Network Working Group J. Linn
Request for Comments: 1508 Geer Zolot Associates
 September 1993
 Generic Security Service Application Program Interface
Status of this Memo
 This RFC 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" for the standardization state and status
 of this protocol. Distribution of this memo is unlimited.
Abstract
 This Generic Security Service Application Program Interface (GSS-API)
 definition provides security services to callers in a generic
 fashion, supportable with a range of underlying mechanisms and
 technologies and hence allowing source-level portability of
 applications to different environments. This specification defines
 GSS-API services and primitives at a level independent of underlying
 mechanism and programming language environment, and is to be
 complemented by other, related specifications:
 documents defining specific parameter bindings for particular
 language environments
 documents defining token formats, protocols, and procedures to
 be implemented in order to realize GSS-API services atop
 particular security mechanisms
Table of Contents
 1. GSS-API Characteristics and Concepts ....................... 2
 1.1. GSS-API Constructs ....................................... 5
 1.1.1. Credentials ........................................... 5
 1.1.2. Tokens ................................................ 6
 1.1.3. Security Contexts ..................................... 7
 1.1.4. Mechanism Types ....................................... 8
 1.1.5. Naming ................................................ 9
 1.1.6. Channel Bindings ...................................... 10
 1.2. GSS-API Features and Issues ............................. 11
 1.2.1. Status Reporting ...................................... 11
 1.2.2. Per-Message Security Service Availability ............. 12
 1.2.3. Per-Message Replay Detection and Sequencing ........... 13
 1.2.4. Quality of Protection ................................. 15
 2. Interface Descriptions ..................................... 15
 2.1. Credential management calls ............................. 17
 2.1.1. GSS_Acquire_cred call ................................. 17
 2.1.2. GSS_Release_cred call ................................. 19
 2.1.3. GSS_Inquire_cred call ................................. 20
 2.2. Context-level calls ..................................... 21
 2.2.1. GSS_Init_sec_context call ............................. 21
 2.2.2. GSS_Accept_sec_context call ........................... 26
 2.2.3. GSS_Delete_sec_context call ........................... 29
 2.2.4. GSS_Process_context_token call ........................ 30
 2.2.5. GSS_Context_time call ................................. 31
 2.3. Per-message calls ....................................... 32
 2.3.1. GSS_Sign call ......................................... 32
 2.3.2. GSS_Verify call ....................................... 33
 2.3.3. GSS_Seal call ......................................... 35
 2.3.4. GSS_Unseal call ....................................... 36
 2.4. Support calls ........................................... 37
 2.4.1. GSS_Display_status call ............................... 37
 2.4.2. GSS_Indicate_mechs call ............................... 38
 2.4.3. GSS_Compare_name call ................................. 38
 2.4.4. GSS_Display_name call ................................. 39
 2.4.5. GSS_Import_name call .................................. 40
 2.4.6. GSS_Release_name call ................................. 41
 2.4.7. GSS_Release_buffer call ............................... 41
 2.4.8. GSS_Release_oid_set call .............................. 42
 3. Mechanism-Specific Example Scenarios ....................... 42
 3.1. Kerberos V5, single-TGT ................................. 43
 3.2. Kerberos V5, double-TGT ................................. 43
 3.3. X.509 Authentication Framework .......................... 44
 4. Related Activities ......................................... 45
 5. Acknowledgments ............................................ 46
 6. Security Considerations .................................... 46
 7. Author's Address ........................................... 46
 Appendix A .................................................... 47
 Appendix B .................................................... 48
 Appendix C .................................................... 49
1. GSS-API Characteristics and Concepts
 The operational paradigm in which GSS-API operates is as follows. A
 typical GSS-API caller is itself a communications protocol, calling
 on GSS-API in order to protect its communications with
 authentication, integrity, and/or confidentiality security services.
 A GSS-API caller accepts tokens provided to it by its local GSS-API
 implementation and transfers the tokens to a peer on a remote system;
 that peer passes the received tokens to its local GSS-API
 implementation for processing. The security services available
 through GSS-API in this fashion are implementable (and have been
 implemented) over a range of underlying mechanisms based on secret-
 key and public-key cryptographic technologies.
 The GSS-API separates the operations of initializing a security
 context between peers, achieving peer entity authentication (This
 security service definition, and other definitions used in this
 document, corresponds to that provided in International Standard ISO
 7498年2月19日88(E), Security Architecture.) (GSS_Init_sec_context() and
 GSS_Accept_sec_context() calls), from the operations of providing
 per-message data origin authentication and data integrity protection
 (GSS_Sign() and GSS_Verify() calls) for messages subsequently
 transferred in conjunction with that context. Per-message GSS_Seal()
 and GSS_Unseal() calls provide the data origin authentication and
 data integrity services which GSS_Sign() and GSS_Verify() offer, and
 also support selection of confidentiality services as a caller
 option. Additional calls provide supportive functions to the GSS-
 API's users.
 The following paragraphs provide an example illustrating the
 dataflows involved in use of the GSS-API by a client and server in a
 mechanism-independent fashion, establishing a security context and
 transferring a protected message. The example assumes that credential
 acquisition has already been completed. The example assumes that the
 underlying authentication technology is capable of authenticating a
 client to a server using elements carried within a single token, and
 of authenticating the server to the client (mutual authentication)
 with a single returned token; this assumption holds for presently-
 documented CAT mechanisms but is not necessarily true for other
 cryptographic technologies and associated protocols.
 The client calls GSS_Init_sec_context() to establish a security
 context to the server identified by targ_name, and elects to set the
 mutual_req_flag so that mutual authentication is performed in the
 course of context establishment. GSS_Init_sec_context() returns an
 output_token to be passed to the server, and indicates
 GSS_CONTINUE_NEEDED status pending completion of the mutual
 authentication sequence. Had mutual_req_flag not been set, the
 initial call to GSS_Init_sec_context() would have returned
 GSS_COMPLETE status. The client sends the output_token to the server.
 The server passes the received token as the input_token parameter to
 GSS_Accept_sec_context(). GSS_Accept_sec_context indicates
 GSS_COMPLETE status, provides the client's authenticated identity in
 the src_name result, and provides an output_token to be passed to the
 client. The server sends the output_token to the client.
 The client passes the received token as the input_token parameter to
 a successor call to GSS_Init_sec_context(), which processes data
 included in the token in order to achieve mutual authentication from
 the client's viewpoint. This call to GSS_Init_sec_context() returns
 GSS_COMPLETE status, indicating successful mutual authentication and
 the completion of context establishment for this example.
 The client generates a data message and passes it to GSS_Seal().
 GSS_Seal() performs data origin authentication, data integrity, and
 (optionally) confidentiality processing on the message and
 encapsulates the result into output_message, indicating GSS_COMPLETE
 status. The client sends the output_message to the server.
 The server passes the received message to GSS_Unseal(). GSS_Unseal
 inverts the encapsulation performed by GSS_Seal(), deciphers the
 message if the optional confidentiality feature was applied, and
 validates the data origin authentication and data integrity checking
 quantities. GSS_Unseal() indicates successful validation by
 returning GSS_COMPLETE status along with the resultant
 output_message.
 For purposes of this example, we assume that the server knows by
 out-of-band means that this context will have no further use after
 one protected message is transferred from client to server. Given
 this premise, the server now calls GSS_Delete_sec_context() to flush
 context-level information. GSS_Delete_sec_context() returns a
 context_token for the server to pass to the client.
 The client passes the returned context_token to
 GSS_Process_context_token(), which returns GSS_COMPLETE status after
 deleting context-level information at the client system.
 The GSS-API design assumes and addresses several basic goals,
 including:
 Mechanism independence: The GSS-API defines an interface to
 cryptographically implemented strong authentication and other
 security services at a generic level which is independent of
 particular underlying mechanisms. For example, GSS-API-provided
 services can be implemented by secret-key technologies (e.g.,
 Kerberos) or public-key approaches (e.g., X.509).
 Protocol environment independence: The GSS-API is independent of
 the communications protocol suites with which it is employed,
 permitting use in a broad range of protocol environments. In
 appropriate environments, an intermediate implementation "veneer"
 which is oriented to a particular communication protocol (e.g.,
 Remote Procedure Call (RPC)) may be interposed between
 applications which call that protocol and the GSS-API, thereby
 invoking GSS-API facilities in conjunction with that protocol's
 communications invocations.
 Protocol association independence: The GSS-API's security context
 construct is independent of communications protocol association
 constructs. This characteristic allows a single GSS-API
 implementation to be utilized by a variety of invoking protocol
 modules on behalf of those modules' calling applications. GSS-API
 services can also be invoked directly by applications, wholly
 independent of protocol associations.
 Suitability to a range of implementation placements: GSS-API
 clients are not constrained to reside within any Trusted Computing
 Base (TCB) perimeter defined on a system where the GSS-API is
 implemented; security services are specified in a manner suitable
 to both intra-TCB and extra-TCB callers.
1.1. GSS-API Constructs
 This section describes the basic elements comprising the GSS-API.
1.1.1. Credentials
 Credentials structures provide the prerequisites enabling peers to
 establish security contexts with each other. A caller may designate
 that its default credential be used for context establishment calls
 without presenting an explicit handle to that credential.
 Alternately, those GSS-API callers which need to make explicit
 selection of particular credentials structures may make references to
 those credentials through GSS-API-provided credential handles
 ("cred_handles").
 A single credential structure may be used for initiation of outbound
 contexts and acceptance of inbound contexts. Callers needing to
 operate in only one of these modes may designate this fact when
 credentials are acquired for use, allowing underlying mechanisms to
 optimize their processing and storage requirements. The credential
 elements defined by a particular mechanism may contain multiple
 cryptographic keys, e.g., to enable authentication and message
 encryption to be performed with different algorithms.
 A single credential structure may accommodate credential information
 associated with multiple underlying mechanisms (mech_types); a
 credential structure's contents will vary depending on the set of
 mech_types supported by a particular GSS-API implementation.
 Commonly, a single mech_type will be used for all security contexts
 established by a particular initiator to a particular target; the
 primary motivation for supporting credential sets representing
 multiple mech_types is to allow initiators on systems which are
 equipped to handle multiple types to initiate contexts to targets on
 other systems which can accommodate only a subset of the set
 supported at the initiator's system.
 It is the responsibility of underlying system-specific mechanisms and
 OS functions below the GSS-API to ensure that the ability to acquire
 and use credentials associated with a given identity is constrained
 to appropriate processes within a system. This responsibility should
 be taken seriously by implementors, as the ability for an entity to
 utilize a principal's credentials is equivalent to the entity's
 ability to successfully assert that principal's identity.
 Once a set of GSS-API credentials is established, the transferability
 of that credentials set to other processes or analogous constructs
 within a system is a local matter, not defined by the GSS-API. An
 example local policy would be one in which any credentials received
 as a result of login to a given user account, or of delegation of
 rights to that account, are accessible by, or transferable to,
 processes running under that account.
 The credential establishment process (particularly when performed on
 behalf of users rather than server processes) is likely to require
 access to passwords or other quantities which should be protected
 locally and exposed for the shortest time possible. As a result, it
 will often be appropriate for preliminary credential establishment to
 be performed through local means at user login time, with the
 result(s) cached for subsequent reference. These preliminary
 credentials would be set aside (in a system-specific fashion) for
 subsequent use, either:
 to be accessed by an invocation of the GSS-API GSS_Acquire_cred()
 call, returning an explicit handle to reference that credential
 as the default credentials installed on behalf of a process
1.1.2. Tokens
 Tokens are data elements transferred between GSS-API callers, and are
 divided into two classes. Context-level tokens are exchanged in order
 to establish and manage a security context between peers. Per-message
 tokens are exchanged in conjunction with an established context to
 provide protective security services for corresponding data messages.
 The internal contents of both classes of tokens are specific to the
 particular underlying mechanism used to support the GSS-API; Appendix
 B of this document provides a uniform recommendation for designers of
 GSS-API support mechanisms, encapsulating mechanism-specific
 information along with a globally-interpretable mechanism identifier.
 Tokens are opaque from the viewpoint of GSS-API callers. They are
 generated within the GSS-API implementation at an end system,
 provided to a GSS-API caller to be transferred to the peer GSS-API
 caller at a remote end system, and processed by the GSS-API
 implementation at that remote end system. Tokens may be output by
 GSS-API primitives (and are to be transferred to GSS-API peers)
 independent of the status indications which those primitives
 indicate. Token transfer may take place in an in-band manner,
 integrated into the same protocol stream used by the GSS-API callers
 for other data transfers, or in an out-of-band manner across a
 logically separate channel.
 Development of GSS-API support primitives based on a particular
 underlying cryptographic technique and protocol does not necessarily
 imply that GSS-API callers invoking that GSS-API mechanism type will
 be able to interoperate with peers invoking the same technique and
 protocol outside the GSS-API paradigm. For example, the format of
 GSS-API tokens defined in conjunction with a particular mechanism,
 and the techniques used to integrate those tokens into callers'
 protocols, may not be the same as those used by non-GSS-API callers
 of the same underlying technique.
1.1.3. Security Contexts
 Security contexts are established between peers, using credentials
 established locally in conjunction with each peer or received by
 peers via delegation. Multiple contexts may exist simultaneously
 between a pair of peers, using the same or different sets of
 credentials. Coexistence of multiple contexts using different
 credentials allows graceful rollover when credentials expire.
 Distinction among multiple contexts based on the same credentials
 serves applications by distinguishing different message streams in a
 security sense.
 The GSS-API is independent of underlying protocols and addressing
 structure, and depends on its callers to transport GSS-API-provided
 data elements. As a result of these factors, it is a caller
 responsibility to parse communicated messages, separating GSS-API-
 related data elements from caller-provided data. The GSS-API is
 independent of connection vs. connectionless orientation of the
 underlying communications service.
 No correlation between security context and communications protocol
 association is dictated. (The optional channel binding facility,
 discussed in Section 1.1.6 of this document, represents an
 intentional exception to this rule, supporting additional protection
 features within GSS-API supporting mechanisms.) This separation
 allows the GSS-API to be used in a wide range of communications
 environments, and also simplifies the calling sequences of the
 individual calls. In many cases (depending on underlying security
 protocol, associated mechanism, and availability of cached
 information), the state information required for context setup can be
 sent concurrently with initial signed user data, without interposing
 additional message exchanges.
1.1.4. Mechanism Types
 In order to successfully establish a security context with a target
 peer, it is necessary to identify an appropriate underlying mechanism
 type (mech_type) which both initiator and target peers support. The
 definition of a mechanism embodies not only the use of a particular
 cryptographic technology (or a hybrid or choice among alternative
 cryptographic technologies), but also definition of the syntax and
 semantics of data element exchanges which that mechanism will employ
 in order to support security services.
 It is recommended that callers initiating contexts specify the
 "default" mech_type value, allowing system-specific functions within
 or invoked by the GSS-API implementation to select the appropriate
 mech_type, but callers may direct that a particular mech_type be
 employed when necessary.
 The means for identifying a shared mech_type to establish a security
 context with a peer will vary in different environments and
 circumstances; examples include (but are not limited to):
 use of a fixed mech_type, defined by configuration, within an
 environment
 syntactic convention on a target-specific basis, through
 examination of a target's name
 lookup of a target's name in a naming service or other database in
 order to identify mech_types supported by that target
 explicit negotiation between GSS-API callers in advance of
 security context setup
 When transferred between GSS-API peers, mech_type specifiers (per
 Appendix B, represented as Object Identifiers (OIDs)) serve to
 qualify the interpretation of associated tokens. (The structure and
 encoding of Object Identifiers is defined in ISO/IEC 8824,
 "Specification of Abstract Syntax Notation One (ASN.1)" and in
 ISO/IEC 8825, "Specification of Basic Encoding Rules for Abstract
 Syntax Notation One (ASN.1)".) Use of hierarchically structured OIDs
 serves to preclude ambiguous interpretation of mech_type specifiers.
 The OID representing the DASS MechType, for example, is
 1.3.12.2.1011年7月5日.
1.1.5. Naming
 The GSS-API avoids prescription of naming structures, treating the
 names transferred across the interface in order to initiate and
 accept security contexts as opaque octet string quantities. This
 approach supports the GSS-API's goal of implementability atop a range
 of underlying security mechanisms, recognizing the fact that
 different mechanisms process and authenticate names which are
 presented in different forms. Generalized services offering
 translation functions among arbitrary sets of naming environments are
 outside the scope of the GSS-API; availability and use of local
 conversion functions to translate among the naming formats supported
 within a given end system is anticipated.
 Two distinct classes of name representations are used in conjunction
 with different GSS-API parameters:
 a printable form (denoted by OCTET STRING), for acceptance from
 and presentation to users; printable name forms are accompanied by
 OID tags identifying the namespace to which they correspond
 an internal form (denoted by INTERNAL NAME), opaque to callers and
 defined by individual GSS-API implementations; GSS-API
 implementations supporting multiple namespace types are
 responsible for maintaining internal tags to disambiguate the
 interpretation of particular names
 Tagging of printable names allows GSS-API callers and underlying
 GSS-API mechanisms to disambiguate name types and to determine
 whether an associated name's type is one which they are capable of
 processing, avoiding aliasing problems which could result from
 misinterpreting a name of one type as a name of another type.
 In addition to providing means for names to be tagged with types,
 this specification defines primitives to support a level of naming
 environment independence for certain calling applications. To provide
 basic services oriented towards the requirements of callers which
 need not themselves interpret the internal syntax and semantics of
 names, GSS-API calls for name comparison (GSS_Compare_name()),
 human-readable display (GSS_Display_name()), input conversion
 (GSS_Import_name()), and internal name deallocation
 (GSS_Release_name()) functions are defined. (It is anticipated that
 these proposed GSS-API calls will be implemented in many end systems
 based on system-specific name manipulation primitives already extant
 within those end systems; inclusion within the GSS-API is intended to
 offer GSS-API callers a portable means to perform specific
 operations, supportive of authorization and audit requirements, on
 authenticated names.)
 GSS_Import_name() implementations can, where appropriate, support
 more than one printable syntax corresponding to a given namespace
 (e.g., alternative printable representations for X.500 Distinguished
 Names), allowing flexibility for their callers to select among
 alternative representations. GSS_Display_name() implementations
 output a printable syntax selected as appropriate to their
 operational environments; this selection is a local matter. Callers
 desiring portability across alternative printable syntaxes should
 refrain from implementing comparisons based on printable name forms
 and should instead use the GSS_Compare_name() call to determine
 whether or not one internal-format name matches another.
1.1.6. Channel Bindings
 The GSS-API accommodates the concept of caller-provided channel
 binding ("chan_binding") information, used by GSS-API callers to bind
 the establishment of a security context to relevant characteristics
 (e.g., addresses, transformed representations of encryption keys) of
 the underlying communications channel and of protection mechanisms
 applied to that communications channel. Verification by one peer of
 chan_binding information provided by the other peer to a context
 serves to protect against various active attacks. The caller
 initiating a security context must determine the chan_binding values
 before making the GSS_Init_sec_context() call, and consistent values
 must be provided by both peers to a context. Callers should not
 assume that underlying mechanisms provide confidentiality protection
 for channel binding information.
 Use or non-use of the GSS-API channel binding facility is a caller
 option, and GSS-API supporting mechanisms can support operation in an
 environment where NULL channel bindings are presented. When non-NULL
 channel bindings are used, certain mechanisms will offer enhanced
 security value by interpreting the bindings' content (rather than
 simply representing those bindings, or signatures computed on them,
 within tokens) and will therefore depend on presentation of specific
 data in a defined format. To this end, agreements among mechanism
 implementors are defining conventional interpretations for the
 contents of channel binding arguments, including address specifiers
 (with content dependent on communications protocol environment) for
 context initiators and acceptors. (These conventions are being
 incorporated into related documents.) In order for GSS-API callers to
 be portable across multiple mechanisms and achieve the full security
 functionality available from each mechanism, it is strongly
 recommended that GSS-API callers provide channel bindings consistent
 with these conventions and those of the networking environment in
 which they operate.
1.2. GSS-API Features and Issues
 This section describes aspects of GSS-API operations, of the security
 services which the GSS-API provides, and provides commentary on
 design issues.
1.2.1. Status Reporting
 Each GSS-API call provides two status return values. Major_status
 values provide a mechanism-independent indication of call status
 (e.g., GSS_COMPLETE, GSS_FAILURE, GSS_CONTINUE_NEEDED), sufficient to
 drive normal control flow within the caller in a generic fashion.
 Table 1 summarizes the defined major_status return codes in tabular
 fashion.
 Table 1: GSS-API Major Status Codes
 FATAL ERROR CODES
 GSS_BAD_BINDINGS channel binding mismatch
 GSS_BAD_MECH unsupported mechanism requested
 GSS_BAD_NAME invalid name provided
 GSS_BAD_NAMETYPE name of unsupported type provided
 GSS_BAD_STATUS invalid input status selector
 GSS_BAD_SIG token had invalid signature
 GSS_CONTEXT_EXPIRED specified security context expired
 GSS_CREDENTIALS_EXPIRED expired credentials detected
 GSS_DEFECTIVE_CREDENTIAL defective credential detected
 GSS_DEFECTIVE_TOKEN defective token detected
 GSS_FAILURE failure, unspecified at GSS-API
 level
 GSS_NO_CONTEXT no valid security context specified
 GSS_NO_CRED no valid credentials provided
 INFORMATORY STATUS CODES
 GSS_COMPLETE normal completion
 GSS_CONTINUE_NEEDED continuation call to routine
 required
 GSS_DUPLICATE_TOKEN duplicate per-message token
 detected
 GSS_OLD_TOKEN timed-out per-message token
 detected
 GSS_UNSEQ_TOKEN out-of-order per-message token
 detected
 Minor_status provides more detailed status information which may
 include status codes specific to the underlying security mechanism.
 Minor_status values are not specified in this document.
 GSS_CONTINUE_NEEDED major_status returns, and optional message
 outputs, are provided in GSS_Init_sec_context() and
 GSS_Accept_sec_context() calls so that different mechanisms'
 employment of different numbers of messages within their
 authentication sequences need not be reflected in separate code paths
 within calling applications. Instead, such cases are accomodated with
 sequences of continuation calls to GSS_Init_sec_context() and
 GSS_Accept_sec_context(). The same mechanism is used to encapsulate
 mutual authentication within the GSS-API's context initiation calls.
 For mech_types which require interactions with third-party servers in
 order to establish a security context, GSS-API context establishment
 calls may block pending completion of such third-party interactions.
 On the other hand, no GSS-API calls pend on serialized interactions
 with GSS-API peer entities. As a result, local GSS-API status
 returns cannot reflect unpredictable or asynchronous exceptions
 occurring at remote peers, and reflection of such status information
 is a caller responsibility outside the GSS-API.
1.2.2. Per-Message Security Service Availability
 When a context is established, two flags are returned to indicate the
 set of per-message protection security services which will be
 available on the context:
 the integ_avail flag indicates whether per-message integrity and
 data origin authentication services are available
 the conf_avail flag indicates whether per-message confidentiality
 services are available, and will never be returned TRUE unless the
 integ_avail flag is also returned TRUE
 GSS-API callers desiring per-message security services should
 check the values of these flags at context establishment time, and
 must be aware that a returned FALSE value for integ_avail means
 that invocation of GSS_Sign() or GSS_Seal() primitives on the
 associated context will apply no cryptographic protection to user
 data messages.
 The GSS-API per-message protection service primitives, as the
 category name implies, are oriented to operation at the granularity
 of protocol data units. They perform cryptographic operations on the
 data units, transfer cryptographic control information in tokens,
 and, in the case of GSS_Seal(), encapsulate the protected data unit.
 As such, these primitives are not oriented to efficient data
 protection for stream-paradigm protocols (e.g., Telnet) if
 cryptography must be applied on an octet-by-octet basis.
1.2.3. Per-Message Replay Detection and Sequencing
 Certain underlying mech_types are expected to offer support for
 replay detection and/or sequencing of messages transferred on the
 contexts they support. These optionally-selectable protection
 features are distinct from replay detection and sequencing features
 applied to the context establishment operation itself; the presence
 or absence of context-level replay or sequencing features is wholly a
 function of the underlying mech_type's capabilities, and is not
 selected or omitted as a caller option.
 The caller initiating a context provides flags (replay_det_req_flag
 and sequence_req_flag) to specify whether the use of per-message
 replay detection and sequencing features is desired on the context
 being established. The GSS-API implementation at the initiator system
 can determine whether these features are supported (and whether they
 are optionally selectable) as a function of mech_type, without need
 for bilateral negotiation with the target. When enabled, these
 features provide recipients with indicators as a result of GSS-API
 processing of incoming messages, identifying whether those messages
 were detected as duplicates or out-of-sequence. Detection of such
 events does not prevent a suspect message from being provided to a
 recipient; the appropriate course of action on a suspect message is a
 matter of caller policy.
 The semantics of the replay detection and sequencing services applied
 to received messages, as visible across the interface which the GSS-
 API provides to its clients, are as follows:
 When replay_det_state is TRUE, the possible major_status returns for
 well-formed and correctly signed messages are as follows:
 1. GSS_COMPLETE indicates that the message was within the window
 (of time or sequence space) allowing replay events to be detected,
 and that the message was not a replay of a previously-processed
 message within that window.
 2. GSS_DUPLICATE_TOKEN indicates that the signature on the
 received message was correct, but that the message was recognized
 as a duplicate of a previously-processed message.
 3. GSS_OLD_TOKEN indicates that the signature on the received
 message was correct, but that the message is too old to be checked
 for duplication.
 When sequence_state is TRUE, the possible major_status returns for
 well-formed and correctly signed messages are as follows:
 1. GSS_COMPLETE indicates that the message was within the window
 (of time or sequence space) allowing replay events to be detected,
 and that the message was not a replay of a previously-processed
 message within that window.
 2. GSS_DUPLICATE_TOKEN indicates that the signature on the
 received message was correct, but that the message was recognized
 as a duplicate of a previously-processed message.
 3. GSS_OLD_TOKEN indicates that the signature on the received
 message was correct, but that the token is too old to be checked
 for duplication.
 4. GSS_UNSEQ_TOKEN indicates that the signature on the received
 message was correct, but that it is earlier in a sequenced stream
 than a message already processed on the context. [Note:
 Mechanisms can be architected to provide a stricter form of
 sequencing service, delivering particular messages to recipients
 only after all predecessor messages in an ordered stream have been
 delivered. This type of support is incompatible with the GSS-API
 paradigm in which recipients receive all messages, whether in
 order or not, and provide them (one at a time, without intra-GSS-
 API message buffering) to GSS-API routines for validation. GSS-
 API facilities provide supportive functions, aiding clients to
 achieve strict message stream integrity in an efficient manner in
 conjunction with sequencing provisions in communications
 protocols, but the GSS-API does not offer this level of message
 stream integrity service by itself.]
 As the message stream integrity features (especially sequencing) may
 interfere with certain applications' intended communications
 paradigms, and since support for such features is likely to be
 resource intensive, it is highly recommended that mech_types
 supporting these features allow them to be activated selectively on
 initiator request when a context is established. A context initiator
 and target are provided with corresponding indicators
 (replay_det_state and sequence_state), signifying whether these
 features are active on a given context.
 An example mech_type supporting per-message replay detection could
 (when replay_det_state is TRUE) implement the feature as follows: The
 underlying mechanism would insert timestamps in data elements output
 by GSS_Sign() and GSS_Seal(), and would maintain (within a time-
 limited window) a cache (qualified by originator-recipient pair)
 identifying received data elements processed by GSS_Verify() and
 GSS_Unseal(). When this feature is active, exception status returns
 (GSS_DUPLICATE_TOKEN, GSS_ OLD_TOKEN) will be provided when
 GSS_Verify() or GSS_Unseal() is presented with a message which is
 either a detected duplicate of a prior message or which is too old to
 validate against a cache of recently received messages.
1.2.4. Quality of Protection
 Some mech_types will provide their users with fine granularity
 control over the means used to provide per-message protection,
 allowing callers to trade off security processing overhead
 dynamically against the protection requirements of particular
 messages. A per-message quality-of-protection parameter (analogous to
 quality-of-service, or QOS) selects among different QOP options
 supported by that mechanism. On context establishment for a multi-QOP
 mech_type, context-level data provides the prerequisite data for a
 range of protection qualities.
 It is expected that the majority of callers will not wish to exert
 explicit mechanism-specific QOP control and will therefore request
 selection of a default QOP. Definitions of, and choices among, non-
 default QOP values are mechanism-specific, and no ordered sequences
 of QOP values can be assumed equivalent across different mechanisms.
 Meaningful use of non-default QOP values demands that callers be
 familiar with the QOP definitions of an underlying mechanism or
 mechanisms, and is therefore a non-portable construct.
2. Interface Descriptions
 This section describes the GSS-API's service interface, dividing the
 set of calls offered into four groups. Credential management calls
 are related to the acquisition and release of credentials by
 principals. Context-level calls are related to the management of
 security contexts between principals. Per-message calls are related
 to the protection of individual messages on established security
 contexts. Support calls provide ancillary functions useful to GSS-API
 callers. Table 2 groups and summarizes the calls in tabular fashion.
 Table 2: GSS-API Calls
 CREDENTIAL MANAGEMENT
 GSS_Acquire_cred acquire credentials for use
 GSS_Release_cred release credentials after use
 GSS_Inquire_cred display information about
 credentials
 CONTEXT-LEVEL CALLS
 GSS_Init_sec_context initiate outbound security context
 GSS_Accept_sec_context accept inbound security context
 GSS_Delete_sec_context flush context when no longer needed
 GSS_Process_context_token process received control token on
 context
 GSS_Context_time indicate validity time remaining on
 context
 PER-MESSAGE CALLS
 GSS_Sign apply signature, receive as token
 separate from message
 GSS_Verify validate signature token along with
 message
 GSS_Seal sign, optionally encrypt,
 encapsulate
 GSS_Unseal decapsulate, decrypt if needed,
 validate signature
 SUPPORT CALLS
 GSS_Display_status translate status codes to printable
 form
 GSS_Indicate_mechs indicate mech_types supported on
 local system
 GSS_Compare_name compare two names for equality
 GSS_Display_name translate name to printable form
 GSS_Import_name convert printable name to
 normalized form
 GSS_Release_name free storage of normalized-form
 name
 GSS_Release_buffer free storage of printable name
 GSS_Release_oid_set free storage of OID set object
2.1. Credential management calls
 These GSS-API calls provide functions related to the management of
 credentials. Their characterization with regard to whether or not
 they may block pending exchanges with other network entities (e.g.,
 directories or authentication servers) depends in part on OS-specific
 (extra-GSS-API) issues, so is not specified in this document.
 The GSS_Acquire_cred() call is defined within the GSS-API in support
 of application portability, with a particular orientation towards
 support of portable server applications. It is recognized that (for
 certain systems and mechanisms) credentials for interactive users may
 be managed differently from credentials for server processes; in such
 environments, it is the GSS-API implementation's responsibility to
 distinguish these cases and the procedures for making this
 distinction are a local matter. The GSS_Release_cred() call provides
 a means for callers to indicate to the GSS-API that use of a
 credentials structure is no longer required. The GSS_Inquire_cred()
 call allows callers to determine information about a credentials
 structure.
2.1.1. GSS_Acquire_cred call
 Inputs:
 o desired_name INTERNAL NAME, -NULL requests locally-determined
 default
 o lifetime_req INTEGER,-in seconds; 0 requests default
 o desired_mechs SET OF OBJECT IDENTIFIER,-empty set requests
 system-selected default
 o cred_usage INTEGER-0=INITIATE-AND-ACCEPT, 1=INITIATE-ONLY,
 2=ACCEPT-ONLY
 Outputs:
 o major_status INTEGER,
 o minor_status INTEGER,
 o output_cred_handle OCTET STRING,
 o actual_mechs SET OF OBJECT IDENTIFIER,
 o lifetime_rec INTEGER -in seconds, or reserved value for
 INDEFINITE
 Return major_status codes:
 o GSS_COMPLETE indicates that requested credentials were
 successfully established, for the duration indicated in
 lifetime_rec, suitable for the usage requested in cred_usage, for
 the set of mech_types indicated in actual_mechs, and that those
 credentials can be referenced for subsequent use with the handle
 returned in output_cred_handle.
 o GSS_BAD_MECH indicates that a mech_type unsupported by the GSS-API
 implementation type was requested, causing the credential
 establishment operation to fail.
 o GSS_BAD_NAMETYPE indicates that the provided desired_name is
 uninterpretable or of a type unsupported by the supporting GSS-API
 implementation, so no credentials could be established for the
 accompanying desired_name.
 o GSS_BAD_NAME indicates that the provided desired_name is
 inconsistent in terms of internally-incorporated type specifier
 information, so no credentials could be established for the
 accompanying desired_name.
 o GSS_FAILURE indicates that credential establishment failed for
 reasons unspecified at the GSS-API level, including lack of
 authorization to establish and use credentials associated with the
 identity named in the input desired_name argument.
 GSS_Acquire_cred() is used to acquire credentials so that a
 principal can (as a function of the input cred_usage parameter)
 initiate and/or accept security contexts under the identity
 represented by the desired_name input argument. On successful
 completion, the returned output_cred_handle result provides a handle
 for subsequent references to the acquired credentials. Typically,
 single-user client processes using only default credentials for
 context establishment purposes will have no need to invoke this call.
 A caller may provide the value NULL for desired_name, signifying a
 request for credentials corresponding to a default principal
 identity. The procedures used by GSS-API implementations to select
 the appropriate principal identity in response to this form of
 request are local matters. It is possible that multiple pre-
 established credentials may exist for the same principal identity
 (for example, as a result of multiple user login sessions) when
 GSS_Acquire_cred() is called; the means used in such cases to select
 a specific credential are local matters. The input lifetime_req
 argument to GSS_Acquire_cred() may provide useful information for
 local GSS-API implementations to employ in making this disambiguation
 in a manner which will best satisfy a caller's intent.
 The lifetime_rec result indicates the length of time for which the
 acquired credentials will be valid, as an offset from the present. A
 mechanism may return a reserved value indicating INDEFINITE if no
 constraints on credential lifetime are imposed. A caller of
 GSS_Acquire_cred() can request a length of time for which acquired
 credentials are to be valid (lifetime_req argument), beginning at the
 present, or can request credentials with a default validity interval.
 (Requests for postdated credentials are not supported within the
 GSS-API.) Certain mechanisms and implementations may bind in
 credential validity period specifiers at a point preliminary to
 invocation of the GSS_Acquire_cred() call (e.g., in conjunction with
 user login procedures). As a result, callers requesting non-default
 values for lifetime_req must recognize that such requests cannot
 always be honored and must be prepared to accommodate the use of
 returned credentials with different lifetimes as indicated in
 lifetime_rec.
 The caller of GSS_Acquire_cred() can explicitly specify a set of
 mech_types which are to be accommodated in the returned credentials
 (desired_mechs argument), or can request credentials for a system-
 defined default set of mech_types. Selection of the system-specified
 default set is recommended in the interests of application
 portability. The actual_mechs return value may be interrogated by the
 caller to determine the set of mechanisms with which the returned
 credentials may be used.
2.1.2. GSS_Release_cred call
 Input:
 o cred_handle OCTET STRING-NULL specifies default credentials
 Outputs:
 o major_status INTEGER,
 o minor_status INTEGER
 Return major_status codes:
 o GSS_COMPLETE indicates that the credentials referenced by the
 input cred_handle were released for purposes of subsequent access
 by the caller. The effect on other processes which may be
 authorized shared access to such credentials is a local matter.
 o GSS_NO_CRED indicates that no release operation was performed,
 either because the input cred_handle was invalid or because the
 caller lacks authorization to access the referenced credentials.
 o GSS_FAILURE indicates that the release operation failed for
 reasons unspecified at the GSS-API level.
 Provides a means for a caller to explicitly request that credentials
 be released when their use is no longer required. Note that system-
 specific credential management functions are also likely to exist,
 for example to assure that credentials shared among processes are
 properly deleted when all affected processes terminate, even if no
 explicit release requests are issued by those processes. Given the
 fact that multiple callers are not precluded from gaining authorized
 access to the same credentials, invocation of GSS_Release_cred()
 cannot be assumed to delete a particular set of credentials on a
 system-wide basis.
2.1.3. GSS_Inquire_cred call
 Input:
 o cred_handle OCTET STRING -NULL specifies default credentials
 Outputs:
 o major_status INTEGER,
 o minor_status INTEGER,
 o cred_name INTERNAL NAME,
 o lifetime_rec INTEGER -in seconds, or reserved value for
 INDEFINITE
 o cred_usage INTEGER, -0=INITIATE-AND-ACCEPT, 1=INITIATE-ONLY,
 2=ACCEPT-ONLY
 o mech_set SET OF OBJECT IDENTIFIER
 Return major_status codes:
 o GSS_COMPLETE indicates that the credentials referenced by the
 input cred_handle argument were valid, and that the output
 cred_name, lifetime_rec, and cred_usage values represent,
 respectively, the credentials' associated principal name,
 remaining lifetime, suitable usage modes, and supported
 mechanism types.
 o GSS_NO_CRED indicates that no information could be returned
 about the referenced credentials, either because the input
 cred_handle was invalid or because the caller lacks
 authorization to access the referenced credentials.
 o GSS_FAILURE indicates that the release operation failed for
 reasons unspecified at the GSS-API level.
 The GSS_Inquire_cred() call is defined primarily for the use of
 those callers which make use of default credentials rather than
 acquiring credentials explicitly with GSS_Acquire_cred(). It enables
 callers to determine a credential structure's associated principal
 name, remaining validity period, usability for security context
 initiation and/or acceptance, and supported mechanisms.
2.2. Context-level calls
 This group of calls is devoted to the establishment and management of
 security contexts between peers. A context's initiator calls
 GSS_Init_sec_context(), resulting in generation of a token which the
 caller passes to the target. At the target, that token is passed to
 GSS_Accept_sec_context(). Depending on the underlying mech_type and
 specified options, additional token exchanges may be performed in the
 course of context establishment; such exchanges are accommodated by
 GSS_CONTINUE_NEEDED status returns from GSS_Init_sec_context() and
 GSS_Accept_sec_context(). Either party to an established context may
 invoke GSS_Delete_sec_context() to flush context information when a
 context is no longer required. GSS_Process_context_token() is used
 to process received tokens carrying context-level control
 information. GSS_Context_time() allows a caller to determine the
 length of time for which an established context will remain valid.
2.2.1. GSS_Init_sec_context call
 Inputs:
 o claimant_cred_handle OCTET STRING, -NULL specifies "use
 default"
 o input_context_handle INTEGER, -0 specifies "none assigned
 yet"
 o targ_name INTERNAL NAME,
 o mech_type OBJECT IDENTIFIER, -NULL parameter specifies "use
 default"
 o deleg_req_flag BOOLEAN,
 o mutual_req_flag BOOLEAN,
 o replay_det_req_flag BOOLEAN,
 o sequence_req_flag BOOLEAN,
 o lifetime_req INTEGER,-0 specifies default lifetime
 o chan_bindings OCTET STRING,
 o input_token OCTET STRING-NULL or token received from target
 Outputs:
 o major_status INTEGER,
 o minor_status INTEGER,
 o output_context_handle INTEGER,
 o mech_type OBJECT IDENTIFIER, -actual mechanism always
 indicated, never NULL
 o output_token OCTET STRING, -NULL or token to pass to context
 target
 o deleg_state BOOLEAN,
 o mutual_state BOOLEAN,
 o replay_det_state BOOLEAN,
 o sequence_state BOOLEAN,
 o conf_avail BOOLEAN,
 o integ_avail BOOLEAN,
 o lifetime_rec INTEGER - in seconds, or reserved value for
 INDEFINITE
 This call may block pending network interactions for those mech_types
 in which an authentication server or other network entity must be
 consulted on behalf of a context initiator in order to generate an
 output_token suitable for presentation to a specified target.
 Return major_status codes:
 o GSS_COMPLETE indicates that context-level information was
 successfully initialized, and that the returned output_token will
 provide sufficient information for the target to perform per-
 message processing on the newly-established context.
 o GSS_CONTINUE_NEEDED indicates that control information in the
 returned output_token must be sent to the target, and that a reply
 must be received and passed as the input_token argument to a
 continuation call to GSS_Init_sec_context(), before per-message
 processing can be performed in conjunction with this context.
 o GSS_DEFECTIVE_TOKEN indicates that consistency checks performed on
 the input_token failed, preventing further processing from being
 performed based on that token.
 o GSS_DEFECTIVE_CREDENTIAL indicates that consistency checks
 performed on the credential structure referenced by
 claimant_cred_handle failed, preventing further processing from
 being performed using that credential structure.
 o GSS_BAD_SIG indicates that the received input_token contains an
 incorrect signature, so context setup cannot be accomplished.
 o GSS_NO_CRED indicates that no context was established, either
 because the input cred_handle was invalid, because the referenced
 credentials are valid for context acceptor use only, or because
 the caller lacks authorization to access the referenced
 credentials.
 o GSS_CREDENTIALS_EXPIRED indicates that the credentials provided
 through the input claimant_cred_handle argument are no longer
 valid, so context establishment cannot be completed.
 o GSS_BAD_BINDINGS indicates that a mismatch between the caller-
 provided chan_bindings and those extracted from the input_token
 was detected, signifying a security-relevant event and preventing
 context establishment. (This result will be returned by
 GSS_Init_sec_context only for contexts where mutual_state is
 TRUE.)
 o GSS_NO_CONTEXT indicates that no valid context was recognized for
 the input context_handle provided; this major status will be
 returned only for successor calls following GSS_CONTINUE_NEEDED
 status returns.
 o GSS_BAD_NAMETYPE indicates that the provided targ_name is of a
 type uninterpretable or unsupported by the supporting GSS-API
 implementation, so context establishment cannot be completed.
 o GSS_BAD_NAME indicates that the provided targ_name is inconsistent
 in terms of internally-incorporated type specifier information, so
 context establishment cannot be accomplished.
 o GSS_FAILURE indicates that context setup could not be accomplished
 for reasons unspecified at the GSS-API level, and that no
 interface-defined recovery action is available.
 This routine is used by a context initiator, and ordinarily emits one
 (or, for the case of a multi-step exchange, more than one)
 output_token suitable for use by the target within the selected
 mech_type's protocol. Using information in the credentials structure
 referenced by claimant_cred_handle, GSS_Init_sec_context()
 initializes the data structures required to establish a security
 context with target targ_name. The claimant_cred_handle must
 correspond to the same valid credentials structure on the initial
 call to GSS_Init_sec_context() and on any successor calls resulting
 from GSS_CONTINUE_NEEDED status returns; different protocol sequences
 modeled by the GSS_CONTINUE_NEEDED mechanism will require access to
 credentials at different points in the context establishment
 sequence.
 The input_context_handle argument is 0, specifying "not yet
 assigned", on the first GSS_Init_sec_context() call relating to a
 given context. That call returns an output_context_handle for future
 references to this context. When continuation attempts to
 GSS_Init_sec_context() are needed to perform context establishment,
 the previously-returned non-zero handle value is entered into the
 input_context_handle argument and will be echoed in the returned
 output_context_handle argument. On such continuation attempts (and
 only on continuation attempts) the input_token value is used, to
 provide the token returned from the context's target.
 The chan_bindings argument is used by the caller to provide
 information binding the security context to security-related
 characteristics (e.g., addresses, cryptographic keys) of the
 underlying communications channel. See Section 1.1.6 of this document
 for more discussion of this argument's usage.
 The input_token argument contains a message received from the target,
 and is significant only on a call to GSS_Init_sec_context() which
 follows a previous return indicating GSS_CONTINUE_NEEDED
 major_status.
 It is the caller's responsibility to establish a communications path
 to the target, and to transmit any returned output_token (independent
 of the accompanying returned major_status value) to the target over
 that path. The output_token can, however, be transmitted along with
 the first application-provided input message to be processed by
 GSS_Sign() or GSS_Seal() in conjunction with a successfully-
 established context.
 The initiator may request various context-level functions through
 input flags: the deleg_req_flag requests delegation of access rights,
 the mutual_req_flag requests mutual authentication, the
 replay_det_req_flag requests that replay detection features be
 applied to messages transferred on the established context, and the
 sequence_req_flag requests that sequencing be enforced. (See Section
 1.2.3 for more information on replay detection and sequencing
 features.)
 Not all of the optionally-requestable features will be available in
 all underlying mech_types; the corresponding return state values
 (deleg_state, mutual_state, replay_det_state, sequence_state)
 indicate, as a function of mech_type processing capabilities and
 initiator-provided input flags, the set of features which will be
 active on the context. These state indicators' values are undefined
 unless the routine's major_status indicates COMPLETE. Failure to
 provide the precise set of features requested by the caller does not
 cause context establishment to fail; it is the caller's prerogative
 to delete the context if the feature set provided is unsuitable for
 the caller's use. The returned mech_type value indicates the
 specific mechanism employed on the context, and will never indicate
 the value for "default".
 The conf_avail return value indicates whether the context supports
 per-message confidentiality services, and so informs the caller
 whether or not a request for encryption through the conf_req_flag
 input to GSS_Seal() can be honored. In similar fashion, the
 integ_avail return value indicates whether per-message integrity
 services are available (through either GSS_Sign() or GSS_Seal()) on
 the established context.
 The lifetime_req input specifies a desired upper bound for the
 lifetime of the context to be established, with a value of 0 used to
 request a default lifetime. The lifetime_rec return value indicates
 the length of time for which the context will be valid, expressed as
 an offset from the present; depending on mechanism capabilities,
 credential lifetimes, and local policy, it may not correspond to the
 value requested in lifetime_req. If no constraints on context
 lifetime are imposed, this may be indicated by returning a reserved
 value representing INDEFINITE lifetime_req. The values of conf_avail,
 integ_avail, and lifetime_rec are undefined unless the routine's
 major_status indicates COMPLETE.
 If the mutual_state is TRUE, this fact will be reflected within the
 output_token. A call to GSS_Accept_sec_context() at the target in
 conjunction with such a context will return a token, to be processed
 by a continuation call to GSS_Init_sec_context(), in order to achieve
 mutual authentication.
2.2.2. GSS_Accept_sec_context call
 Inputs:
 o acceptor_cred_handle OCTET STRING,-NULL specifies "use
 default"
 o input_context_handle INTEGER, -0 specifies "not yet assigned"
 o chan_bindings OCTET STRING,
 o input_token OCTET STRING
 Outputs:
 o major_status INTEGER,
 o minor_status INTEGER,
 o src_name INTERNAL NAME,
 o mech_type OBJECT IDENTIFIER,
 o output_context_handle INTEGER,
 o deleg_state BOOLEAN,
 o mutual_state BOOLEAN,
 o replay_det_state BOOLEAN,
 o sequence_state BOOLEAN,
 o conf_avail BOOLEAN,
 o integ_avail BOOLEAN,
 o lifetime_rec INTEGER, - in seconds, or reserved value for
 INDEFINITE
 o delegated_cred_handle OCTET STRING,
 o output_token OCTET STRING -NULL or token to pass to context
 initiator
 This call may block pending network interactions for those mech_types
 in which a directory service or other network entity must be
 consulted on behalf of a context acceptor in order to validate a
 received input_token.
 Return major_status codes:
 o GSS_COMPLETE indicates that context-level data structures were
 successfully initialized, and that per-message processing can now
 be performed in conjunction with this context.
 o GSS_CONTINUE_NEEDED indicates that control information in the
 returned output_token must be sent to the initiator, and that a
 response must be received and passed as the input_token argument
 to a continuation call to GSS_Accept_sec_context(), before per-
 message processing can be performed in conjunction with this
 context.
 o GSS_DEFECTIVE_TOKEN indicates that consistency checks performed on
 the input_token failed, preventing further processing from being
 performed based on that token.
 o GSS_DEFECTIVE_CREDENTIAL indicates that consistency checks
 performed on the credential structure referenced by
 acceptor_cred_handle failed, preventing further processing from
 being performed using that credential structure.
 o GSS_BAD_SIG indicates that the received input_token contains an
 incorrect signature, so context setup cannot be accomplished.
 o GSS_DUPLICATE_TOKEN indicates that the signature on the received
 input_token was correct, but that the input_token was recognized
 as a duplicate of an input_token already processed. No new context
 is established.
 o GSS_OLD_TOKEN indicates that the signature on the received
 input_token was correct, but that the input_token is too old to be
 checked for duplication against previously-processed input_tokens.
 No new context is established.
 o GSS_NO_CRED indicates that no context was established, either
 because the input cred_handle was invalid, because the referenced
 credentials are valid for context initiator use only, or because
 the caller lacks authorization to access the referenced
 credentials.
 o GSS_CREDENTIALS_EXPIRED indicates that the credentials provided
 through the input acceptor_cred_handle argument are no longer
 valid, so context establishment cannot be completed.
 o GSS_BAD_BINDINGS indicates that a mismatch between the caller-
 provided chan_bindings and those extracted from the input_token
 was detected, signifying a security-relevant event and preventing
 context establishment.
 o GSS_NO_CONTEXT indicates that no valid context was recognized for
 the input context_handle provided; this major status will be
 returned only for successor calls following GSS_CONTINUE_NEEDED
 status returns.
 o GSS_FAILURE indicates that context setup could not be accomplished
 for reasons unspecified at the GSS-API level, and that no
 interface-defined recovery action is available.
 The GSS_Accept_sec_context() routine is used by a context target.
 Using information in the credentials structure referenced by the
 input acceptor_cred_handle, it verifies the incoming input_token and
 (following the successful completion of a context establishment
 sequence) returns the authenticated src_name and the mech_type used.
 The acceptor_cred_handle must correspond to the same valid
 credentials structure on the initial call to GSS_Accept_sec_context()
 and on any successor calls resulting from GSS_CONTINUE_NEEDED status
 returns; different protocol sequences modeled by the
 GSS_CONTINUE_NEEDED mechanism will require access to credentials at
 different points in the context establishment sequence.
 The input_context_handle argument is 0, specifying "not yet
 assigned", on the first GSS_Accept_sec_context() call relating to a
 given context. That call returns an output_context_handle for future
 references to this context; when continuation attempts to
 GSS_Accept_sec_context() are needed to perform context
 establishment, that handle value will be entered into the
 input_context_handle argument.
 The chan_bindings argument is used by the caller to provide
 information binding the security context to security-related
 characteristics (e.g., addresses, cryptographic keys) of the
 underlying communications channel. See Section 1.1.6 of this document
 for more discussion of this argument's usage.
 The returned state results (deleg_state, mutual_state,
 replay_det_state, and sequence_state) reflect the same context state
 values as returned to GSS_Init_sec_context()'s caller at the
 initiator system.
 The conf_avail return value indicates whether the context supports
 per-message confidentiality services, and so informs the caller
 whether or not a request for encryption through the conf_req_flag
 input to GSS_Seal() can be honored. In similar fashion, the
 integ_avail return value indicates whether per-message integrity
 services are available (through either GSS_Sign() or GSS_Seal()) on
 the established context.
 The lifetime_rec return value indicates the length of time for which
 the context will be valid, expressed as an offset from the present.
 The values of deleg_state, mutual_state, replay_det_state,
 sequence_state, conf_avail, integ_avail, and lifetime_rec are
 undefined unless the accompanying major_status indicates COMPLETE.
 The delegated_cred_handle result is significant only when deleg_state
 is TRUE, and provides a means for the target to reference the
 delegated credentials. The output_token result, when non-NULL,
 provides a context-level token to be returned to the context
 initiator to continue a multi-step context establishment sequence. As
 noted with GSS_Init_sec_context(), any returned token should be
 transferred to the context's peer (in this case, the context
 initiator), independent of the value of the accompanying returned
 major_status.
 Note: A target must be able to distinguish a context-level
 input_token, which is passed to GSS_Accept_sec_context(), from the
 per-message data elements passed to GSS_Verify() or GSS_Unseal().
 These data elements may arrive in a single application message, and
 GSS_Accept_sec_context() must be performed before per-message
 processing can be performed successfully.
2.2.3. GSS_Delete_sec_context call
 Input:
 o context_handle INTEGER
 Outputs:
 o major_status INTEGER,
 o minor_status INTEGER,
 o output_context_token OCTET STRING
 Return major_status codes:
 o GSS_COMPLETE indicates that the context was recognized, that
 relevant context-specific information was flushed, and that the
 returned output_context_token is ready for transfer to the
 context's peer.
 o GSS_NO_CONTEXT indicates that no valid context was recognized for
 the input context_handle provide, so no deletion was performed.
 o GSS_FAILURE indicates that the context is recognized, but that the
 GSS_Delete_sec_context() operation could not be performed for
 reasons unspecified at the GSS-API level.
 This call may block pending network interactions for mech_types in
 which active notification must be made to a central server when a
 security context is to be deleted.
 This call can be made by either peer in a security context, to flush
 context-specific information and to return an output_context_token
 which can be passed to the context's peer informing it that the
 peer's corresponding context information can also be flushed. (Once a
 context is established, the peers involved are expected to retain
 cached credential and context-related information until the
 information's expiration time is reached or until a
 GSS_Delete_sec_context() call is made.) Attempts to perform per-
 message processing on a deleted context will result in error returns.
2.2.4. GSS_Process_context_token call
 Inputs:
 o context_handle INTEGER,
 o input_context_token OCTET STRING
 Outputs:
 o major_status INTEGER,
 o minor_status INTEGER,
 Return major_status codes:
 o GSS_COMPLETE indicates that the input_context_token was
 successfully processed in conjunction with the context referenced
 by context_handle.
 o GSS_DEFECTIVE_TOKEN indicates that consistency checks performed on
 the received context_token failed, preventing further processing
 from being performed with that token.
 o GSS_NO_CONTEXT indicates that no valid context was recognized for
 the input context_handle provided.
 o GSS_FAILURE indicates that the context is recognized, but that the
 GSS_Process_context_token() operation could not be performed for
 reasons unspecified at the GSS-API level.
 This call is used to process context_tokens received from a peer once
 a context has been established, with corresponding impact on
 context-level state information. One use for this facility is
 processing of the context_tokens generated by
 GSS_Delete_sec_context(); GSS_Process_context_token() will not block
 pending network interactions for that purpose. Another use is to
 process tokens indicating remote-peer context establishment failures
 after the point where the local GSS-API implementation has already
 indicated GSS_COMPLETE status.
2.2.5. GSS_Context_time call
 Input:
 o context_handle INTEGER,
 Outputs:
 o major_status INTEGER,
 o minor_status INTEGER,
 o lifetime_rec INTEGER - in seconds, or reserved value for
 INDEFINITE
 Return major_status codes:
 o GSS_COMPLETE indicates that the referenced context is valid, and
 will remain valid for the amount of time indicated in
 lifetime_rec.
 o GSS_CONTEXT_EXPIRED indicates that data items related to the
 referenced context have expired.
 o GSS_CREDENTIALS_EXPIRED indicates that the context is recognized,
 but that its associated credentials have expired.
 o GSS_NO_CONTEXT indicates that no valid context was recognized for
 the input context_handle provided.
 o GSS_FAILURE indicates that the requested operation failed for
 reasons unspecified at the GSS-API level.
 This call is used to determine the amount of time for which a
 currently established context will remain valid.
2.3. Per-message calls
 This group of calls is used to perform per-message protection
 processing on an established security context. None of these calls
 block pending network interactions. These calls may be invoked by a
 context's initiator or by the context's target. The four members of
 this group should be considered as two pairs; the output from
 GSS_Sign() is properly input to GSS_Verify(), and the output from
 GSS_Seal() is properly input to GSS_Unseal().
 GSS_Sign() and GSS_Verify() support data origin authentication and
 data integrity services. When GSS_Sign() is invoked on an input
 message, it yields a per-message token containing data items which
 allow underlying mechanisms to provide the specified security
 services. The original message, along with the generated per-message
 token, is passed to the remote peer; these two data elements are
 processed by GSS_Verify(), which validates the message in
 conjunction with the separate token.
 GSS_Seal() and GSS_Unseal() support caller-requested confidentiality
 in addition to the data origin authentication and data integrity
 services offered by GSS_Sign() and GSS_Verify(). GSS_Seal() outputs
 a single data element, encapsulating optionally enciphered user data
 as well as associated token data items. The data element output from
 GSS_Seal() is passed to the remote peer and processed by
 GSS_Unseal() at that system. GSS_Unseal() combines decipherment (as
 required) with validation of data items related to authentication and
 integrity.
2.3.1. GSS_Sign call
 Inputs:
 o context_handle INTEGER,
 o qop_req INTEGER,-0 specifies default QOP
 o message OCTET STRING
 Outputs:
 o major_status INTEGER,
 o minor_status INTEGER,
 o per_msg_token OCTET STRING
 Return major_status codes:
 o GSS_COMPLETE indicates that a signature, suitable for an
 established security context, was successfully applied and that
 the message and corresponding per_msg_token are ready for
 transmission.
 o GSS_CONTEXT_EXPIRED indicates that context-related data items have
 expired, so that the requested operation cannot be performed.
 o GSS_CREDENTIALS_EXPIRED indicates that the context is recognized,
 but that its associated credentials have expired, so that the
 requested operation cannot be performed.
 o GSS_NO_CONTEXT indicates that no valid context was recognized for
 the input context_handle provided.
 o GSS_FAILURE indicates that the context is recognized, but that the
 requested operation could not be performed for reasons unspecified
 at the GSS-API level.
 Using the security context referenced by context_handle, apply a
 signature to the input message (along with timestamps and/or other
 data included in support of mech_type-specific mechanisms) and return
 the result in per_msg_token. The qop_req parameter allows quality-
 of-protection control. The caller passes the message and the
 per_msg_token to the target.
 The GSS_Sign() function completes before the message and
 per_msg_token is sent to the peer; successful application of
 GSS_Sign() does not guarantee that a corresponding GSS_Verify() has
 been (or can necessarily be) performed successfully when the message
 arrives at the destination.
2.3.2. GSS_Verify call
 Inputs:
 o context_handle INTEGER,
 o message OCTET STRING,
 o per_msg_token OCTET STRING
 Outputs:
 o qop_state INTEGER,
 o major_status INTEGER,
 o minor_status INTEGER,
 Return major_status codes:
 o GSS_COMPLETE indicates that the message was successfully verified.
 o GSS_DEFECTIVE_TOKEN indicates that consistency checks performed on
 the received per_msg_token failed, preventing further processing
 from being performed with that token.
 o GSS_BAD_SIG indicates that the received per_msg_token contains an
 incorrect signature for the message.
 o GSS_DUPLICATE_TOKEN, GSS_OLD_TOKEN, and GSS_UNSEQ_TOKEN values
 appear in conjunction with the optional per-message replay
 detection features described in Section 1.2.3; their semantics are
 described in that section.
 o GSS_CONTEXT_EXPIRED indicates that context-related data items have
 expired, so that the requested operation cannot be performed.
 o GSS_CREDENTIALS_EXPIRED indicates that the context is recognized,
 but that its associated credentials have expired, so that the
 requested operation cannot be performed.
 o GSS_NO_CONTEXT indicates that no valid context was recognized for
 the input context_handle provided.
 o GSS_FAILURE indicates that the context is recognized, but that the
 GSS_Verify() operation could not be performed for reasons
 unspecified at the GSS-API level.
 Using the security context referenced by context_handle, verify that
 the input per_msg_token contains an appropriate signature for the
 input message, and apply any active replay detection or sequencing
 features. Return an indication of the quality-of-protection applied
 to the processed message in the qop_state result.
2.3.3. GSS_Seal call
 Inputs:
 o context_handle INTEGER,
 o conf_req_flag BOOLEAN,
 o qop_req INTEGER,-0 specifies default QOP
 o input_message OCTET STRING
 Outputs:
 o major_status INTEGER,
 o minor_status INTEGER,
 o conf_state BOOLEAN,
 o output_message OCTET STRING
 Return major_status codes:
 o GSS_COMPLETE indicates that the input_message was successfully
 processed and that the output_message is ready for transmission.
 o GSS_CONTEXT_EXPIRED indicates that context-related data items have
 expired, so that the requested operation cannot be performed.
 o GSS_CREDENTIALS_EXPIRED indicates that the context is recognized,
 but that its associated credentials have expired, so that the
 requested operation cannot be performed.
 o GSS_NO_CONTEXT indicates that no valid context was recognized for
 the input context_handle provided.
 o GSS_FAILURE indicates that the context is recognized, but that the
 GSS_Seal() operation could not be performed for reasons
 unspecified at the GSS-API level.
 Performs the data origin authentication and data integrity functions
 of GSS_Sign(). If the input conf_req_flag is TRUE, requests that
 confidentiality be applied to the input_message. Confidentiality may
 not be supported in all mech_types or by all implementations; the
 returned conf_state flag indicates whether confidentiality was
 provided for the input_message. The qop_req parameter allows
 quality-of-protection control.
 In all cases, the GSS_Seal() call yields a single output_message
 data element containing (optionally enciphered) user data as well as
 control information.
2.3.4. GSS_Unseal call
 Inputs:
 o context_handle INTEGER,
 o input_message OCTET STRING
 Outputs:
 o conf_state BOOLEAN,
 o qop_state INTEGER,
 o major_status INTEGER,
 o minor_status INTEGER,
 o output_message OCTET STRING
 Return major_status codes:
 o GSS_COMPLETE indicates that the input_message was successfully
 processed and that the resulting output_message is available.
 o GSS_DEFECTIVE_TOKEN indicates that consistency checks performed on
 the per_msg_token extracted from the input_message failed,
 preventing further processing from being performed.
 o GSS_BAD_SIG indicates that an incorrect signature was detected for
 the message.
 o GSS_DUPLICATE_TOKEN, GSS_OLD_TOKEN, and GSS_UNSEQ_TOKEN values
 appear in conjunction with the optional per-message replay
 detection features described in Section 1.2.3; their semantics are
 described in that section.
 o GSS_CONTEXT_EXPIRED indicates that context-related data items have
 expired, so that the requested operation cannot be performed.
 o GSS_CREDENTIALS_EXPIRED indicates that the context is recognized,
 but that its associated credentials have expired, so that the
 requested operation cannot be performed.
 o GSS_NO_CONTEXT indicates that no valid context was recognized for
 the input context_handle provided.
 o GSS_FAILURE indicates that the context is recognized, but that the
 GSS_Unseal() operation could not be performed for reasons
 unspecified at the GSS-API level.
 Processes a data element generated (and optionally enciphered) by
 GSS_Seal(), provided as input_message. The returned conf_state value
 indicates whether confidentiality was applied to the input_message.
 If conf_state is TRUE, GSS_Unseal() deciphers the input_message.
 Returns an indication of the quality-of-protection applied to the
 processed message in the qop_state result. GSS_Seal() performs the
 data integrity and data origin authentication checking functions of
 GSS_Verify() on the plaintext data. Plaintext data is returned in
 output_message.
2.4. Support calls
 This group of calls provides support functions useful to GSS-API
 callers, independent of the state of established contexts. Their
 characterization with regard to blocking or non-blocking status in
 terms of network interactions is unspecified.
2.4.1. GSS_Display_status call
 Inputs:
 o status_value INTEGER,-GSS-API major_status or minor_status
 return value
 o status_type INTEGER,-1 if major_status, 2 if minor_status
 o mech_type OBJECT IDENTIFIER-mech_type to be used for minor_
 status translation
 Outputs:
 o major_status INTEGER,
 o minor_status INTEGER,
 o status_string_set SET OF OCTET STRING
 Return major_status codes:
 o GSS_COMPLETE indicates that a valid printable status
 representation (possibly representing more than one status event
 encoded within the status_value) is available in the returned
 status_string_set.
 o GSS_BAD_MECH indicates that translation in accordance with an
 unsupported mech_type was requested, so translation could not be
 performed.
 o GSS_BAD_STATUS indicates that the input status_value was invalid,
 or that the input status_type carried a value other than 1 or 2,
 so translation could not be performed.
 o GSS_FAILURE indicates that the requested operation could not be
 performed for reasons unspecified at the GSS-API level.
 Provides a means for callers to translate GSS-API-returned major and
 minor status codes into printable string representations.
2.4.2. GSS_Indicate_mechs call
 Input:
 o (none)
 Outputs:
 o major_status INTEGER,
 o minor_status INTEGER,
 o mech_set SET OF OBJECT IDENTIFIER
 Return major_status codes:
 o GSS_COMPLETE indicates that a set of available mechanisms has
 been returned in mech_set.
 o GSS_FAILURE indicates that the requested operation could not
 be performed for reasons unspecified at the GSS-API level.
 Allows callers to determine the set of mechanism types available on
 the local system. This call is intended for support of specialized
 callers who need to request non-default mech_type sets from
 GSS_Acquire_cred(), and should not be needed by other callers.
2.4.3. GSS_Compare_name call
 Inputs:
 o name1 INTERNAL NAME,
 o name2 INTERNAL NAME
 Outputs:
 o major_status INTEGER,
 o minor_status INTEGER,
 o name_equal BOOLEAN
 Return major_status codes:
 o GSS_COMPLETE indicates that name1 and name2 were comparable, and
 that the name_equal result indicates whether name1 and name2 were
 equal or unequal.
 o GSS_BAD_NAMETYPE indicates that one or both of name1 and name2
 contained internal type specifiers uninterpretable by the
 supporting GSS-API implementation, or that the two names' types
 are different and incomparable, so the equality comparison could
 not be completed.
 o GSS_BAD_NAME indicates that one or both of the input names was
 ill-formed in terms of its internal type specifier, so the
 equality comparison could not be completed.
 o GSS_FAILURE indicates that the requested operation could not be
 performed for reasons unspecified at the GSS-API level.
 Allows callers to compare two internal name representations for
 equality.
2.4.4. GSS_Display_name call
 Inputs:
 o name INTERNAL NAME
 Outputs:
 o major_status INTEGER,
 o minor_status INTEGER,
 o name_string OCTET STRING,
 o name_type OBJECT IDENTIFIER
 Return major_status codes:
 o GSS_COMPLETE indicates that a valid printable name representation
 is available in the returned name_string.
 o GSS_BAD_NAMETYPE indicates that the provided name was of a type
 uninterpretable by the supporting GSS-API implementation, so no
 printable representation could be generated.
 o GSS_BAD_NAME indicates that the contents of the provided name were
 inconsistent with the internally-indicated name type, so no
 printable representation could be generated.
 o GSS_FAILURE indicates that the requested operation could not be
 performed for reasons unspecified at the GSS-API level.
 Allows callers to translate an internal name representation into a
 printable form with associated namespace type descriptor. The syntax
 of the printable form is a local matter.
2.4.5. GSS_Import_name call
 Inputs:
 o input_name_string OCTET STRING,
 o input_name_type OBJECT IDENTIFIER
 Outputs:
 o major_status INTEGER,
 o minor_status INTEGER,
 o output_name INTERNAL NAME
 Return major_status codes:
 o GSS_COMPLETE indicates that a valid name representation is output
 in output_name and described by the type value in
 output_name_type.
 o GSS_BAD_NAMETYPE indicates that the input_name_type is unsupported
 by the GSS-API implementation, so the import operation could not
 be completed.
 o GSS_BAD_NAME indicates that the provided input_name_string is
 ill-formed in terms of the input_name_type, so the import
 operation could not be completed.
 o GSS_FAILURE indicates that the requested operation could not be
 performed for reasons unspecified at the GSS-API level.
 Allows callers to provide a printable name representation, designate
 the type of namespace in conjunction with which it should be parsed,
 and convert that printable representation to an internal form
 suitable for input to other GSS-API routines. The syntax of the
 input_name is a local matter.
2.4.6. GSS_Release_name call
 Inputs:
 o name INTERNAL NAME
 Outputs:
 o major_status INTEGER,
 o minor_status INTEGER
 Return major_status codes:
 o GSS_COMPLETE indicates that the storage associated with the input
 name was successfully released.
 o GSS_BAD_NAME indicates that the input name argument did not
 contain a valid name.
 o GSS_FAILURE indicates that the requested operation could not be
 performed for reasons unspecified at the GSS-API level.
 Allows callers to release the storage associated with an internal
 name representation.
2.4.7. GSS_Release_buffer call
 Inputs:
 o buffer OCTET STRING
 Outputs:
 o major_status INTEGER,
 o minor_status INTEGER
 Return major_status codes:
 o GSS_COMPLETE indicates that the storage associated with the input
 buffer was successfully released.
 o GSS_FAILURE indicates that the requested operation could not be
 performed for reasons unspecified at the GSS-API level.
 Allows callers to release the storage associated with an OCTET STRING
 buffer allocated by another GSS-API call.
2.4.8. GSS_Release_oid_set call
 Inputs:
 o buffer SET OF OBJECT IDENTIFIER
 Outputs:
 o major_status INTEGER,
 o minor_status INTEGER
 Return major_status codes:
 o GSS_COMPLETE indicates that the storage associated with the input
 object identifier set was successfully released.
 o GSS_FAILURE indicates that the requested operation could not be
 performed for reasons unspecified at the GSS-API level.
 Allows callers to release the storage associated with an object
 identifier set object allocated by another GSS-API call.
3. Mechanism-Specific Example Scenarios
 This section provides illustrative overviews of the use of various
 candidate mechanism types to support the GSS-API. These discussions
 are intended primarily for readers familiar with specific security
 technologies, demonstrating how GSS-API functions can be used and
 implemented by candidate underlying mechanisms. They should not be
 regarded as constrictive to implementations or as defining the only
 means through which GSS-API functions can be realized with a
 particular underlying technology, and do not demonstrate all GSS-API
 features with each technology.
3.1. Kerberos V5, single-TGT
 OS-specific login functions yield a TGT to the local realm Kerberos
 server; TGT is placed in a credentials structure for the client.
 Client calls GSS_Acquire_cred() to acquire a cred_handle in order to
 reference the credentials for use in establishing security contexts.
 Client calls GSS_Init_sec_context(). If the requested service is
 located in a different realm, GSS_Init_sec_context() gets the
 necessary TGT/key pairs needed to traverse the path from local to
 target realm; these data are placed in the owner's TGT cache. After
 any needed remote realm resolution, GSS_Init_sec_context() yields a
 service ticket to the requested service with a corresponding session
 key; these data are stored in conjunction with the context. GSS-API
 code sends KRB_TGS_REQ request(s) and receives KRB_TGS_REP
 response(s) (in the successful case) or KRB_ERROR.
 Assuming success, GSS_Init_sec_context() builds a Kerberos-formatted
 KRB_AP_REQ message, and returns it in output_token. The client sends
 the output_token to the service.
 The service passes the received token as the input_token argument to
 GSS_Accept_sec_context(), which verifies the authenticator, provides
 the service with the client's authenticated name, and returns an
 output_context_handle.
 Both parties now hold the session key associated with the service
 ticket, and can use this key in subsequent GSS_Sign(), GSS_Verify(),
 GSS_Seal(), and GSS_Unseal() operations.
3.2. Kerberos V5, double-TGT
 TGT acquisition as above.
 Note: To avoid unnecessary frequent invocations of error paths when
 implementing the GSS-API atop Kerberos V5, it seems appropriate to
 represent "single-TGT K-V5" and "double-TGT K-V5" with separate
 mech_types, and this discussion makes that assumption.
 Based on the (specified or defaulted) mech_type,
 GSS_Init_sec_context() determines that the double-TGT protocol
 should be employed for the specified target. GSS_Init_sec_context()
 returns GSS_CONTINUE_NEEDED major_status, and its returned
 output_token contains a request to the service for the service's TGT.
 (If a service TGT with suitably long remaining lifetime already
 exists in a cache, it may be usable, obviating the need for this
 step.) The client passes the output_token to the service. Note: this
 scenario illustrates a different use for the GSS_CONTINUE_NEEDED
 status return facility than for support of mutual authentication;
 note that both uses can coexist as successive operations within a
 single context establishment operation.
 The service passes the received token as the input_token argument to
 GSS_Accept_sec_context(), which recognizes it as a request for TGT.
 (Note that current Kerberos V5 defines no intra-protocol mechanism to
 represent such a request.) GSS_Accept_sec_context() returns
 GSS_CONTINUE_NEEDED major_status and provides the service's TGT in
 its output_token. The service sends the output_token to the client.
 The client passes the received token as the input_token argument to a
 continuation of GSS_Init_sec_context(). GSS_Init_sec_context() caches
 the received service TGT and uses it as part of a service ticket
 request to the Kerberos authentication server, storing the returned
 service ticket and session key in conjunction with the context.
 GSS_Init_sec_context() builds a Kerberos-formatted authenticator,
 and returns it in output_token along with GSS_COMPLETE return
 major_status. The client sends the output_token to the service.
 Service passes the received token as the input_token argument to a
 continuation call to GSS_Accept_sec_context().
 GSS_Accept_sec_context() verifies the authenticator, provides the
 service with the client's authenticated name, and returns
 major_status GSS_COMPLETE.
 GSS_Sign(), GSS_Verify(), GSS_Seal(), and GSS_Unseal() as above.
3.3. X.509 Authentication Framework
 This example illustrates use of the GSS-API in conjunction with
 public-key mechanisms, consistent with the X.509 Directory
 Authentication Framework.
 The GSS_Acquire_cred() call establishes a credentials structure,
 making the client's private key accessible for use on behalf of the
 client.
 The client calls GSS_Init_sec_context(), which interrogates the
 Directory to acquire (and validate) a chain of public-key
 certificates, thereby collecting the public key of the service. The
 certificate validation operation determines that suitable signatures
 were applied by trusted authorities and that those certificates have
 not expired. GSS_Init_sec_context() generates a secret key for use
 in per-message protection operations on the context, and enciphers
 that secret key under the service's public key.
 The enciphered secret key, along with an authenticator quantity
 signed with the client's private key, is included in the output_token
 from GSS_Init_sec_context(). The output_token also carries a
 certification path, consisting of a certificate chain leading from
 the service to the client; a variant approach would defer this path
 resolution to be performed by the service instead of being asserted
 by the client. The client application sends the output_token to the
 service.
 The service passes the received token as the input_token argument to
 GSS_Accept_sec_context(). GSS_Accept_sec_context() validates the
 certification path, and as a result determines a certified binding
 between the client's distinguished name and the client's public key.
 Given that public key, GSS_Accept_sec_context() can process the
 input_token's authenticator quantity and verify that the client's
 private key was used to sign the input_token. At this point, the
 client is authenticated to the service. The service uses its private
 key to decipher the enciphered secret key provided to it for per-
 message protection operations on the context.
 The client calls GSS_Sign() or GSS_Seal() on a data message, which
 causes per-message authentication, integrity, and (optional)
 confidentiality facilities to be applied to that message. The service
 uses the context's shared secret key to perform corresponding
 GSS_Verify() and GSS_Unseal() calls.
4. Related Activities
 In order to implement the GSS-API atop existing, emerging, and future
 security mechanisms:
 object identifiers must be assigned to candidate GSS-API
 mechanisms and the name types which they support
 concrete data element formats must be defined for candidate
 mechanisms
 Calling applications must implement formatting conventions which will
 enable them to distinguish GSS-API tokens from other data carried in
 their application protocols.
 Concrete language bindings are required for the programming
 environments in which the GSS-API is to be employed; such bindings
 for the C language are available in an associated RFC.
5. Acknowledgments
 This proposal is the result of a collaborative effort.
 Acknowledgments are due to the many members of the IETF Security Area
 Advisory Group (SAAG) and the Common Authentication Technology (CAT)
 Working Group for their contributions at meetings and by electronic
 mail. Acknowledgments are also due to Kannan Alagappan, Doug Barlow,
 Bill Brown, Cliff Kahn, Charlie Kaufman, Butler Lampson, Richard
 Pitkin, Joe Tardo, and John Wray of Digital Equipment Corporation,
 and John Carr, John Kohl, Jon Rochlis, Jeff Schiller, and Ted T'so of
 MIT and Project Athena. Joe Pato and Bill Sommerfeld of HP/Apollo,
 Walt Tuvell of OSF, and Bill Griffith and Mike Merritt of AT&T,
 provided inputs which helped to focus and clarify directions.
 Precursor work by Richard Pitkin, presented to meetings of the
 Trusted Systems Interoperability Group (TSIG), helped to demonstrate
 the value of a generic, mechanism-independent security service API.
6. Security Considerations
 Security issues are discussed throughout this memo.
7. Author's Address
 John Linn
 Geer Zolot Associates
 One Main St.
 Cambridge, MA 02142 USA
 Phone: +1 617.374.3700
 Email: Linn@gza.com
APPENDIX A
PACS AND AUTHORIZATION SERVICES
 Consideration has been given to modifying the GSS-API service
 interface to recognize and manipulate Privilege Attribute
 Certificates (PACs) as in ECMA 138, carrying authorization data as a
 side effect of establishing a security context, but no such
 modifications have been incorporated at this time. This appendix
 provides rationale for this decision and discusses compatibility
 alternatives between PACs and the GSS-API which do not require that
 PACs be made visible to GSS-API callers.
 Existing candidate mechanism types such as Kerberos and X.509 do not
 incorporate PAC manipulation features, and exclusion of such
 mechanisms from the set of candidates equipped to fully support the
 GSS-API seems inappropriate. Inclusion (and GSS-API visibility) of a
 feature supported by only a limited number of mechanisms could
 encourage the development of ostensibly portable applications which
 would in fact have only limited portability.
 The status quo, in which PACs are not visible across the GSS-API
 interface, does not preclude implementations in which PACs are
 carried transparently, within the tokens defined and used for certain
 mech_types, and stored within peers' credentials and context-level
 data structures. While invisible to API callers, such PACs could be
 used by operating system or other local functions as inputs in the
 course of mediating access requests made by callers. This course of
 action allows dynamic selection of PAC contents, if such selection is
 administratively-directed rather than caller-directed.
 In a distributed computing environment, authentication must span
 different systems; the need for such authentication provides
 motivation for GSS-API definition and usage. Heterogeneous systems in
 a network can intercommunicate, with globally authenticated names
 comprising the common bond between locally defined access control
 policies. Access control policies to which authentication provides
 inputs are often local, or specific to particular operating systems
 or environments. If the GSS-API made particular authorization models
 visible across its service interface, its scope of application would
 become less general. The current GSS-API paradigm is consistent with
 the precedent set by Kerberos, neither defining the interpretation of
 authorization-related data nor enforcing access controls based on
 such data.
 The GSS-API is a general interface, whose callers may reside inside
 or outside any defined TCB or NTCB boundaries. Given this
 characteristic, it appears more realistic to provide facilities which
 provide "value-added" security services to its callers than to offer
 facilities which enforce restrictions on those callers. Authorization
 decisions must often be mediated below the GSS-API level in a local
 manner against (or in spite of) applications, and cannot be
 selectively invoked or omitted at those applications' discretion.
 Given that the GSS-API's placement prevents it from providing a
 comprehensive solution to the authorization issue, the value of a
 partial contribution specific to particular authorization models is
 debatable.
APPENDIX B
MECHANISM-INDEPENDENT TOKEN FORMAT
 This appendix specifies a mechanism-independent level of
 encapsulating representation for the initial token of a GSS-API
 context establishment sequence, incorporating an identifier of the
 mechanism type to be used on that context. Use of this format (with
 ASN.1-encoded data elements represented in BER, constrained in the
 interests of parsing simplicity to the Distinguished Encoding Rule
 (DER) BER subset defined in X.509, clause 8.7) is recommended to the
 designers of GSS-API implementations based on various mechanisms, so
 that tokens can be interpreted unambiguously at GSS-API peers. There
 is no requirement that the mechanism-specific innerContextToken,
 innerMsgToken, and sealedUserData data elements be encoded in ASN.1
 BER.
 -- optional top-level token definitions to
 -- frame different mechanisms
 GSS-API DEFINITIONS ::=
 BEGIN
 MechType ::= OBJECT IDENTIFIER
 -- data structure definitions
 -- callers must be able to distinguish among
 -- InitialContextToken, SubsequentContextToken,
 -- PerMsgToken, and SealedMessage data elements
 -- based on the usage in which they occur
 InitialContextToken ::=
 -- option indication (delegation, etc.) indicated within
 -- mechanism-specific token
 [APPLICATION 0] IMPLICIT SEQUENCE {
 thisMech MechType,
 innerContextToken ANY DEFINED BY thisMech
 -- contents mechanism-specific
 }
 SubsequentContextToken ::= innerContextToken ANY
 -- interpretation based on predecessor InitialContextToken
 PerMsgToken ::=
 -- as emitted by GSS_Sign and processed by GSS_Verify
 innerMsgToken ANY
 SealedMessage ::=
 -- as emitted by GSS_Seal and processed by GSS_Unseal
 -- includes internal, mechanism-defined indicator
 -- of whether or not encrypted
 sealedUserData ANY
 END
APPENDIX C
MECHANISM DESIGN CONSTRAINTS
 The following constraints on GSS-API mechanism designs are adopted in
 response to observed caller protocol requirements, and adherence
 thereto is anticipated in subsequent descriptions of GSS-API
 mechanisms to be documented in standards-track Internet
 specifications.
 Use of the approach defined in Appendix B of this specification,
 applying a mechanism type tag to the InitialContextToken, is
 required.
 It is strongly recommended that mechanisms offering per-message
 protection services also offer at least one of the replay detection
 and sequencing services, as mechanisms offering neither of the latter
 will fail to satisfy recognized requirements of certain candidate
 caller protocols.