IPsec M. Myers
Internet-Draft TraceRoute Security LLC
Expires: March 29, 2006 H. Tschofenig
Siemens
September 25, 2005
OCSP Extensions to IKEv2
draft-myers-ikev2-ocsp-00.txt
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Copyright (C) The Internet Society (2005).
Abstract
While IKEv2 supports public key based authentication (PKI), the
corresponding use of in-band CRLs is problematic due to unbounded CRL
size. The size of an OCSP response is however well-bounded and
small. This document defines two extensions to IKEv2 which enable
the use of OCSP for in-band signaling of certificate revocation
status. Two new content encodings are defined for use in the CERTREQ
and CERT payloads: OCSP Responder Hash and OCSP Response. An OCSP
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Responder Hash CERTREQ payload triggers transmission of an OCSP
Response CERT payload.
When certificates are used with IKEv2, the communicating peers need a
mechanism to determine the revocation status of the peer's
certificate. OCSP is one such mechanism. This document applies when
OCSP is desired and security policy prevents one of the IKEv2 peers
from accessing the relevant OCSP responder directly. Firewalls are
often deployed in a manner that prevents such access by IKEv2 peers
outside of an enterprise network.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Extension Definition . . . . . . . . . . . . . . . . . . . . . 5
3.1. OCSP Responder Hash . . . . . . . . . . . . . . . . . . . 5
3.2. OCSP Response . . . . . . . . . . . . . . . . . . . . . . 5
4. Extension Requirements . . . . . . . . . . . . . . . . . . . . 6
4.1. OCSP Responder Hash . . . . . . . . . . . . . . . . . . . 6
4.2. OCSP Response . . . . . . . . . . . . . . . . . . . . . . 6
5. Examples and Discussion . . . . . . . . . . . . . . . . . . . 8
5.1. Baseline . . . . . . . . . . . . . . . . . . . . . . . . . 8
5.2. Extended Authentication Protocol (EAP) . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 10
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
9. Normative References . . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
Intellectual Property and Copyright Statements . . . . . . . . . . 14
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1. Introduction
Version 2 of the Internet Key Exchange protocol [IKEv2] supports a
range of authentication mechanisms, including the use of public key
based authentication. Confirmation of certificate reliability is
essential to achieve the security assurances public key cryptography
provides. One fundamental element of such confirmation is reference
to certificate revocation status (see [RFC3280] for additional
detail).
The historic means of determining certificate revocation status is
through the use of Certificate Revocation Lists (CRLs). IKEv2 allows
CRLs to be exchanged in-band via the CERT payload.
CRLs can however grow unbounded in size. Many real-world examples
exist to demonstrate the impracticality of including a multi-megabyte
file in an IKE exchange. This constraint is particularly acute in
bandwidth limited environments (e.g. mobile communications). The net
effect is exclusion of in-band CRLs in favor of out-of-band (OOB)
acquisition of these data, should they even be used at all.
Reliance on OOB methods can be further complicated if access to
revocation data requires use of IPsec (and therefore IKE) to
establish secure and authorized access to the CRLs of an IKE
participant. Such network access deadlock further contributes to a
reduced reliance on certificate revocation status in favor of blind
trust.
OCSP [RFC2560] offers a useful alternative. The size of an OCSP
response is bounded and small and therefore suitable for in-band
IKEv2 signaling of a certificate's revocation status.
This document defines two extensions to IKEv2 that enable the use of
OCSP for in-band signaling of certificate revocation status. Two new
content encodings are defined for use in the CERTREQ and CERT
payloads: OCSP Responder Hash and OCSP Response. An OCSP Responder
Hash CERTREQ payload triggers transmission of an OCSP Response CERT
payload.
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2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
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3. Extension Definition
With reference to Section 3.6 of [IKEv2], the values for the Cert
Encoding field of the CERT payload are extended as follows (see also
the IANA Considerations section of this document):
Certificate Encoding Value
-------------------- -----
OCSP Responder Hash 14
OCSP Response 15
3.1. OCSP Responder Hash
A value of OCSP Responder Hash (14) in the Cert Encoding field of a
CERTREQ Payload indicates the presence of an OCSP Responder
certificate hash in the Certificate Authority field of the CERTREQ
payload.
The presence of the OCSP Responder Hash in a CERTREQ message:
1. identifies an OCSP responder trusted by the sender;
2. notifies the recipient of sender's support for the OCSP extension
to IKEv2; and
3. notifies the recipient of sender's desire to receive OCSP
confirmation in a subsequent CERT payload.
3.2. OCSP Response
A value of OCSP Response (15) in the Cert Encoding field of a CERT
Payload indicates the presence of an OCSP Response in the Certificate
Data field of the CERT payload.
Correlation between an OCSP Response CERT payload and a corresponding
CERT payload carrying a certificate can be achieved by matching the
OCSP response CertID field to the certificate. See [RFC2560] for the
definition of OCSP response content.
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4. Extension Requirements
[IKEv2] allows for multiple CERT and CERTREQ payloads in an exchange.
4.1. OCSP Responder Hash
Section 3.7 of [IKEv2] allows for the concatenation of trust anchor
hashes as the Certification Authority value of a single CERTREQ
message. There is no means however to indicate which among those
hashes relates to the certificate of a trusted OCSP responder.
Therefore an OCSP Responder Hash CERTREQ SHALL be transmitted
separate from any other CERTREQ payloads in an IKEv2 exchange.
Where it is useful to identify more than one trusted OCSP responder,
each such identification SHALL be transmitted via separate OCSP
Responder Hash CERTREQ payloads.
The Certification Authority value in an OCSP Responder CERTREQ SHALL
be computed and produced in a manner identical to that of trust
anchor hashes as documented in Section 3.7 of [IKEv2] with the
exception that each such hash SHALL be expressed in a separate
CERTREQ payload.
Upon receipt of an OCSP Response CERT payload corresponding to a
prior OCSP Responder Hash CERTREQ, the CERTREQ sender SHALL
incorporate the OCSP response into path validation logic defined by
[RFC3280].
The sender of an OCSP Responder Hash CERTREQ MAY abort an IKEv2
exchange if either:
1. the corresponding OCSP Response CERT payload indicates that the
subject certificate is revoked; OR
2. the corresponding OCSP Response CERT payload indicates an OCSP
error (e.g. malformedRequest, internalError, tryLater,
sigRequired, unauthorized, etc.).
The sender of an OCSP Responder Hash CERTREQ SHOULD accept an IKEv2
exchange if a corresponding OCSP Response CERT payload is not
received. This might be an indication that this OCSP extension is
not supported.
4.2. OCSP Response
Upon receipt of an OCSP Responder Hash CERTREQ payload, the recipient
SHOULD acquire the related OCSP-based assertion and produce and
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transmit an OCSP Response CERT payload corresponding to the
certificate needed to verify its signature on IKEv2 payloads.
An OCSP Response CERT payload SHALL be transmitted separate from any
other CERT payload in an IKEv2 exchange.
Where multiple OCSP responses are useful to an environment, each such
SHALL be transmitted via separate OCSP Response CERT payloads.
The means by which an OCSP response may be acquired for production of
an OCSP Response CERT payload is out of scope of this document.
The structure and encoding of the Certificate Data field of an OCSP
Response CERT payload SHALL be identical to that defined in
[RFC2560].
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5. Examples and Discussion
This section shows the standard IKEv2 message examples with both
peers, the initiator and the responder, using public key based
authentication, CERTREQ and CERT payloads. The first instance
corresponds to Section 1.2 of [IKEv2], the illustrations of which are
reproduced below for reference.
5.1. Baseline
Application of the IKEv2 extensions defined in this document to the
baseline exchange defined in Section 1.2 of [IKEv2] is as follows.
Messages are numbered for ease of reference.
Initiator Responder
----------- -----------
(1) HDR, SAi1, KEi, Ni -->
(2) <-- HDR, SAr1, KEr, Nr,
CERTREQ(OCSP Responder Hash)
(3) HDR, SK {IDi, CERT(certificate),-->
CERT(OCSP Response),
CERTREQ(OCSP Responder Hash),
[IDr,] AUTH, SAi2, TSi, TSr}
(4) <-- HDR, SK {IDr,
CERT(certificate),
CERT(OCSP Response),
AUTH, SAr2, TSi, TSr}
In (2) Responder sends an OCSP Responder Hash CERTREQ payload
identifying an OCSP responder trusted by Responder. In response,
Initiator sends in (3) both a CERT payload carrying its certificate
and an OCSP Response CERT payload covering that certificate. In (3)
Initiator also requests an OCSP response via the OCSP Responder Hash
CERTREQ payload. In (4) Responder returns its certificate and a
separate OCSP Response CERT payload covering that certificate.
It is important to note that in this scenario, the Responder in (2)
does not yet possess the Initiator's certificate and therefore cannot
form an OCSP request. [RFC2560] allows for pre-produced responses.
It is thus easily inferred that OCSP responses can be produced in the
absence of a corresponding request (OCSP nonces notwithstanding). In
such instances OCSP Requests are simply index values into these data.
It is also important in extending IKEv2 towards OCSP in this scenario
that the Initiator has certain knowledge that the Responder is
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capable of and willing to participate in the extension. Yet the
Responder will only trust one or more OCSP responder signatures.
These factors motivate the definition of OCSP Responder Hash
extension.
5.2. Extended Authentication Protocol (EAP)
Another scenario of pressing interest is the use of EAP to
accommodate multiple end users seeking enterprise access to an IPsec
gateway. As with the preceding section, the following illustration
is extracted from [IKEv2]. In the event of a conflict between this
document and[IKEv2] regarding these illustrations, [IKEv2] SHALL
dominate.
Initiator Responder
----------- -----------
(1) HDR, SAi1, KEi, Ni -->
(2) <-- HDR, SAr1, KEr, Nr
(3) HDR, SK {IDi, -->
CERTREQ(OCSP Responder Hash),
[IDr,] AUTH, SAi2, TSi, TSr}
(4) <-- HDR, SK {IDr,
CERT(certificate),
CERT(OCSP Response),
AUTH, EAP}
(5) HDR, SK {EAP} -->
(6) <-- HDR, SK {EAP (success)}
(7) HDR, SK {AUTH} -->
(8) <-- HDR, SK {AUTH, SAr2, TSi,
TSr }
In the EAP scenario, messages (5) through (8) are not relevant to
this document. Note that while [IKEv2] allows for the optional
inclusion of a CERTREQ in (2), this document asserts no need of its
use. It is assumed that environments including this optional payload
and yet wishing to implement the OCSP extension to IKEv2 are
sufficiently robust as to accommodate this redundant payload.
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6. Security Considerations
For the reasons noted above, OCSP Responder Hash is used in place of
OCSP request syntax to trigger production and transmission of an OCSP
response. OCSP as defined in [RFC2560] may contain a nonce request
extension to improve security against replay attacks (see Section
4.4.1 of [RFC2560] for further details). The OCSP Responder Hash
does not contain such a nonce. [RFC2560] deals with this aspect by
allowing pre-produced responses.
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7. IANA Considerations
This document defines two new field types for use in the IKEv2 Cert
Encoding field of the Certificate Payload format. Official values
for "OCSP Responder Hash" and "OCSP Response" extensions to the Cert
Encoding table of Section 3.6 of [IKEv2] need to be acquired from
IANA.
Certificate Encoding Value
-------------------- -----
OCSP Responder Hash 14
OCSP Response 15
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8. Acknowledgements
The authors would like to thank Russ Housley for this support and
Pasi Eronen for his review.
9. Normative References
[IKEv2] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
draft-ietf-ipsec-ikev2-17 (work in progress),
October 2004.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2560] Myers, M., Ankney, R., Malpani, A., Galperin, S., and C.
Adams, "X.509 Internet Public Key Infrastructure Online
Certificate Status Protocol - OCSP", RFC 2560, June 1999.
[RFC3280] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
X.509 Public Key Infrastructure Certificate and
Certificate Revocation List (CRL) Profile", RFC 3280,
April 2002.
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Authors' Addresses
Michael Myers
TraceRoute Security LLC
Email: mmyers@fastq.com
Hannes Tschofenig
Siemens
Otto-Hahn-Ring 6
Munich, Bavaria 81739
Germany
Email: Hannes.Tschofenig@siemens.com
URI: http://www.tschofenig.com
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