SEQUENCE OF UsmUserEntry
 MAX-ACCESS not-accessible
 STATUS current
 DESCRIPTION "The table of users configured in the SNMP engine's
 Local Configuration Datastore (LCD).
 To create a new user (i.e., to instantiate a new
 conceptual row in this table), it is recommended to
 follow this procedure:
 1) GET(usmUserSpinLock.0) and save in sValue.
 2) SET(usmUserSpinLock.0=sValue,
 usmUserCloneFrom=templateUser,
 usmUserStatus=createAndWait)
 You should use a template user to clone from
 which has the proper auth/priv protocol defined.
 If the new user is to use privacy:
 3) generate the keyChange value based on the secret
 privKey of the clone-from user and the secret key
 to be used for the new user. Let us call this
 pkcValue.
 4) GET(usmUserSpinLock.0) and save in sValue.
 5) SET(usmUserSpinLock.0=sValue,
 usmUserPrivKeyChange=pkcValue
 usmUserPublic=randomValue1)
 6) GET(usmUserPulic) and check it has randomValue1.
 If not, repeat steps 4-6.
 If the new user will never use privacy:
 7) SET(usmUserPrivProtocol=usmNoPrivProtocol)
 If the new user is to use authentication:
 8) generate the keyChange value based on the secret
 authKey of the clone-from user and the secret key
 to be used for the new user. Let us call this
 akcValue.
 9) GET(usmUserSpinLock.0) and save in sValue.
 10) SET(usmUserSpinLock.0=sValue,
 usmUserAuthKeyChange=akcValue
 usmUserPublic=randomValue2)
 11) GET(usmUserPulic) and check it has randomValue2.
 If not, repeat steps 9-11.
 If the new user will never use authentication:
 12) SET(usmUserAuthProtocol=usmNoAuthProtocol)
 Finally, activate the new user:
 13) SET(usmUserStatus=active)
 The new user should now be available and ready to be
 used for SNMPv3 communication. Note however that access
 to MIB data must be provided via configuration of the
 SNMP-VIEW-BASED-ACM-MIB.
 The use of usmUserSpinlock is to avoid conflicts with
 another SNMP command generator application which may
 also be acting on the usmUserTable.
 "
 ::= { usmUser 2 }
usmUserEntry OBJECT-TYPE
 SYNTAX UsmUserEntry
 MAX-ACCESS not-accessible
 STATUS current
 DESCRIPTION "A user configured in the SNMP engine's Local
 Configuration Datastore (LCD) for the User-based
 Security Model.
 "
 INDEX { usmUserEngineID,
 usmUserName
 }
 ::= { usmUserTable 1 }
UsmUserEntry ::= SEQUENCE
 {
 usmUserEngineID SnmpEngineID,
 usmUserName SnmpAdminString,
 usmUserSecurityName SnmpAdminString,
 usmUserCloneFrom RowPointer,
 usmUserAuthProtocol AutonomousType,
 usmUserAuthKeyChange KeyChange,
 usmUserOwnAuthKeyChange KeyChange,
 usmUserPrivProtocol AutonomousType,
 usmUserPrivKeyChange KeyChange,
 usmUserOwnPrivKeyChange KeyChange,
 usmUserPublic OCTET STRING,
 usmUserStorageType StorageType,
 usmUserStatus RowStatus
 }
usmUserEngineID OBJECT-TYPE
 SYNTAX SnmpEngineID
 MAX-ACCESS not-accessible
 STATUS current
 DESCRIPTION "An SNMP engine's administratively-unique identifier.
 In a simple agent, this value is always that agent's
 own snmpEngineID value.
 The value can also take the value of the snmpEngineID
 of a remote SNMP engine with which this user can
 communicate.
 "
 ::= { usmUserEntry 1 }
usmUserName OBJECT-TYPE
 SYNTAX SnmpAdminString (SIZE(1..32))
 MAX-ACCESS not-accessible
 STATUS current
 DESCRIPTION "A human readable string representing the name of
 the user.
 This is the (User-based Security) Model dependent
 security ID.
 "
 ::= { usmUserEntry 2 }
usmUserSecurityName OBJECT-TYPE
 SYNTAX SnmpAdminString
 MAX-ACCESS read-only
 STATUS current
 DESCRIPTION "A human readable string representing the user in
 Security Model independent format.
 The default transformation of the User-based Security
 Model dependent security ID to the securityName and
 vice versa is the identity function so that the
 securityName is the same as the userName.
 "
 ::= { usmUserEntry 3 }
usmUserCloneFrom OBJECT-TYPE
 SYNTAX RowPointer
 MAX-ACCESS read-create
 STATUS current
 DESCRIPTION "A pointer to another conceptual row in this
 usmUserTable. The user in this other conceptual
 row is called the clone-from user.
 When a new user is created (i.e., a new conceptual
 row is instantiated in this table), the privacy and
 authentication parameters of the new user must be
 cloned from its clone-from user. These parameters are:
 - authentication protocol (usmUserAuthProtocol)
 - privacy protocol (usmUserPrivProtocol)
 They will be copied regardless of what the current
 value is.
 Cloning also causes the initial values of the secret
 authentication key (authKey) and the secret encryption
 key (privKey) of the new user to be set to the same
 values as the corresponding secrets of the clone-from
 user to allow the KeyChange process to occur as
 required during user creation.
 The first time an instance of this object is set by
 a management operation (either at or after its
 instantiation), the cloning process is invoked.
 Subsequent writes are successful but invoke no
 action to be taken by the receiver.
 The cloning process fails with an 'inconsistentName'
 error if the conceptual row representing the
 clone-from user does not exist or is not in an active
 state when the cloning process is invoked.
 When this object is read, the ZeroDotZero OID
 is returned.
 "
 ::= { usmUserEntry 4 }
usmUserAuthProtocol OBJECT-TYPE
 SYNTAX AutonomousType
 MAX-ACCESS read-create
 STATUS current
 DESCRIPTION "An indication of whether messages sent on behalf of
 this user to/from the SNMP engine identified by
 usmUserEngineID, can be authenticated, and if so,
 the type of authentication protocol which is used.
 An instance of this object is created concurrently
 with the creation of any other object instance for
 the same user (i.e., as part of the processing of
 the set operation which creates the first object
 instance in the same conceptual row).
 If an initial set operation (i.e. at row creation time)
 tries to set a value for an unknown or unsupported
 protocol, then a 'wrongValue' error must be returned.
 The value will be overwritten/set when a set operation
 is performed on the corresponding instance of
 usmUserCloneFrom.
 Once instantiated, the value of such an instance of
 this object can only be changed via a set operation to
 the value of the usmNoAuthProtocol.
 If a set operation tries to change the value of an
 existing instance of this object to any value other
 than usmNoAuthProtocol, then an 'inconsistentValue'
 error must be returned.
 If a set operation tries to set the value to the
 usmNoAuthProtocol while the usmUserPrivProtocol value
 in the same row is not equal to usmNoPrivProtocol,
 then an 'inconsistentValue' error must be returned.
 That means that an SNMP command generator application
 must first ensure that the usmUserPrivProtocol is set
 to the usmNoPrivProtocol value before it can set
 the usmUserAuthProtocol value to usmNoAuthProtocol.
 "
 DEFVAL { usmNoAuthProtocol }
 ::= { usmUserEntry 5 }
usmUserAuthKeyChange OBJECT-TYPE
 SYNTAX KeyChange -- typically (SIZE (0 | 32)) for HMACMD5
 -- typically (SIZE (0 | 40)) for HMACSHA
 MAX-ACCESS read-create
 STATUS current
 DESCRIPTION "An object, which when modified, causes the secret
 authentication key used for messages sent on behalf
 of this user to/from the SNMP engine identified by
 usmUserEngineID, to be modified via a one-way
 function.
 The associated protocol is the usmUserAuthProtocol.
 The associated secret key is the user's secret
 authentication key (authKey). The associated hash
 algorithm is the algorithm used by the user's
 usmUserAuthProtocol.
 When creating a new user, it is an 'inconsistentName'
 error for a set operation to refer to this object
 unless it is previously or concurrently initialized
 through a set operation on the corresponding instance
 of usmUserCloneFrom.
 When the value of the corresponding usmUserAuthProtocol
 is usmNoAuthProtocol, then a set is successful, but
 effectively is a no-op.
 When this object is read, the zero-length (empty)
 string is returned.
 The recommended way to do a key change is as follows:
 1) GET(usmUserSpinLock.0) and save in sValue.
 2) generate the keyChange value based on the old
 (existing) secret key and the new secret key,
 let us call this kcValue.
 If you do the key change on behalf of another user:
 3) SET(usmUserSpinLock.0=sValue,
 usmUserAuthKeyChange=kcValue
 usmUserPublic=randomValue)
 If you do the key change for yourself:
 4) SET(usmUserSpinLock.0=sValue,
 usmUserOwnAuthKeyChange=kcValue
 usmUserPublic=randomValue)
 If you get a response with error-status of noError,
 then the SET succeeded and the new key is active.
 If you do not get a response, then you can issue a
 GET(usmUserPublic) and check if the value is equal
 to the randomValue you did send in the SET. If so, then
 the key change succeeded and the new key is active
 (probably the response got lost). If not, then the SET
 request probably never reached the target and so you
 can start over with the procedure above.
 "
 DEFVAL { ''H } -- the empty string
 ::= { usmUserEntry 6 }
usmUserOwnAuthKeyChange OBJECT-TYPE
 SYNTAX KeyChange -- typically (SIZE (0 | 32)) for HMACMD5
 -- typically (SIZE (0 | 40)) for HMACSHA
 MAX-ACCESS read-create
 STATUS current
 DESCRIPTION "Behaves exactly as usmUserAuthKeyChange, with one
 notable difference: in order for the set operation
 to succeed, the usmUserName of the operation
 requester must match the usmUserName that
 indexes the row which is targeted by this
 operation.
 In addition, the USM security model must be
 used for this operation.
 The idea here is that access to this column can be
 public, since it will only allow a user to change
 his own secret authentication key (authKey).
 Note that this can only be done once the row is active.
 When a set is received and the usmUserName of the
 requester is not the same as the umsUserName that
 indexes the row which is targeted by this operation,
 then a 'noAccess' error must be returned.
 When a set is received and the security model in use
 is not USM, then a 'noAccess' error must be returned.
 "
 DEFVAL { ''H } -- the empty string
 ::= { usmUserEntry 7 }
usmUserPrivProtocol OBJECT-TYPE
 SYNTAX AutonomousType
 MAX-ACCESS read-create
 STATUS current
 DESCRIPTION "An indication of whether messages sent on behalf of
 this user to/from the SNMP engine identified by
 usmUserEngineID, can be protected from disclosure,
 and if so, the type of privacy protocol which is used.
 An instance of this object is created concurrently
 with the creation of any other object instance for
 the same user (i.e., as part of the processing of
 the set operation which creates the first object
 instance in the same conceptual row).
 If an initial set operation (i.e. at row creation time)
 tries to set a value for an unknown or unsupported
 protocol, then a 'wrongValue' error must be returned.
 The value will be overwritten/set when a set operation
 is performed on the corresponding instance of
 usmUserCloneFrom.
 Once instantiated, the value of such an instance of
 this object can only be changed via a set operation to
 the value of the usmNoPrivProtocol.
 If a set operation tries to change the value of an
 existing instance of this object to any value other
 than usmNoPrivProtocol, then an 'inconsistentValue'
 error must be returned.
 Note that if any privacy protocol is used, then you
 must also use an authentication protocol. In other
 words, if usmUserPrivProtocol is set to anything else
 than usmNoPrivProtocol, then the corresponding instance
 of usmUserAuthProtocol cannot have a value of
 usmNoAuthProtocol. If it does, then an
 'inconsistentValue' error must be returned.
 "
 DEFVAL { usmNoPrivProtocol }
 ::= { usmUserEntry 8 }
usmUserPrivKeyChange OBJECT-TYPE
 SYNTAX KeyChange -- typically (SIZE (0 | 32)) for DES
 MAX-ACCESS read-create
 STATUS current
 DESCRIPTION "An object, which when modified, causes the secret
 encryption key used for messages sent on behalf
 of this user to/from the SNMP engine identified by
 usmUserEngineID, to be modified via a one-way
 function.
 The associated protocol is the usmUserPrivProtocol.
 The associated secret key is the user's secret
 privacy key (privKey). The associated hash
 algorithm is the algorithm used by the user's
 usmUserAuthProtocol.
 When creating a new user, it is an 'inconsistentName'
 error for a set operation to refer to this object
 unless it is previously or concurrently initialized
 through a set operation on the corresponding instance
 of usmUserCloneFrom.
 When the value of the corresponding usmUserPrivProtocol
 is usmNoPrivProtocol, then a set is successful, but
 effectively is a no-op.
 When this object is read, the zero-length (empty)
 string is returned.
 See the description clause of usmUserAuthKeyChange for
 a recommended procedure to do a key change.
 "
 DEFVAL { ''H } -- the empty string
 ::= { usmUserEntry 9 }
usmUserOwnPrivKeyChange OBJECT-TYPE
 SYNTAX KeyChange -- typically (SIZE (0 | 32)) for DES
 MAX-ACCESS read-create
 STATUS current
 DESCRIPTION "Behaves exactly as usmUserPrivKeyChange, with one
 notable difference: in order for the Set operation
 to succeed, the usmUserName of the operation
 requester must match the usmUserName that indexes
 the row which is targeted by this operation.
 In addition, the USM security model must be
 used for this operation.
 The idea here is that access to this column can be
 public, since it will only allow a user to change
 his own secret privacy key (privKey).
 Note that this can only be done once the row is active.
 When a set is received and the usmUserName of the
 requester is not the same as the umsUserName that
 indexes the row which is targeted by this operation,
 then a 'noAccess' error must be returned.
 When a set is received and the security model in use
 is not USM, then a 'noAccess' error must be returned.
 "
 DEFVAL { ''H } -- the empty string
 ::= { usmUserEntry 10 }
usmUserPublic OBJECT-TYPE
 SYNTAX OCTET STRING (SIZE(0..32))
 MAX-ACCESS read-create
 STATUS current
 DESCRIPTION "A publicly-readable value which can be written as part
 of the procedure for changing a user's secret
 authentication and/or privacy key, and later read to
 determine whether the change of the secret was
 effected.
 "
 DEFVAL { ''H } -- the empty string
 ::= { usmUserEntry 11 }
usmUserStorageType OBJECT-TYPE
 SYNTAX StorageType
 MAX-ACCESS read-create
 STATUS current
 DESCRIPTION "The storage type for this conceptual row.
 Conceptual rows having the value 'permanent' must
 allow write-access at a minimum to:
 - usmUserAuthKeyChange, usmUserOwnAuthKeyChange
 and usmUserPublic for a user who employs
 authentication, and
 - usmUserPrivKeyChange, usmUserOwnPrivKeyChange
 and usmUserPublic for a user who employs
 privacy.
 Note that any user who employs authentication or
 privacy must allow its secret(s) to be updated and
 thus cannot be 'readOnly'.
 If an initial set operation tries to set the value to
 'readOnly' for a user who employs authentication or
 privacy, then an 'inconsistentValue' error must be
 returned. Note that if the value has been previously
 set (implicit or explicit) to any value, then the rules
 as defined in the StorageType Textual Convention apply.
 It is an implementation issue to decide if a SET for
 a readOnly or permanent row is accepted at all. In some
 contexts this may make sense, in others it may not. If
 a SET for a readOnly or permanent row is not accepted
 at all, then a 'wrongValue' error must be returned.
 "
 DEFVAL { nonVolatile }
 ::= { usmUserEntry 12 }
usmUserStatus OBJECT-TYPE
 SYNTAX RowStatus
 MAX-ACCESS read-create
 STATUS current
 DESCRIPTION "The status of this conceptual row.
 Until instances of all corresponding columns are
 appropriately configured, the value of the
 corresponding instance of the usmUserStatus column
 is 'notReady'.
 In particular, a newly created row for a user who
 employs authentication, cannot be made active until the
 corresponding usmUserCloneFrom and usmUserAuthKeyChange
 have been set.
 Further, a newly created row for a user who also
 employs privacy, cannot be made active until the
 usmUserPrivKeyChange has been set.
 The RowStatus TC [RFC2579] requires that this
 DESCRIPTION clause states under which circumstances
 other objects in this row can be modified:
 The value of this object has no effect on whether
 other objects in this conceptual row can be modified,
 except for usmUserOwnAuthKeyChange and
 usmUserOwnPrivKeyChange. For these 2 objects, the
 value of usmUserStatus MUST be active.
 "
 ::= { usmUserEntry 13 }
-- Conformance Information *******************************************
usmMIBCompliances OBJECT IDENTIFIER ::= { usmMIBConformance 1 }
usmMIBGroups OBJECT IDENTIFIER ::= { usmMIBConformance 2 }
-- Compliance statements
usmMIBCompliance MODULE-COMPLIANCE
 STATUS current
 DESCRIPTION "The compliance statement for SNMP engines which
 implement the SNMP-USER-BASED-SM-MIB.
 "
 MODULE -- this module
 MANDATORY-GROUPS { usmMIBBasicGroup }
 OBJECT usmUserAuthProtocol
 MIN-ACCESS read-only
 DESCRIPTION "Write access is not required."
 OBJECT usmUserPrivProtocol
 MIN-ACCESS read-only
 DESCRIPTION "Write access is not required."
 ::= { usmMIBCompliances 1 }
-- Units of compliance
usmMIBBasicGroup OBJECT-GROUP
 OBJECTS {
 usmStatsUnsupportedSecLevels,
 usmStatsNotInTimeWindows,
 usmStatsUnknownUserNames,
 usmStatsUnknownEngineIDs,
 usmStatsWrongDigests,
 usmStatsDecryptionErrors,
 usmUserSpinLock,
 usmUserSecurityName,
 usmUserCloneFrom,
 usmUserAuthProtocol,
 usmUserAuthKeyChange,
 usmUserOwnAuthKeyChange,
 usmUserPrivProtocol,
 usmUserPrivKeyChange,
 usmUserOwnPrivKeyChange,
 usmUserPublic,
 usmUserStorageType,
 usmUserStatus
 }
 STATUS current
 DESCRIPTION "A collection of objects providing for configuration
 of an SNMP engine which implements the SNMP
 User-based Security Model.
 "
 ::= { usmMIBGroups 1 }
END
6. HMAC-MD5-96 Authentication Protocol
 This section describes the HMAC-MD5-96 authentication protocol. This
 authentication protocol is the first defined for the User-based
 Security Model. It uses MD5 hash-function which is described in
 [RFC1321], in HMAC mode described in [RFC2104], truncating the output
 to 96 bits.
 This protocol is identified by usmHMACMD5AuthProtocol.
 Over time, other authentication protocols may be defined either as a
 replacement of this protocol or in addition to this protocol.
6.1. Mechanisms
 - In support of data integrity, a message digest algorithm is
 required. A digest is calculated over an appropriate portion of an
 SNMP message and included as part of the message sent to the
 recipient.
 - In support of data origin authentication and data integrity, a
 secret value is prepended to SNMP message prior to computing the
 digest; the calculated digest is partially inserted into the SNMP
 message prior to transmission, and the prepended value is not
 transmitted. The secret value is shared by all SNMP engines
 authorized to originate messages on behalf of the appropriate user.
6.1.1. Digest Authentication Mechanism
 The Digest Authentication Mechanism defined in this memo provides
 for:
 - verification of the integrity of a received message, i.e., the
 message received is the message sent.
 The integrity of the message is protected by computing a digest
 over an appropriate portion of the message. The digest is computed
 by the originator of the message, transmitted with the message, and
 verified by the recipient of the message.
 - verification of the user on whose behalf the message was generated.
 A secret value known only to SNMP engines authorized to generate
 messages on behalf of a user is used in HMAC mode (see [RFC2104]).
 It also recommends the hash-function output used as Message
 Authentication Code, to be truncated.
 This protocol uses the MD5 [RFC1321] message digest algorithm. A
 128-bit MD5 digest is calculated in a special (HMAC) way over the
 designated portion of an SNMP message and the first 96 bits of this
 digest is included as part of the message sent to the recipient. The
 size of the digest carried in a message is 12 octets. The size of
 the private authentication key (the secret) is 16 octets. For the
 details see section 6.3.
6.2. Elements of the Digest Authentication Protocol
 This section contains definitions required to realize the
 authentication module defined in this section of this memo.
6.2.1. Users
 Authentication using this authentication protocol makes use of a
 defined set of userNames. For any user on whose behalf a message
 must be authenticated at a particular SNMP engine, that SNMP engine
 must have knowledge of that user. An SNMP engine that wishes to
 communicate with another SNMP engine must also have knowledge of a
 user known to that engine, including knowledge of the applicable
 attributes of that user.
 A user and its attributes are defined as follows:
 <userName>
 A string representing the name of the user.
 <authKey>
 A user's secret key to be used when calculating a digest.
 It MUST be 16 octets long for MD5.
6.2.2. msgAuthoritativeEngineID
 The msgAuthoritativeEngineID value contained in an authenticated
 message specifies the authoritative SNMP engine for that particular
 message (see the definition of SnmpEngineID in the SNMP Architecture
 document [RFC3411]).
 The user's (private) authentication key is normally different at each
 authoritative SNMP engine and so the snmpEngineID is used to select
 the proper key for the authentication process.
6.2.3. SNMP Messages Using this Authentication Protocol
 Messages using this authentication protocol carry a
 msgAuthenticationParameters field as part of the
 msgSecurityParameters. For this protocol, the
 msgAuthenticationParameters field is the serialized OCTET STRING
 representing the first 12 octets of the HMAC-MD5-96 output done over
 the wholeMsg.
 The digest is calculated over the wholeMsg so if a message is
 authenticated, that also means that all the fields in the message are
 intact and have not been tampered with.
6.2.4. Services provided by the HMAC-MD5-96 Authentication Module
 This section describes the inputs and outputs that the HMAC-MD5-96
 Authentication module expects and produces when the User-based
 Security module calls the HMAC-MD5-96 Authentication module for
 services.
6.2.4.1. Services for Generating an Outgoing SNMP Message
 The HMAC-MD5-96 authentication protocol assumes that the selection of
 the authKey is done by the caller and that the caller passes the
 secret key to be used.
 Upon completion the authentication module returns statusInformation
 and, if the message digest was correctly calculated, the wholeMsg
 with the digest inserted at the proper place. The abstract service
 primitive is:
 statusInformation = -- success or failure
 authenticateOutgoingMsg(
 IN authKey -- secret key for authentication
 IN wholeMsg -- unauthenticated complete message
 OUT authenticatedWholeMsg -- complete authenticated message
 )
 The abstract data elements are:
 statusInformation
 An indication of whether the authentication process was successful.
 If not it is an indication of the problem.
 authKey
 The secret key to be used by the authentication algorithm. The
 length of this key MUST be 16 octets.
 wholeMsg
 The message to be authenticated.
 authenticatedWholeMsg
 The authenticated message (including inserted digest) on output.
 Note, that authParameters field is filled by the authentication
 module and this module and this field should be already present in
 the wholeMsg before the Message Authentication Code (MAC) is
 generated.
6.2.4.2. Services for Processing an Incoming SNMP Message
 The HMAC-MD5-96 authentication protocol assumes that the selection of
 the authKey is done by the caller and that the caller passes the
 secret key to be used.
 Upon completion the authentication module returns statusInformation
 and, if the message digest was correctly calculated, the wholeMsg as
 it was processed. The abstract service primitive is:
 statusInformation = -- success or failure
 authenticateIncomingMsg(
 IN authKey -- secret key for authentication
 IN authParameters -- as received on the wire
 IN wholeMsg -- as received on the wire
 OUT authenticatedWholeMsg -- complete authenticated message
 )
 The abstract data elements are:
 statusInformation
 An indication of whether the authentication process was successful.
 If not it is an indication of the problem.
 authKey
 The secret key to be used by the authentication algorithm. The
 length of this key MUST be 16 octets.
 authParameters
 The authParameters from the incoming message.
 wholeMsg
 The message to be authenticated on input and the authenticated
 message on output.
 authenticatedWholeMsg
 The whole message after the authentication check is complete.
6.3. Elements of Procedure
 This section describes the procedures for the HMAC-MD5-96
 authentication protocol.
6.3.1. Processing an Outgoing Message
 This section describes the procedure followed by an SNMP engine
 whenever it must authenticate an outgoing message using the
 usmHMACMD5AuthProtocol.
 1) The msgAuthenticationParameters field is set to the serialization,
 according to the rules in [RFC3417], of an OCTET STRING containing
 12 zero octets.
 2) From the secret authKey, two keys K1 and K2 are derived:
 a) extend the authKey to 64 octets by appending 48 zero octets;
 save it as extendedAuthKey
 b) obtain IPAD by replicating the octet 0x36 64 times;
 c) obtain K1 by XORing extendedAuthKey with IPAD;
 d) obtain OPAD by replicating the octet 0x5C 64 times;
 e) obtain K2 by XORing extendedAuthKey with OPAD.
 3) Prepend K1 to the wholeMsg and calculate MD5 digest over it
 according to [RFC1321].
 4) Prepend K2 to the result of the step 4 and calculate MD5 digest
 over it according to [RFC1321]. Take the first 12 octets of the
 final digest - this is Message Authentication Code (MAC).
 5) Replace the msgAuthenticationParameters field with MAC obtained in
 the step 4.
 6) The authenticatedWholeMsg is then returned to the caller together
 with statusInformation indicating success.
6.3.2. Processing an Incoming Message
 This section describes the procedure followed by an SNMP engine
 whenever it must authenticate an incoming message using the
 usmHMACMD5AuthProtocol.
 1) If the digest received in the msgAuthenticationParameters field is
 not 12 octets long, then an failure and an errorIndication
 (authenticationError) is returned to the calling module.
 2) The MAC received in the msgAuthenticationParameters field is
 saved.
 3) The digest in the msgAuthenticationParameters field is replaced by
 the 12 zero octets.
 4) From the secret authKey, two keys K1 and K2 are derived:
 a) extend the authKey to 64 octets by appending 48 zero octets;
 save it as extendedAuthKey
 b) obtain IPAD by replicating the octet 0x36 64 times;
 c) obtain K1 by XORing extendedAuthKey with IPAD;
 d) obtain OPAD by replicating the octet 0x5C 64 times;
 e) obtain K2 by XORing extendedAuthKey with OPAD.
 5) The MAC is calculated over the wholeMsg:
 a) prepend K1 to the wholeMsg and calculate the MD5 digest over
 it;
 b) prepend K2 to the result of step 5.a and calculate the MD5
 digest over it;
 c) first 12 octets of the result of step 5.b is the MAC.
 The msgAuthenticationParameters field is replaced with the MAC
 value that was saved in step 2.
 6) Then the newly calculated MAC is compared with the MAC saved in
 step 2. If they do not match, then an failure and an
 errorIndication (authenticationFailure) is returned to the calling
 module.
 7) The authenticatedWholeMsg and statusInformation indicating success
 are then returned to the caller.
7. HMAC-SHA-96 Authentication Protocol
 This section describes the HMAC-SHA-96 authentication protocol. This
 protocol uses the SHA hash-function which is described in [SHA-NIST],
 in HMAC mode described in [RFC2104], truncating the output to 96
 bits.
 This protocol is identified by usmHMACSHAAuthProtocol.
 Over time, other authentication protocols may be defined either as a
 replacement of this protocol or in addition to this protocol.
7.1. Mechanisms
 - In support of data integrity, a message digest algorithm is
 required. A digest is calculated over an appropriate portion of an
 SNMP message and included as part of the message sent to the
 recipient.
 - In support of data origin authentication and data integrity, a
 secret value is prepended to the SNMP message prior to computing
 the digest; the calculated digest is then partially inserted into
 the message prior to transmission. The prepended secret is not
 transmitted. The secret value is shared by all SNMP engines
 authorized to originate messages on behalf of the appropriate user.
7.1.1. Digest Authentication Mechanism
 The Digest Authentication Mechanism defined in this memo provides
 for:
 - verification of the integrity of a received message, i.e., the
 message received is the message sent.
 The integrity of the message is protected by computing a digest
 over an appropriate portion of the message. The digest is computed
 by the originator of the message, transmitted with the message, and
 verified by the recipient of the message.
 - verification of the user on whose behalf the message was generated.
 A secret value known only to SNMP engines authorized to generate
 messages on behalf of a user is used in HMAC mode (see [RFC2104]).
 It also recommends the hash-function output used as Message
 Authentication Code, to be truncated.
 This mechanism uses the SHA [SHA-NIST] message digest algorithm. A
 160-bit SHA digest is calculated in a special (HMAC) way over the
 designated portion of an SNMP message and the first 96 bits of this
 digest is included as part of the message sent to the recipient. The
 size of the digest carried in a message is 12 octets. The size of
 the private authentication key (the secret) is 20 octets. For the
 details see section 7.3.
7.2. Elements of the HMAC-SHA-96 Authentication Protocol
 This section contains definitions required to realize the
 authentication module defined in this section of this memo.
7.2.1. Users
 Authentication using this authentication protocol makes use of a
 defined set of userNames. For any user on whose behalf a message
 must be authenticated at a particular SNMP engine, that SNMP engine
 must have knowledge of that user. An SNMP engine that wishes to
 communicate with another SNMP engine must also have knowledge of a
 user known to that engine, including knowledge of the applicable
 attributes of that user.
 A user and its attributes are defined as follows:
 <userName>
 A string representing the name of the user.
 <authKey>
 A user's secret key to be used when calculating a digest.
 It MUST be 20 octets long for SHA.
7.2.2. msgAuthoritativeEngineID
 The msgAuthoritativeEngineID value contained in an authenticated
 message specifies the authoritative SNMP engine for that particular
 message (see the definition of SnmpEngineID in the SNMP Architecture
 document [RFC3411]).
 The user's (private) authentication key is normally different at each
 authoritative SNMP engine and so the snmpEngineID is used to select
 the proper key for the authentication process.
7.2.3. SNMP Messages Using this Authentication Protocol
 Messages using this authentication protocol carry a
 msgAuthenticationParameters field as part of the
 msgSecurityParameters. For this protocol, the
 msgAuthenticationParameters field is the serialized OCTET STRING
 representing the first 12 octets of HMAC-SHA-96 output done over the
 wholeMsg.
 The digest is calculated over the wholeMsg so if a message is
 authenticated, that also means that all the fields in the message are
 intact and have not been tampered with.
7.2.4. Services Provided by the HMAC-SHA-96 Authentication Module
 This section describes the inputs and outputs that the HMAC-SHA-96
 Authentication module expects and produces when the User-based
 Security module calls the HMAC-SHA-96 Authentication module for
 services.
7.2.4.1. Services for Generating an Outgoing SNMP Message
 HMAC-SHA-96 authentication protocol assumes that the selection of the
 authKey is done by the caller and that the caller passes the secret
 key to be used.
 Upon completion the authentication module returns statusInformation
 and, if the message digest was correctly calculated, the wholeMsg
 with the digest inserted at the proper place. The abstract service
 primitive is:
 statusInformation = -- success or failure
 authenticateOutgoingMsg(
 IN authKey -- secret key for authentication
 IN wholeMsg -- unauthenticated complete message
 OUT authenticatedWholeMsg -- complete authenticated message
 )
 The abstract data elements are:
 statusInformation
 An indication of whether the authentication process was successful.
 If not it is an indication of the problem.
 authKey
 The secret key to be used by the authentication algorithm. The
 length of this key MUST be 20 octets.
 wholeMsg
 The message to be authenticated.
 authenticatedWholeMsg
 The authenticated message (including inserted digest) on output.
 Note, that authParameters field is filled by the authentication
 module and this field should be already present in the wholeMsg
 before the Message Authentication Code (MAC) is generated.
7.2.4.2. Services for Processing an Incoming SNMP Message
 HMAC-SHA-96 authentication protocol assumes that the selection of the
 authKey is done by the caller and that the caller passes the secret
 key to be used.
 Upon completion the authentication module returns statusInformation
 and, if the message digest was correctly calculated, the wholeMsg as
 it was processed. The abstract service primitive is:
 statusInformation = -- success or failure
 authenticateIncomingMsg(
 IN authKey -- secret key for authentication
 IN authParameters -- as received on the wire
 IN wholeMsg -- as received on the wire
 OUT authenticatedWholeMsg -- complete authenticated message
 )
 The abstract data elements are:
 statusInformation
 An indication of whether the authentication process was successful.
 If not it is an indication of the problem.
 authKey
 The secret key to be used by the authentication algorithm. The
 length of this key MUST be 20 octets.
 authParameters
 The authParameters from the incoming message.
 wholeMsg
 The message to be authenticated on input and the authenticated
 message on output.
 authenticatedWholeMsg
 The whole message after the authentication check is complete.
7.3. Elements of Procedure
 This section describes the procedures for the HMAC-SHA-96
 authentication protocol.
7.3.1. Processing an Outgoing Message
 This section describes the procedure followed by an SNMP engine
 whenever it must authenticate an outgoing message using the
 usmHMACSHAAuthProtocol.
 1) The msgAuthenticationParameters field is set to the serialization,
 according to the rules in [RFC3417], of an OCTET STRING containing
 12 zero octets.
 2) From the secret authKey, two keys K1 and K2 are derived:
 a) extend the authKey to 64 octets by appending 44 zero octets;
 save it as extendedAuthKey
 b) obtain IPAD by replicating the octet 0x36 64 times;
 c) obtain K1 by XORing extendedAuthKey with IPAD;
 d) obtain OPAD by replicating the octet 0x5C 64 times;
 e) obtain K2 by XORing extendedAuthKey with OPAD.
 3) Prepend K1 to the wholeMsg and calculate the SHA digest over it
 according to [SHA-NIST].
 4) Prepend K2 to the result of the step 4 and calculate SHA digest
 over it according to [SHA-NIST]. Take the first 12 octets of the
 final digest - this is Message Authentication Code (MAC).
 5) Replace the msgAuthenticationParameters field with MAC obtained in
 the step 5.
 6) The authenticatedWholeMsg is then returned to the caller together
 with statusInformation indicating success.
7.3.2. Processing an Incoming Message
 This section describes the procedure followed by an SNMP engine
 whenever it must authenticate an incoming message using the
 usmHMACSHAAuthProtocol.
 1) If the digest received in the msgAuthenticationParameters field is
 not 12 octets long, then an failure and an errorIndication
 (authenticationError) is returned to the calling module.
 2) The MAC received in the msgAuthenticationParameters field is
 saved.
 3) The digest in the msgAuthenticationParameters field is replaced by
 the 12 zero octets.
 4) From the secret authKey, two keys K1 and K2 are derived:
 a) extend the authKey to 64 octets by appending 44 zero octets;
 save it as extendedAuthKey
 b) obtain IPAD by replicating the octet 0x36 64 times;
 c) obtain K1 by XORing extendedAuthKey with IPAD;
 d) obtain OPAD by replicating the octet 0x5C 64 times;
 e) obtain K2 by XORing extendedAuthKey with OPAD.
 5) The MAC is calculated over the wholeMsg:
 a) prepend K1 to the wholeMsg and calculate the SHA digest over
 it;
 b) prepend K2 to the result of step 5.a and calculate the SHA
 digest over it;
 c) first 12 octets of the result of step 5.b is the MAC.
 The msgAuthenticationParameters field is replaced with the MAC
 value that was saved in step 2.
 6) The the newly calculated MAC is compared with the MAC saved in
 step 2. If they do not match, then a failure and an
 errorIndication (authenticationFailure) are returned to the
 calling module.
 7) The authenticatedWholeMsg and statusInformation indicating success
 are then returned to the caller.
8. CBC-DES Symmetric Encryption Protocol
 This section describes the CBC-DES Symmetric Encryption Protocol.
 This protocol is the first privacy protocol defined for the
 User-based Security Model.
 This protocol is identified by usmDESPrivProtocol.
 Over time, other privacy protocols may be defined either as a
 replacement of this protocol or in addition to this protocol.
8.1. Mechanisms
 - In support of data confidentiality, an encryption algorithm is
 required. An appropriate portion of the message is encrypted prior
 to being transmitted. The User-based Security Model specifies that
 the scopedPDU is the portion of the message that needs to be
 encrypted.
 - A secret value in combination with a timeliness value is used to
 create the en/decryption key and the initialization vector. The
 secret value is shared by all SNMP engines authorized to originate
 messages on behalf of the appropriate user.
8.1.1. Symmetric Encryption Protocol
 The Symmetric Encryption Protocol defined in this memo provides
 support for data confidentiality. The designated portion of an SNMP
 message is encrypted and included as part of the message sent to the
 recipient.
 Two organizations have published specifications defining the DES:
 the National Institute of Standards and Technology (NIST) [DES-NIST]
 and the American National Standards Institute [DES-ANSI]. There is a
 companion Modes of Operation specification for each definition
 ([DESO-NIST] and [DESO-ANSI], respectively).
 The NIST has published three additional documents that implementors
 may find useful.
 - There is a document with guidelines for implementing and using the
 DES, including functional specifications for the DES and its modes
 of operation [DESG-NIST].
 - There is a specification of a validation test suite for the DES
 [DEST-NIST]. The suite is designed to test all aspects of the DES
 and is useful for pinpointing specific problems.
 - There is a specification of a maintenance test for the DES [DESM-
 NIST]. The test utilizes a minimal amount of data and processing
 to test all components of the DES. It provides a simple yes-or-no
 indication of correct operation and is useful to run as part of an
 initialization step, e.g., when a computer re-boots.
8.1.1.1. DES key and Initialization Vector
 The first 8 octets of the 16-octet secret (private privacy key) are
 used as a DES key. Since DES uses only 56 bits, the Least
 Significant Bit in each octet is disregarded.
 The Initialization Vector for encryption is obtained using the
 following procedure.
 The last 8 octets of the 16-octet secret (private privacy key) are
 used as pre-IV.
 In order to ensure that the IV for two different packets encrypted by
 the same key, are not the same (i.e., the IV does not repeat) we need
 to "salt" the pre-IV with something unique per packet. An 8-octet
 string is used as the "salt". The concatenation of the generating
 SNMP engine's 32-bit snmpEngineBoots and a local 32-bit integer, that
 the encryption engine maintains, is input to the "salt". The 32-bit
 integer is initialized to an arbitrary value at boot time.
 The 32-bit snmpEngineBoots is converted to the first 4 octets (Most
 Significant Byte first) of our "salt". The 32-bit integer is then
 converted to the last 4 octet (Most Significant Byte first) of our
 "salt". The resulting "salt" is then XOR-ed with the pre-IV to
 obtain the IV. The 8-octet "salt" is then put into the
 privParameters field encoded as an OCTET STRING. The "salt" integer
 is then modified. We recommend that it be incremented by one and
 wrap when it reaches the maximum value.
 How exactly the value of the "salt" (and thus of the IV) varies, is
 an implementation issue, as long as the measures are taken to avoid
 producing a duplicate IV.
 The "salt" must be placed in the privParameters field to enable the
 receiving entity to compute the correct IV and to decrypt the
 message.
8.1.1.2. Data Encryption
 The data to be encrypted is treated as sequence of octets. Its
 length should be an integral multiple of 8 - and if it is not, the
 data is padded at the end as necessary. The actual pad value is
 irrelevant.
 The data is encrypted in Cipher Block Chaining mode.
 The plaintext is divided into 64-bit blocks.
 The plaintext for each block is XOR-ed with the ciphertext of the
 previous block, the result is encrypted and the output of the
 encryption is the ciphertext for the block. This procedure is
 repeated until there are no more plaintext blocks.
 For the very first block, the Initialization Vector is used instead
 of the ciphertext of the previous block.
8.1.1.3. Data Decryption
 Before decryption, the encrypted data length is verified. If the
 length of the OCTET STRING to be decrypted is not an integral
 multiple of 8 octets, the decryption process is halted and an
 appropriate exception noted. When decrypting, the padding is
 ignored.
 The first ciphertext block is decrypted, the decryption output is
 XOR-ed with the Initialization Vector, and the result is the first
 plaintext block.
 For each subsequent block, the ciphertext block is decrypted, the
 decryption output is XOR-ed with the previous ciphertext block and
 the result is the plaintext block.
8.2. Elements of the DES Privacy Protocol
 This section contains definitions required to realize the privacy
 module defined by this memo.
8.2.1. Users
 Data en/decryption using this Symmetric Encryption Protocol makes use
 of a defined set of userNames. For any user on whose behalf a
 message must be en/decrypted at a particular SNMP engine, that SNMP
 engine must have knowledge of that user. An SNMP engine that wishes
 to communicate with another SNMP engine must also have knowledge of a
 user known to that SNMP engine, including knowledge of the applicable
 attributes of that user.
 A user and its attributes are defined as follows:
 <userName>
 An octet string representing the name of the user.
 <privKey>
 A user's secret key to be used as input for the DES key and IV.
 The length of this key MUST be 16 octets.
8.2.2. msgAuthoritativeEngineID
 The msgAuthoritativeEngineID value contained in an authenticated
 message specifies the authoritative SNMP engine for that particular
 message (see the definition of SnmpEngineID in the SNMP Architecture
 document [RFC3411]).
 The user's (private) privacy key is normally different at each
 authoritative SNMP engine and so the snmpEngineID is used to select
 the proper key for the en/decryption process.
8.2.3. SNMP Messages Using this Privacy Protocol
 Messages using this privacy protocol carry a msgPrivacyParameters
 field as part of the msgSecurityParameters. For this protocol, the
 msgPrivacyParameters field is the serialized OCTET STRING
 representing the "salt" that was used to create the IV.
8.2.4. Services Provided by the DES Privacy Module
 This section describes the inputs and outputs that the DES Privacy
 module expects and produces when the User-based Security module
 invokes the DES Privacy module for services.
8.2.4.1. Services for Encrypting Outgoing Data
 This DES privacy protocol assumes that the selection of the privKey
 is done by the caller and that the caller passes the secret key to be
 used.
 Upon completion the privacy module returns statusInformation and, if
 the encryption process was successful, the encryptedPDU and the
 msgPrivacyParameters encoded as an OCTET STRING. The abstract
 service primitive is:
 statusInformation = -- success of failure
 encryptData(
 IN encryptKey -- secret key for encryption
 IN dataToEncrypt -- data to encrypt (scopedPDU)
 OUT encryptedData -- encrypted data (encryptedPDU)
 OUT privParameters -- filled in by service provider
 )
 The abstract data elements are:
 statusInformation
 An indication of the success or failure of the encryption process.
 In case of failure, it is an indication of the error.
 encryptKey
 The secret key to be used by the encryption algorithm. The length
 of this key MUST be 16 octets.
 dataToEncrypt
 The data that must be encrypted.
 encryptedData
 The encrypted data upon successful completion.
 privParameters
 The privParameters encoded as an OCTET STRING.
8.2.4.2. Services for Decrypting Incoming Data
 This DES privacy protocol assumes that the selection of the privKey
 is done by the caller and that the caller passes the secret key to be
 used.
 Upon completion the privacy module returns statusInformation and, if
 the decryption process was successful, the scopedPDU in plain text.
 The abstract service primitive is:
 statusInformation =
 decryptData(
 IN decryptKey -- secret key for decryption
 IN privParameters -- as received on the wire
 IN encryptedData -- encrypted data (encryptedPDU)
 OUT decryptedData -- decrypted data (scopedPDU)
 )
 The abstract data elements are:
 statusInformation
 An indication whether the data was successfully decrypted and if
 not an indication of the error.
 decryptKey
 The secret key to be used by the decryption algorithm. The length
 of this key MUST be 16 octets.
 privParameters
 The "salt" to be used to calculate the IV.
 encryptedData
 The data to be decrypted.
 decryptedData
 The decrypted data.
8.3. Elements of Procedure.
 This section describes the procedures for the DES privacy protocol.
8.3.1. Processing an Outgoing Message
 This section describes the procedure followed by an SNMP engine
 whenever it must encrypt part of an outgoing message using the
 usmDESPrivProtocol.
 1) The secret cryptKey is used to construct the DES encryption key,
 the "salt" and the DES pre-IV (from which the IV is computed as
 described in section 8.1.1.1).
 2) The privParameters field is set to the serialization according to
 the rules in [RFC3417] of an OCTET STRING representing the "salt"
 string.
 3) The scopedPDU is encrypted (as described in section 8.1.1.2)
 and the encrypted data is serialized according to the rules in
 [RFC3417] as an OCTET STRING.
 4) The serialized OCTET STRING representing the encrypted scopedPDU
 together with the privParameters and statusInformation indicating
 success is returned to the calling module.
8.3.2. Processing an Incoming Message
 This section describes the procedure followed by an SNMP engine
 whenever it must decrypt part of an incoming message using the
 usmDESPrivProtocol.
 1) If the privParameters field is not an 8-octet OCTET STRING, then
 an error indication (decryptionError) is returned to the calling
 module.
 2) The "salt" is extracted from the privParameters field.
 3) The secret cryptKey and the "salt" are then used to construct the
 DES decryption key and pre-IV (from which the IV is computed as
 described in section 8.1.1.1).
 4) The encryptedPDU is then decrypted (as described in section
 8.1.1.3).
 5) If the encryptedPDU cannot be decrypted, then an error indication
 (decryptionError) is returned to the calling module.
 6) The decrypted scopedPDU and statusInformation indicating success
 are returned to the calling module.
9. Intellectual Property
 The IETF takes no position regarding the validity or scope of any
 intellectual property or other rights that might be claimed to
 pertain to the implementation or use of the technology described in
 this document or the extent to which any license under such rights
 might or might not be available; neither does it represent that it
 has made any effort to identify any such rights. Information on the
 IETF's procedures with respect to rights in standards-track and
 standards-related documentation can be found in BCP-11. Copies of
 claims of rights made available for publication and any assurances of
 licenses to be made available, or the result of an attempt made to
 obtain a general license or permission for the use of such
 proprietary rights by implementors or users of this specification can
 be obtained from the IETF Secretariat.
 The IETF invites any interested party to bring to its attention any
 copyrights, patents or patent applications, or other proprietary
 rights which may cover technology that may be required to practice
 this standard. Please address the information to the IETF Executive
 Director.
10. Acknowledgements
 This document is the result of the efforts of the SNMPv3 Working
 Group. Some special thanks are in order to the following SNMPv3 WG
 members:
 Harald Tveit Alvestrand (Maxware)
 Dave Battle (SNMP Research, Inc.)
 Alan Beard (Disney Worldwide Services)
 Paul Berrevoets (SWI Systemware/Halcyon Inc.)
 Martin Bjorklund (Ericsson)
 Uri Blumenthal (IBM T.J. Watson Research Center)
 Jeff Case (SNMP Research, Inc.)
 John Curran (BBN)
 Mike Daniele (Compaq Computer Corporation))
 T. Max Devlin (Eltrax Systems)
 John Flick (Hewlett Packard)
 Rob Frye (MCI)
 Wes Hardaker (U.C.Davis, Information Technology - D.C.A.S.)
 David Harrington (Cabletron Systems Inc.)
 Lauren Heintz (BMC Software, Inc.)
 N.C. Hien (IBM T.J. Watson Research Center)
 Michael Kirkham (InterWorking Labs, Inc.)
 Dave Levi (SNMP Research, Inc.)
 Louis A Mamakos (UUNET Technologies Inc.)
 Joe Marzot (Nortel Networks)
 Paul Meyer (Secure Computing Corporation)
 Keith McCloghrie (Cisco Systems)
 Bob Moore (IBM)
 Russ Mundy (TIS Labs at Network Associates)
 Bob Natale (ACE*COMM Corporation)
 Mike O'Dell (UUNET Technologies Inc.)
 Dave Perkins (DeskTalk)
 Peter Polkinghorne (Brunel University)
 Randy Presuhn (BMC Software, Inc.)
 David Reeder (TIS Labs at Network Associates)
 David Reid (SNMP Research, Inc.)
 Aleksey Romanov (Quality Quorum)
 Shawn Routhier (Epilogue)
 Juergen Schoenwaelder (TU Braunschweig)
 Bob Stewart (Cisco Systems)
 Mike Thatcher (Independent Consultant)
 Bert Wijnen (IBM T.J. Watson Research Center)
 The document is based on recommendations of the IETF Security and
 Administrative Framework Evolution for SNMP Advisory Team. Members
 of that Advisory Team were:
 David Harrington (Cabletron Systems Inc.)
 Jeff Johnson (Cisco Systems)
 David Levi (SNMP Research Inc.)
 John Linn (Openvision)
 Russ Mundy (Trusted Information Systems) chair
 Shawn Routhier (Epilogue)
 Glenn Waters (Nortel)
 Bert Wijnen (IBM T. J. Watson Research Center)
 As recommended by the Advisory Team and the SNMPv3 Working Group
 Charter, the design incorporates as much as practical from previous
 RFCs and drafts. As a result, special thanks are due to the authors
 of previous designs known as SNMPv2u and SNMPv2*:
 Jeff Case (SNMP Research, Inc.)
 David Harrington (Cabletron Systems Inc.)
 David Levi (SNMP Research, Inc.)
 Keith McCloghrie (Cisco Systems)
 Brian O'Keefe (Hewlett Packard)
 Marshall T. Rose (Dover Beach Consulting)
 Jon Saperia (BGS Systems Inc.)
 Steve Waldbusser (International Network Services)
 Glenn W. Waters (Bell-Northern Research Ltd.)
11. Security Considerations
11.1. Recommended Practices
 This section describes practices that contribute to the secure,
 effective operation of the mechanisms defined in this memo.
 - An SNMP engine must discard SNMP Response messages that do not
 correspond to any currently outstanding Request message. It is the
 responsibility of the Message Processing module to take care of
 this. For example it can use a msgID for that.
 An SNMP Command Generator Application must discard any Response
 Class PDU for which there is no currently outstanding Confirmed
 Class PDU; for example for SNMPv2 [RFC3416] PDUs, the request-id
 component in the PDU can be used to correlate Responses to
 outstanding Requests.
 Although it would be typical for an SNMP engine and an SNMP Command
 Generator Application to do this as a matter of course, when using
 these security protocols it is significant due to the possibility
 of message duplication (malicious or otherwise).
 - If an SNMP engine uses a msgID for correlating Response messages to
 outstanding Request messages, then it MUST use different msgIDs in
 all such Request messages that it sends out during a Time Window
 (150 seconds) period.
 A Command Generator or Notification Originator Application MUST use
 different request-ids in all Request PDUs that it sends out during
 a TimeWindow (150 seconds) period.
 This must be done to protect against the possibility of message
 duplication (malicious or otherwise).
 For example, starting operations with a msgID and/or request-id
 value of zero is not a good idea. Initializing them with an
 unpredictable number (so they do not start out the same after each
 reboot) and then incrementing by one would be acceptable.
 - An SNMP engine should perform time synchronization using
 authenticated messages in order to protect against the possibility
 of message duplication (malicious or otherwise).
 - When sending state altering messages to a managed authoritative
 SNMP engine, a Command Generator Application should delay sending
 successive messages to that managed SNMP engine until a positive
 acknowledgement is received for the previous message or until the
 previous message expires.
 No message ordering is imposed by the SNMP. Messages may be
 received in any order relative to their time of generation and each
 will be processed in the ordered received. Note that when an
 authenticated message is sent to a managed SNMP engine, it will be
 valid for a period of time of approximately 150 seconds under
 normal circumstances, and is subject to replay during this period.
 Indeed, an SNMP engine and SNMP Command Generator Applications must
 cope with the loss and re-ordering of messages resulting from
 anomalies in the network as a matter of course.
 However, a managed object, snmpSetSerialNo [RFC3418], is
 specifically defined for use with SNMP Set operations in order to
 provide a mechanism to ensure that the processing of SNMP messages
 occurs in a specific order.
 - The frequency with which the secrets of a User-based Security Model
 user should be changed is indirectly related to the frequency of
 their use.
 Protecting the secrets from disclosure is critical to the overall
 security of the protocols. Frequent use of a secret provides a
 continued source of data that may be useful to a cryptanalyst in
 exploiting known or perceived weaknesses in an algorithm. Frequent
 changes to the secret avoid this vulnerability.
 Changing a secret after each use is generally regarded as the most
 secure practice, but a significant amount of overhead may be
 associated with that approach.
 Note, too, in a local environment the threat of disclosure may be
 less significant, and as such the changing of secrets may be less
 frequent. However, when public data networks are used as the
 communication paths, more caution is prudent.
11.2 Defining Users
 The mechanisms defined in this document employ the notion of users on
 whose behalf messages are sent. How "users" are defined is subject
 to the security policy of the network administration. For example,
 users could be individuals (e.g., "joe" or "jane"), or a particular
 role (e.g., "operator" or "administrator"), or a combination (e.g.,
 "joe-operator", "jane-operator" or "joe-admin"). Furthermore, a user
 may be a logical entity, such as an SNMP Application or a set of SNMP
 Applications, acting on behalf of an individual or role, or set of
 individuals, or set of roles, including combinations.
 Appendix A describes an algorithm for mapping a user "password" to a
 16/20 octet value for use as either a user's authentication key or
 privacy key (or both). Note however, that using the same password
 (and therefore the same key) for both authentication and privacy is
 very poor security practice and should be strongly discouraged.
 Passwords are often generated, remembered, and input by a human.
 Human-generated passwords may be less than the 16/20 octets required
 by the authentication and privacy protocols, and brute force attacks
 can be quite easy on a relatively short ASCII character set.
 Therefore, the algorithm is Appendix A performs a transformation on
 the password. If the Appendix A algorithm is used, SNMP
 implementations (and SNMP configuration applications) must ensure
 that passwords are at least 8 characters in length. Please note that
 longer passwords with repetitive strings may result in exactly the
 same key. For example, a password 'bertbert' will result in exactly
 the same key as password 'bertbertbert'.
 Because the Appendix A algorithm uses such passwords (nearly)
 directly, it is very important that they not be easily guessed. It
 is suggested that they be composed of mixed-case alphanumeric and
 punctuation characters that don't form words or phrases that might be
 found in a dictionary. Longer passwords improve the security of the
 system. Users may wish to input multiword phrases to make their
 password string longer while ensuring that it is memorable.
 Since it is infeasible for human users to maintain different
 passwords for every SNMP engine, but security requirements strongly
 discourage having the same key for more than one SNMP engine, the
 User-based Security Model employs a compromise proposed in
 [Localized-key]. It derives the user keys for the SNMP engines from
 user's password in such a way that it is practically impossible to
 either determine the user's password, or user's key for another SNMP
 engine from any combination of user's keys on SNMP engines.
 Note however, that if user's password is disclosed, then key
 localization will not help and network security may be compromised in
 this case. Therefore a user's password or non-localized key MUST NOT
 be stored on a managed device/node. Instead the localized key SHALL
 be stored (if at all), so that, in case a device does get
 compromised, no other managed or managing devices get compromised.
11.3. Conformance
 To be termed a "Secure SNMP implementation" based on the User-based
 Security Model, an SNMP implementation MUST:
 - implement one or more Authentication Protocol(s). The HMAC-MD5-96
 and HMAC-SHA-96 Authentication Protocols defined in this memo are
 examples of such protocols.
 - to the maximum extent possible, prohibit access to the secret(s) of
 each user about which it maintains information in a Local
 Configuration Datastore (LCD) under all circumstances except as
 required to generate and/or validate SNMP messages with respect to
 that user.
 - implement the key-localization mechanism.
 - implement the SNMP-USER-BASED-SM-MIB.
 In addition, an authoritative SNMP engine SHOULD provide initial
 configuration in accordance with Appendix A.1.
 Implementation of a Privacy Protocol (the DES Symmetric Encryption
 Protocol defined in this memo is one such protocol) is optional.
11.4. Use of Reports
 The use of unsecure reports (i.e., sending them with a securityLevel
 of noAuthNoPriv) potentially exposes a non-authoritative SNMP engine
 to some form of attacks. Some people consider these denial of
 service attacks, others don't. An installation should evaluate the
 risk involved before deploying unsecure Report PDUs.
11.5 Access to the SNMP-USER-BASED-SM-MIB
 The objects in this MIB may be considered sensitive in many
 environments. Specifically the objects in the usmUserTable contain
 information about users and their authentication and privacy
 protocols. It is important to closely control (both read and write)
 access to these MIB objects by using appropriately configured Access
 Control models (for example the View-based Access Control Model as
 specified in [RFC3415]).
12. References
12.1 Normative References
 [RFC1321] Rivest, R., "Message Digest Algorithm MD5", RFC 1321,
 April 1992.
 [RFC2104] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC:
 Keyed-Hashing for Message Authentication", RFC 2104,
 February 1997.
 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
 Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2578] McCloghrie, K., Perkins, D., Schoenwaelder, J., Case,
 J., Rose, M. and S. Waldbusser, "Structure of
 Management Information Version 2 (SMIv2)", STD 58,
 RFC 2578, April 1999.
 [RFC2579] McCloghrie, K., Perkins, D., Schoenwaelder, J., Case,
 J., Rose, M. and S. Waldbusser, "Textual Conventions
 for SMIv2", STD 58, RFC 2579, April 1999.
 [RFC2580] McCloghrie, K., Perkins, D., Schoenwaelder, J., Case,
 J., Rose, M. and S. Waldbusser, "Conformance
 Statements for SMIv2", STD 58, RFC 2580, April 1999.
 [RFC3411] Harrington, D., Presuhn, R. and B. Wijnen, "An
 Architecture for Describing Simple Network Management
 Protocol (SNMP) Management Frameworks", STD 62, RFC
 3411, December 2002.
 [RFC3412] Case, J., Harrington, D., Presuhn, R. and B. Wijnen,
 "Message Processing and Dispatching for the Simple
 Network Management Protocol (SNMP)", STD 62, RFC
 3412, December 2002.
 [RFC3415] Wijnen, B., Presuhn, R. and K. McCloghrie, "View-
 based Access Control Model (VACM) for the Simple
 Network Management Protocol (SNMP)", STD 62, RFC
 3415, December 2002.
 [RFC3416] Presuhn, R., Case, J., McCloghrie, K., Rose, M. and
 S. Waldbusser, "Version 2 of the Protocol Operations
 for the Simple Network Management Protocol (SNMP)",
 STD 62, RFC 3416, December 2002.
 [RFC3417] Presuhn, R., Case, J., McCloghrie, K., Rose, M. and
 S. Waldbusser, "Transport Mappings for the Simple
 Network Management Protocol (SNMP)", STD 62, RFC
 3417, December 2002.
 [RFC3418] Presuhn, R., Case, J., McCloghrie, K., Rose, M. and
 S. Waldbusser, "Management Information Base (MIB) for
 the Simple Network Management Protocol (SNMP)", STD
 62, RFC 3418, December 2002.
 [DES-NIST] Data Encryption Standard, National Institute of
 Standards and Technology. Federal Information
 Processing Standard (FIPS) Publication 46-1.
 Supersedes FIPS Publication 46, (January, 1977;
 reaffirmed January, 1988).
 [DESO-NIST] DES Modes of Operation, National Institute of
 Standards and Technology. Federal Information
 Processing Standard (FIPS) Publication 81, (December,
 1980).
 [SHA-NIST] Secure Hash Algorithm. NIST FIPS 180-1, (April, 1995)
 http://csrc.nist.gov/fips/fip180-1.txt (ASCII)
 http://csrc.nist.gov/fips/fip180-1.ps (Postscript)
12.1 Informative References
 [Localized-Key] U. Blumenthal, N. C. Hien, B. Wijnen "Key Derivation
 for Network Management Applications" IEEE Network
 Magazine, April/May issue, 1997.
 [DES-ANSI] Data Encryption Algorithm, American National
 Standards Institute. ANSI X3.92-1981, (December,
 1980).
 [DESO-ANSI] Data Encryption Algorithm - Modes of Operation,
 American National Standards Institute. ANSI X3.106-
 1983, (May 1983).
 [DESG-NIST] Guidelines for Implementing and Using the NBS Data
 Encryption Standard, National Institute of Standards
 and Technology. Federal Information Processing
 Standard (FIPS) Publication 74, (April, 1981).
 [DEST-NIST] Validating the Correctness of Hardware
 Implementations of the NBS Data Encryption Standard,
 National Institute of Standards and Technology.
 Special Publication 500-20.
 [DESM-NIST] Maintenance Testing for the Data Encryption Standard,
 National Institute of Standards and Technology.
 Special Publication 500-61, (August, 1980).
 [RFC3174] Eastlake, D. 3rd and P. Jones, "US Secure Hash
 Algorithm 1 (SHA1)", RFC 3174, September 2001.
APPENDIX A - Installation
A.1. SNMP engine Installation Parameters
 During installation, an authoritative SNMP engine SHOULD (in the
 meaning as defined in [RFC2119]) be configured with several initial
 parameters. These include:
 1) A Security Posture
 The choice of security posture determines if initial configuration
 is implemented and if so how. One of three possible choices is
 selected:
 minimum-secure,
 semi-secure,
 very-secure (i.e., no-initial-configuration)
 In the case of a very-secure posture, there is no initial
 configuration, and so the following steps are irrelevant.
 2) One or More Secrets
 These are the authentication/privacy secrets for the first user to
 be configured.
 One way to accomplish this is to have the installer enter a
 "password" for each required secret. The password is then
 algorithmically converted into the required secret by:
 - forming a string of length 1,048,576 octets by repeating the
 value of the password as often as necessary, truncating
 accordingly, and using the resulting string as the input to the
 MD5 algorithm [RFC1321]. The resulting digest, termed
 "digest1", is used in the next step.
 - a second string is formed by concatenating digest1, the SNMP
 engine's snmpEngineID value, and digest1. This string is used
 as input to the MD5 algorithm [RFC1321].
 The resulting digest is the required secret (see Appendix A.2).
 With these configured parameters, the SNMP engine instantiates the
 following usmUserEntry in the usmUserTable:
 no privacy support privacy support
 ------------------ ---------------
 usmUserEngineID localEngineID localEngineID
 usmUserName "initial" "initial"
 usmUserSecurityName "initial" "initial"
 usmUserCloneFrom ZeroDotZero ZeroDotZero
 usmUserAuthProtocol usmHMACMD5AuthProtocol usmHMACMD5AuthProtocol
 usmUserAuthKeyChange "" ""
 usmUserOwnAuthKeyChange "" ""
 usmUserPrivProtocol none usmDESPrivProtocol
 usmUserPrivKeyChange "" ""
 usmUserOwnPrivKeyChange "" ""
 usmUserPublic "" ""
 usmUserStorageType anyValidStorageType anyValidStorageType
 usmUserStatus active active
 It is recommended to also instantiate a set of template
 usmUserEntries which can be used as clone-from users for newly
 created usmUserEntries. These are the two suggested entries:
 no privacy support privacy support
 ------------------ ---------------
 usmUserEngineID localEngineID localEngineID
 usmUserName "templateMD5" "templateMD5"
 usmUserSecurityName "templateMD5" "templateMD5"
 usmUserCloneFrom ZeroDotZero ZeroDotZero
 usmUserAuthProtocol usmHMACMD5AuthProtocol usmHMACMD5AuthProtocol
 usmUserAuthKeyChange "" ""
 usmUserOwnAuthKeyChange "" ""
 usmUserPrivProtocol none usmDESPrivProtocol
 usmUserPrivKeyChange "" ""
 usmUserOwnPrivKeyChange "" ""
 usmUserPublic "" ""
 usmUserStorageType permanent permanent
 usmUserStatus active active
 no privacy support privacy support
 ------------------ ---------------
 usmUserEngineID localEngineID localEngineID
 usmUserName "templateSHA" "templateSHA"
 usmUserSecurityName "templateSHA" "templateSHA"
 usmUserCloneFrom ZeroDotZero ZeroDotZero
 usmUserAuthProtocol usmHMACSHAAuthProtocol usmHMACSHAAuthProtocol
 usmUserAuthKeyChange "" ""
 usmUserOwnAuthKeyChange "" ""
 usmUserPrivProtocol none usmDESPrivProtocol
 usmUserPrivKeyChange "" ""
 usmUserOwnPrivKeyChange "" ""
 usmUserPublic "" ""
 usmUserStorageType permanent permanent
 usmUserStatus active active
A.2. Password to Key Algorithm
 A sample code fragment (section A.2.1) demonstrates the password to
 key algorithm which can be used when mapping a password to an
 authentication or privacy key using MD5. The reference source code
 of MD5 is available in [RFC1321].
 Another sample code fragment (section A.2.2) demonstrates the
 password to key algorithm which can be used when mapping a password
 to an authentication or privacy key using SHA (documented in SHA-
 NIST).
 An example of the results of a correct implementation is provided
 (section A.3) which an implementor can use to check if his
 implementation produces the same result.
A.2.1. Password to Key Sample Code for MD5
 void password_to_key_md5(
 u_char *password, /* IN */
 u_int passwordlen, /* IN */
 u_char *engineID, /* IN - pointer to snmpEngineID */
 u_int engineLength,/* IN - length of snmpEngineID */
 u_char *key) /* OUT - pointer to caller 16-octet buffer */
 {
 MD5_CTX MD;
 u_char *cp, password_buf[64];
 u_long password_index = 0;
 u_long count = 0, i;
 MD5Init (&MD); /* initialize MD5 */
 /**********************************************/
 /* Use while loop until we've done 1 Megabyte */
 /**********************************************/
 while (count < 1048576) {
 cp = password_buf;
 for (i = 0; i < 64; i++) {
 /*************************************************/
 /* Take the next octet of the password, wrapping */
 /* to the beginning of the password as necessary.*/
 /*************************************************/
 *cp++ = password[password_index++ % passwordlen];
 }
 MD5Update (&MD, password_buf, 64);
 count += 64;
 }
 MD5Final (key, &MD); /* tell MD5 we're done */
 /*****************************************************/
 /* Now localize the key with the engineID and pass */
 /* through MD5 to produce final key */
 /* May want to ensure that engineLength <= 32, */
 /* otherwise need to use a buffer larger than 64 */
 /*****************************************************/
 memcpy(password_buf, key, 16);
 memcpy(password_buf+16, engineID, engineLength);
 memcpy(password_buf+16+engineLength, key, 16);
 MD5Init(&MD);
 MD5Update(&MD, password_buf, 32+engineLength);
 MD5Final(key, &MD);
 return;
 }
A.2.2. Password to Key Sample Code for SHA
 void password_to_key_sha(
 u_char *password, /* IN */
 u_int passwordlen, /* IN */
 u_char *engineID, /* IN - pointer to snmpEngineID */
 u_int engineLength,/* IN - length of snmpEngineID */
 u_char *key) /* OUT - pointer to caller 20-octet buffer */
 {
 SHA_CTX SH;
 u_char *cp, password_buf[72];
 u_long password_index = 0;
 u_long count = 0, i;
 SHAInit (&SH); /* initialize SHA */
 /**********************************************/
 /* Use while loop until we've done 1 Megabyte */
 /**********************************************/
 while (count < 1048576) {
 cp = password_buf;
 for (i = 0; i < 64; i++) {
 /*************************************************/
 /* Take the next octet of the password, wrapping */
 /* to the beginning of the password as necessary.*/
 /*************************************************/
 *cp++ = password[password_index++ % passwordlen];
 }
 SHAUpdate (&SH, password_buf, 64);
 count += 64;
 }
 SHAFinal (key, &SH); /* tell SHA we're done */
 /*****************************************************/
 /* Now localize the key with the engineID and pass */
 /* through SHA to produce final key */
 /* May want to ensure that engineLength <= 32, */
 /* otherwise need to use a buffer larger than 72 */
 /*****************************************************/
 memcpy(password_buf, key, 20);
 memcpy(password_buf+20, engineID, engineLength);
 memcpy(password_buf+20+engineLength, key, 20);
 SHAInit(&SH);
 SHAUpdate(&SH, password_buf, 40+engineLength);
 SHAFinal(key, &SH);
 return;
 }
A.3. Password to Key Sample Results
A.3.1. Password to Key Sample Results using MD5
 The following shows a sample output of the password to key algorithm
 for a 16-octet key using MD5.
 With a password of "maplesyrup" the output of the password to key
 algorithm before the key is localized with the SNMP engine's
 snmpEngineID is:
 '9f af 32 83 88 4e 92 83 4e bc 98 47 d8 ed d9 63'H
 After the intermediate key (shown above) is localized with the
 snmpEngineID value of:
 '00 00 00 00 00 00 00 00 00 00 00 02'H
 the final output of the password to key algorithm is:
 '52 6f 5e ed 9f cc e2 6f 89 64 c2 93 07 87 d8 2b'H
A.3.2. Password to Key Sample Results using SHA
 The following shows a sample output of the password to key algorithm
 for a 20-octet key using SHA.
 With a password of "maplesyrup" the output of the password to key
 algorithm before the key is localized with the SNMP engine's
 snmpEngineID is:
 '9f b5 cc 03 81 49 7b 37 93 52 89 39 ff 78 8d 5d 79 14 52 11'H
 After the intermediate key (shown above) is localized with the
 snmpEngineID value of:
 '00 00 00 00 00 00 00 00 00 00 00 02'H
 the final output of the password to key algorithm is:
 '66 95 fe bc 92 88 e3 62 82 23 5f c7 15 1f 12 84 97 b3 8f 3f'H
A.4. Sample Encoding of msgSecurityParameters
 The msgSecurityParameters in an SNMP message are represented as an
 OCTET STRING. This OCTET STRING should be considered opaque outside
 a specific Security Model.
 The User-based Security Model defines the contents of the OCTET
 STRING as a SEQUENCE (see section 2.4).
 Given these two properties, the following is an example of they
 msgSecurityParameters for the User-based Security Model, encoded as
 an OCTET STRING:
 04 <length>
 30 <length>
 04 <length> <msgAuthoritativeEngineID>
 02 <length> <msgAuthoritativeEngineBoots>
 02 <length> <msgAuthoritativeEngineTime>
 04 <length> <msgUserName>
 04 0c <HMAC-MD5-96-digest>
 04 08 <salt>
 Here is the example once more, but now with real values (except for
 the digest in msgAuthenticationParameters and the salt in
 msgPrivacyParameters, which depend on variable data that we have not
 defined here):
 Hex Data Description
 -------------- -----------------------------------------------
 04 39 OCTET STRING, length 57
 30 37 SEQUENCE, length 55
 04 0c 80000002 msgAuthoritativeEngineID: IBM
 01 IPv4 address
 09840301 9.132.3.1
 02 01 01 msgAuthoritativeEngineBoots: 1
 02 02 0101 msgAuthoritativeEngineTime: 257
 04 04 62657274 msgUserName: bert
 04 0c 01234567 msgAuthenticationParameters: sample value
 89abcdef
 fedcba98
 04 08 01234567 msgPrivacyParameters: sample value
 89abcdef
A.5. Sample keyChange Results
A.5.1. Sample keyChange Results using MD5
 Let us assume that a user has a current password of "maplesyrup" as
 in section A.3.1. and let us also assume the snmpEngineID of 12
 octets:
 '00 00 00 00 00 00 00 00 00 00 00 02'H
 If we now want to change the password to "newsyrup", then we first
 calculate the key for the new password. It is as follows:
 '01 ad d2 73 10 7c 4e 59 6b 4b 00 f8 2b 1d 42 a7'H
 If we localize it for the above snmpEngineID, then the localized new
 key becomes:
 '87 02 1d 7b d9 d1 01 ba 05 ea 6e 3b f9 d9 bd 4a'H
 If we then use a (not so good, but easy to test) random value of:
 '00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00'H
 Then the value we must send for keyChange is:
 '00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 88 05 61 51 41 67 6c c9 19 61 74 e7 42 a3 25 51'H
 If this were for the privacy key, then it would be exactly the same.
A.5.2. Sample keyChange Results using SHA
 Let us assume that a user has a current password of "maplesyrup" as
 in section A.3.2. and let us also assume the snmpEngineID of 12
 octets:
 '00 00 00 00 00 00 00 00 00 00 00 02'H
 If we now want to change the password to "newsyrup", then we first
 calculate the key for the new password. It is as follows:
 '3a 51 a6 d7 36 aa 34 7b 83 dc 4a 87 e3 e5 5e e4 d6 98 ac 71'H
 If we localize it for the above snmpEngineID, then the localized new
 key becomes:
 '78 e2 dc ce 79 d5 94 03 b5 8c 1b ba a5 bf f4 63 91 f1 cd 25'H
 If we then use a (not so good, but easy to test) random value of:
 '00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00'H
 Then the value we must send for keyChange is:
 '00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 9c 10 17 f4 fd 48 3d 2d e8 d5 fa db f8 43 92 cb 06 45 70 51'
 For the key used for privacy, the new nonlocalized key would be:
 '3a 51 a6 d7 36 aa 34 7b 83 dc 4a 87 e3 e5 5e e4 d6 98 ac 71'H
 For the key used for privacy, the new localized key would be (note
 that they localized key gets truncated to 16 octets for DES):
 '78 e2 dc ce 79 d5 94 03 b5 8c 1b ba a5 bf f4 63'H
 If we then use a (not so good, but easy to test) random value of:
 '00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00'H
 Then the value we must send for keyChange for the privacy key is:
 '00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 '7e f8 d8 a4 c9 cd b2 6b 47 59 1c d8 52 ff 88 b5'H
B. Change Log
 Changes made since RFC2574:
 - Updated references
 - Updated contact info
 - Clarifications
 - to first constraint item 1) on page 6.
 - to usmUserCloneFrom DESCRIPTION clause
 - to securityName in section 2.1
 - Fixed "command responder" into "command generator" in last para of
 DESCRIPTION clause of usmUserTable.
 Changes made since RFC2274:
 - Fixed msgUserName to allow size of zero and explain that this can
 be used for snmpEngineID discovery.
 - Clarified section 3.1 steps 4.b, 5, 6 and 8.b.
 - Clarified section 3.2 paragraph 2.
 - Clarified section 3.2 step 7.a last paragraph, step 7.b.1 second
 bullet and step 7.b.2 third bullet.
 - Clarified section 4 to indicate that discovery can use a userName
 of zero length in unAuthenticated messages, whereas a valid
 userName must be used in authenticated messages.
 - Added REVISION clauses to MODULE-IDENTITY
 - Clarified KeyChange TC by adding a note that localized keys must be
 used when calculating a KeyChange value.
 - Added clarifying text to the DESCRIPTION clause of usmUserTable.
 Added text describes a recommended procedure for adding a new user.
 - Clarified the use of usmUserCloneFrom object.
 - Clarified how and under which conditions the usmUserAuthProtocol
 and usmUserPrivProtocol can be initialized and/or changed.
 - Added comment on typical sizes for usmUserAuthKeyChange and
 usmUserPrivKeyChange. Also for usmUserOwnAuthKeyChange and
 usmUserOwnPrivKeyChange.
 - Added clarifications to the DESCRIPTION clauses of
 usmUserAuthKeyChange, usmUserOwnAuthKeychange, usmUserPrivKeyChange
 and usmUserOwnPrivKeychange.
 - Added clarification to DESCRIPTION clause of usmUserStorageType.
 - Added clarification to DESCRIPTION clause of usmUserStatus.
 - Clarified IV generation procedure in section 8.1.1.1 and in
 addition clarified section 8.3.1 step 1 and section 8.3.2. step 3.
 - Clarified section 11.2 and added a warning that different size
 passwords with repetitive strings may result in same key.
 - Added template users to appendix A for cloning process.
 - Fixed C-code examples in Appendix A.
 - Fixed examples of generated keys in Appendix A.
 - Added examples of KeyChange values to Appendix A.
 - Used PDU Classes instead of RFC1905 PDU types.
 - Added text in the security section about Reports and Access Control
 to the MIB.
 - Removed a incorrect note at the end of section 3.2 step 7.
 - Added a note in section 3.2 step 3.
 - Corrected various spelling errors and typos.
 - Corrected procedure for 3.2 step 2.a)
 - various clarifications.
 - Fixed references to new/revised documents
 - Change to no longer cache data that is not used
Editors' Addresses
 Uri Blumenthal
 Lucent Technologies
 67 Whippany Rd.
 Whippany, NJ 07981
 USA
 Phone: +1-973-386-2163
 EMail: uri@lucent.com
 Bert Wijnen
 Lucent Technologies
 Schagen 33
 3461 GL Linschoten
 Netherlands
 Phone: +31-348-480-685
 EMail: bwijnen@lucent.com
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