Network Working Group V. Smyslov
Internet-Draft ELVIS-PLUS
Updates: 4301 (if approved) P. Wouters
Intended status: Standards Track Red Hat
Expires: October 22, 2015 April 20, 2015
The NULL Authentication Method in IKEv2 Protocol
draft-ietf-ipsecme-ikev2-null-auth-06
Abstract
This document specifies the NULL Authentication method and the
ID_NULL Identification Payload ID Type for the IKEv2 Protocol. This
allows two IKE peers to establish single-side authenticated or mutual
unauthenticated IKE sessions for those use cases where a peer is
unwilling or unable to authenticate or identify itself. This ensures
IKEv2 can be used for Opportunistic Security (also known as
Opportunistic Encryption) to defend against Pervasive Monitoring
attacks without the need to sacrifice anonymity.
Status of this Memo
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to this document. Code Components extracted from this document must
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions Used in This Document . . . . . . . . . . . . 3
2. Using the NULL Authentication Method . . . . . . . . . . . . . 5
2.1. Authentication Payload . . . . . . . . . . . . . . . . . . 5
2.2. Identification Payload . . . . . . . . . . . . . . . . . . 5
2.3. INITIAL_CONTACT Notification . . . . . . . . . . . . . . . 6
2.4. Interaction with Peer Authorization Database (PAD) . . . . 6
2.5. Traffic Selectors . . . . . . . . . . . . . . . . . . . . 7
3. Security Considerations . . . . . . . . . . . . . . . . . . . 8
3.1. Audit trail and peer identification . . . . . . . . . . . 8
3.2. Resource management and robustness . . . . . . . . . . . . 8
3.3. IKE configuration selection . . . . . . . . . . . . . . . 9
3.4. Networking topology changes . . . . . . . . . . . . . . . 9
4. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
6.1. Normative References . . . . . . . . . . . . . . . . . . . 12
6.2. Informative References . . . . . . . . . . . . . . . . . . 12
Appendix A. Update of PAD processing in RFC4301 . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14
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1. Introduction
The Internet Key Exchange Protocol version 2 (IKEv2), specified in
[RFC7296], provides a way for two parties to perform an authenticated
key exchange. While the authentication methods used by the peers can
be different, there is no method for one or both parties to remain
unauthenticated and anonymous. This document extends the
authentication methods to support unauthenticated and anonymous IKE
sessions.
In some situations mutual authentication is undesirable, superfluous
or impossible. The following three examples illustrate these
unauthenticated use cases:
o A user wants to establish an anonymous secure connection to a
server. In this situation the user should be able to authenticate
the server without presenting or authenticating to the server with
their own identity. This case uses a single-sided authentication
of the responder.
o A sensor that periodically wakes up from a suspended state wants
to send a measurement (e.g. temperature) to a collecting server.
The sensor must be authenticated by the server to ensure
authenticity of the measurement, but the sensor does not need to
authenticate the server. This case uses a single-sided
authentication of the initiator.
o Two peers without any trust relationship wish to defend against
widespread pervasive monitoring attacks as described in [RFC7258].
Without a trust relationship, the peers cannot authenticate each
other. Opportunistic Security [RFC7435] states that
unauthenticated encrypted communication is preferred over
cleartext communication. The peers want to use IKE to setup an
unauthenticated encrypted connection, that gives them protection
against pervasive monitoring attacks. An attacker that is able
and willing to send packets can still launch a Man-in-the-Middle
attack to obtain a copy of the unencrypted communication. This
case uses a fully unauthenticated key exchange.
To meet these needs, this document introduces the NULL Authentication
method, and the ID_NULL ID type. This allows an IKE peer to
explicitly indicate that it is unwilling or unable to certify its
identity.
1.1. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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document are to be interpreted as described in [RFC2119].
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2. Using the NULL Authentication Method
In IKEv2, each peer independently selects the method to authenticate
itself to the other side. A peer may choose to refrain from
authentication by using the NULL Authentication method. If a peer
that requires authentication receives an AUTH payload containing the
NULL Authentication method type, it MUST return an
AUTHENTICATION_FAILED notification. If an initiator uses EAP, the
responder MUST NOT use the NULL Authentication Method (in conformance
with the section 2.16 of [RFC7296]).
NULL Authentication affects how the Authentication and the
Identification payloads are formed in the IKE_AUTH exchange.
2.1. Authentication Payload
NULL Authentication still requires a properly formed AUTH payload to
be present in the IKE_AUTH exchange messages, as the AUTH payload
cryptographically links the IKE_SA_INIT exchange messages with the
other messages sent over this IKE SA.
When using NULL Authentication, the content of the AUTH payload is
computed using the syntax of pre-shared secret authentication,
described in Section 2.15 of [RFC7296]. The value of SK_pi for the
initiator and SK_pr for the responder is used as the shared secret
for the content of the AUTH payload. Implementers should note this
means that authentication keys used by the two peers are different in
each direction. This is identical to how the content of the two last
AUTH payloads is generated for the non-key-generating EAP methods
(see Section 2.16 of [RFC7296] for details).
The IKEv2 Authentication Method value for NULL Authentication is 13.
2.2. Identification Payload
When a remote peer is not authenticated, any ID presented in the
Identification Data field of the ID payload cannot be validated. To
avoid the need of sending a bogus ID Type with placeholder data, this
specification defines a new ID Type, ID_NULL. The Identification
Data field of the ID payload for this ID Type MUST be empty.
If NULL Authentication is in use and anonymity is a concern then
ID_NULL SHOULD be used in the Identification payload. Some examples
of cases where a non-null identity type and value with NULL
Authentication can be used are logging, troubleshooting and in
scenarios where authentication takes place out of band after the IKE
SA is created (like in [AUTOVPN]). The content of the Identification
payload MUST NOT be used for any trust and policy checking in
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IKE_AUTH exchange when NULL Authentication is employed (see Section
2.4 for details).
ID_NULL is primarily intended to be used with NULL Authentication but
could be used in other situations where the content of the
Identification Payload is not used. For example, ID_NULL could be
used when authentication is performed via raw public keys and the
identities are the keys themselves. These alternative uses of
ID_NULL should be described in their own respective documents.
The IKEv2 Identification Payload ID Type for ID_NULL is 13.
2.3. INITIAL_CONTACT Notification
The identity of a peer using NULL Authentication cannot be used to
find existing IKE SAs created by the same peer, as the peer identity
is not authenticated. For that reason the INITIAL_CONTACT
notifications MUST NOT be used to delete any other IKE SAs based on
the same peer identity without additional verification that the
existing IKE SAs with matching identity are actually stale.
The standard IKE Liveness Check procedure, described in Section 2.4
of [RFC7296], can be used to detect stale IKE SAs created by peers
using NULL Authentication. Inactive unauthenticated IKE SAs should
be checked periodically. Additionally, the event of creating a new
unauthenticated IKE SA can be used to trigger an out-of-order check
on existing unauthenticated IKE SAs, possibly limited to identical or
close-by IP addresses or to identical identities of the just created
IKE SA.
Implementations should weigh the resource consumption of sending
Liveness Checks against the memory usage of possible orphaned IKE
SAs. Implementations may choose to handle situations with thousands
of unauthenticated IKE SAs differently from situations with very few
such SAs.
2.4. Interaction with Peer Authorization Database (PAD)
Section 4.4.3 of [RFC4301] defines the Peer Authorization Database
(PAD), which provides the link between Security Policy Database (SPD)
and the IKEv2. The PAD contains an ordered list of records with
peers' identities along with corresponding authentication data and
Child SA authorization data. When the IKE SA is being established
the PAD is consulted to determine how the peer should be
authenticated and what Child SAs it is authorized to create.
When using NULL Authentication, the peer identity is not
authenticated and cannot be trusted. If ID_NULL is used with NULL
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Authentication, there is no ID at all. The processing of PAD
described in Section 4.4.3 of [RFC4301] is updated for NULL
Authentication as follows.
NULL authentication is added as one of supported authentication
methods. This method does not have any authentication data. ID_NULL
is included into the list of allowed ID types. The matching rule for
ID_NULL consists only of whether this type is used, i.e. no actual ID
matching is done, as ID_NULL contains no identity data.
When using the NULL authentication method those matching rules MUST
include matching of a new flag in the SPD entry specifying whether
unauthenticated users are allowed to use that entry. I.e. each SPD
entry needs to be augmented to have a flag specifying whether it can
be used with NULL authentication or not, and only those rules that
explicitly have that flag turned on can be used with unauthenticated
connections.
The specific updates of text in Section 4.4.3 of [RFC4301] are listed
in Appendix A.
2.5. Traffic Selectors
Traffic Selectors and narrowing allow two IKE peers to mutually agree
on a traffic range for an IPsec SA. An unauthenticated peer must not
be allowed to use this mechanism to steal traffic that an IKE peer
intended to be for another host. This is especially problematic when
supporting anonymous IKE peers behind NAT, as such IKE peers build an
IPsec SA using their pre-NAT IP address that are different from the
source IP of their IKE packets. A rogue IKE peer could use malicious
Traffic Selectors to obtain access to traffic that the host never
intended to hand out. Implementations SHOULD restrict and isolate
all anonymous IKE peers from each other and itself and only allow it
access to itself and possibly its intended network ranges.
One method to achieve this is to always assign internal IP addresses
to unauthenticated IKE clients, as described in Section 2.19 of
[RFC7296]. Implementations may also use other techniques, such as
internal NAT and connection tracking.
Implementations MAY force unauthenticated IKE peers to single host-
to-host IPsec SAs. When using IPv6 this is not always possible, so
implementations MUST be able to assign full /64 address block to the
peer as described in [RFC5739], even if it is not authenticated.
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3. Security Considerations
If authenticated IKE sessions are possible for a certain traffic
selector range between the peers, then unauthenticated IKE SHOULD NOT
be allowed for that traffic selector range. When mixing
authenticated and unauthenticated IKE with the same peer, policy
rules should ensure the highest level of security will be used to
protect the communication between the two peers. See [RFC7435] for
details.
If both peers use NULL Authentication, the entire key exchange
becomes unauthenticated. This makes the IKE session vulnerable to
active Man-in-the-Middle Attacks.
Using an ID Type other than ID_NULL with the NULL Authentication
Method may compromise the client's anonymity in case of an active
MITM attack.
IKE implementations without NULL Authentication have always performed
mutual authentication and were not designed for use with
unauthenticated IKE peers. Implementations might have made
assumptions that remote peers are identified. With NULL
Authentication these assumptions are no longer valid. Furthermore,
the host itself might have made trust assumptions or may not be aware
of the network topology changes that resulted from IPsec SAs from
unauthenticated IKE peers.
3.1. Audit trail and peer identification
With NULL Authentication an established IKE session is no longer
guaranteed to provide a verifiable (authenticated) entity known to
the system or network. Implementers that implement NULL
Authentication should ensure their implementation does not make any
assumptions that depend on IKE peers being "friendly", "trusted" or
"identifiable".
3.2. Resource management and robustness
Section 2.6 of [RFC7296] provides guidance for mitigation of "Denial
of Service" attacks by issuing COOKIES in response to resource
consumption of half-open IKE SAs. Furthermore, [DDOS-PROTECTION]
offers additional counter-measures in an attempt to distinguish
attacking IKE packets from legitimate IKE peers.
These defense mechanisms do not take into account IKE systems that
allow unauthenticated IKE peers. An attacker using NULL
Authentication is a fully legitimate IKE peer that is only
distinguished from authenticated IKE peers by having used NULL
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Authentication.
While implementations should have been written to account for abusive
authenticated clients, any omission or error in handling abusive
clients may have gone unnoticed because abusive clients has been a
rare or non-existent problem. When adding support for
unauthenticated IKE peers, these implementation omissions and errors
will be found and abused by attackers. For example, an
unauthenticated IKE peer could send an abusive amount of Liveness
probes or Delete requests.
3.3. IKE configuration selection
Combining authenticated and unauthenticated IKE peers on a single
host can be dangerous, assuming the authenticated IKE peer gains more
or different access from non-authenticated peers (otherwise, why not
only allow unauthenticated peers). An unauthenticated IKE peer MUST
NOT be able to reach resources only meant for authenticated IKE peers
and MUST NOT be able to replace the Child SAs of an authenticated IKE
peer.
3.4. Networking topology changes
When a host relies on packet filters or firewall software to protect
itself, establishing an IKE SA and installing an IPsec SA might
accidentally circumvent these packet filters and firewall
restrictions, as the encrypted ESP (protocol 50) or ESPinUDP (UDP
port 4500) packets do not match the packet filters defined. IKE
peers supporting unauthenticated IKE MUST pass all decrypted traffic
through the same packet filters and security mechanisms as incoming
plaintext traffic.
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4. Acknowledgments
The authors would like to thank Yaron Sheffer and Tero Kivinen for
their reviews, valuable comments and contributed text.
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5. IANA Considerations
This document defines a new entry in the "IKEv2 Authentication
Method" registry:
13 NULL Authentication
This document also defines a new entry in the "IKEv2 Identification
Payload ID Types" registry:
13 ID_NULL
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6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC5739] Eronen, P., Laganier, J., and C. Madson, "IPv6
Configuration in Internet Key Exchange Protocol Version 2
(IKEv2)", RFC 5739, February 2010.
[RFC7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
Kivinen, "Internet Key Exchange Protocol Version 2
(IKEv2)", STD 79, RFC 7296, October 2014.
6.2. Informative References
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, May 2014.
[RFC7435] Dukhovni, V., "Opportunistic Security: Some Protection
Most of the Time", RFC 7435, December 2014.
[AUTOVPN] Sheffer, Y. and Y. Nir, "The AutoVPN Architecture", Work
in Progress, draft-sheffer-autovpn-00, February 2014.
[DDOS-PROTECTION]
Nir, Y., "Protecting Internet Key Exchange (IKE)
Implementations from Distributed Denial of Service
Attacks", draft-ietf-ipsecme-ddos-protection-00 (work in
progress), October 2014.
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Appendix A. Update of PAD processing in RFC4301
This appendix lists the specific updates of the text in Section 4.4.3
of [RFC4301] that should be followed when implementing NULL
Authentication.
A new item is added to the list of supported ID types in Section
4.4.3.1
o NULL ID (matches ID type only)
and the following text is added at the end of the section:
Added text:
The NULL ID type is defined as having no data. For this name type
the matching function is defined as comparing the ID type only.
A new item is added to the list of authentication data types in
Section 4.4.3.2
- NULL authentication
and the next paragraph is updated as follows:
Old:
For authentication based on an X.509 certificate [...] For
authentication based on a pre-shared secret, the PAD contains the
pre-shared secret to be used by IKE.
New:
For authentication based on an X.509 certificate [...] For
authentication based on a pre-shared secret, the PAD contains the
pre-shared secret to be used by IKE. For NULL authentication the
PAD contains no data.
In addition the following text is added at the end of Section 4.4.3.4
Added text:
When using the NULL authentication method implementations MUST
make sure that they do not mix authenticated and not-authenticated
SPD rules, i.e. implementations need to keep them separately, for
example by adding flag in SPD to tell whether NULL authentication
can be used or not for the entry. I.e. each SPD entry needs to be
augmented to have a flag specifying whether it can be used with
NULL authentication or not, and only those rules that explictly
have that flag set can be used with unauthenticated connections.
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Authors' Addresses
Valery Smyslov
ELVIS-PLUS
PO Box 81
Moscow (Zelenograd) 124460
Russian Federation
Phone: +7 495 276 0211
Email: svan@elvis.ru
Paul Wouters
Red Hat
Email: pwouters@redhat.com
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