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IETF RFC 6277
Online Certificate Status Protocol Algorithm Agility
Last modified on Friday, June 10th, 2011
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Internet Engineering Task Force (IETF) S. Santesson
Request for Comments: 6277 3xA Security
Updates: 2560 P. Hallam-Baker
Category: Standards Track Default Deny Security
ISSN: 2070-1721 June 2011
Online Certificate Status Protocol Algorithm Agility
 Abstract
The Online Certificate Status Protocol (OCSP) requires server
responses to be signed but does not specify a mechanism for selecting
the signature algorithm to be used. This may lead to avoidable
interoperability failures in contexts where multiple signature
algorithms are in use. This document specifies rules for server
signature algorithm selection and an extension that allows a client
to advise a server that specific signature algorithms are supported.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 6277.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Santesson & Hallam-Baker Standards Track PAGE 1 
RFC 6277 OCSP Algorithm Agility June 2011
Table of Contents
1. Introduction ....................................................2
1.1. Requirements Language ......................................3
2. OCSP Algorithm Agility Requirements .............................3
3. Updates to Mandatory and Optional Cryptographic Algorithms ......4
4. Client Indication of Preferred Signature Algorithms .............4
5. Responder Signature Algorithm Selection .........................5
5.1. Dynamic Response ...........................................5
5.2. Static Response ............................................6
6. Acknowledgements ................................................6
7. Security Considerations .........................................6
7.1. Use of Insecure Algorithms .................................6
7.2. Man-in-the-Middle Downgrade Attack .........................7
7.3. Denial-of-Service Attack ...................................7
8. References ......................................................7
8.1. Normative References .......................................7
8.2. Informative References .....................................8
Appendix A. ASN.1 Modules .........................................9
A.1. ASN.1 Module ...............................................9
A.2. 1988 ASN.1 Module .........................................10
1. Introduction
The Online Certificate Status Protocol (OCSP) [RFC 2560] defines a
protocol for obtaining certificate status information from an online
service. An OCSP responder may or may not be issued an OCSP
responder certificate by the certification authority (CA) that issued
the certificate whose status is being queried. An OCSP responder may
provide pre-signed OCSP responses or may sign responses when queried.
RFC 2560 [RFC 2560] specifies a means for an OCSP responder to
indicate the signature and digest algorithms used in a response but
not how those algorithms are specified. The only algorithm
requirements established by that protocol specification are that the
OCSP client SHALL support the Digital Signature Algorithm (DSA) sig-
alg-oid specified in Section 7.2.2 of [RFC 2459] and SHOULD be capable
of processing RSA signatures as specified in Section 7.2.1 of
[RFC 2459]. The only requirement placed on responders by RFC 2560 is
that they SHALL support the SHA1 hashing algorithm.
This document specifies rules for server signature algorithm
selection and an extension that allows a client to advise a server
that specific signature algorithms are supported.
Santesson & Hallam-Baker Standards Track PAGE 2
RFC 6277 OCSP Algorithm Agility June 2011
1.1. Requirements Language
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 [RFC 2119].
2. OCSP Algorithm Agility Requirements
Since algorithms other than those that are mandatory to implement are
allowed and since a client currently has no mechanism to indicate its
algorithm preferences, there is always a risk that a server choosing
a non-mandatory algorithm will generate a response that the client
may not support.
While an OCSP responder may apply rules for algorithm selection,
e.g., using the signature algorithm employed by the CA for signing
certificate revocation lists (CRLs) and certificates, such rules may
fail in common situations:
o The algorithm used to sign the CRLs and certificates may not be
consistent with the key pair being used by the OCSP responder to
sign responses.
o A request for an unknown certificate provides no basis for a
responder to select from among multiple algorithm options.
Without modifying the protocol, the last criterion cannot be resolved
through the information available from in-band signaling using the
protocol described in RFC 2560 [RFC 2560].
In addition, an OCSP responder may wish to employ different signature
algorithms than the one used by the CA to sign certificates and CRLs
for several reasons:
o The responder may employ an algorithm for certificate status
response that is less computationally demanding than for signing
the certificate itself.
o An implementation may wish to guard against the possibility of a
compromise resulting from a signature algorithm compromise by
employing two separate signature algorithms.
This document describes:
o A mechanism that allows a client to indicate the set of preferred
signature algorithms.
Santesson & Hallam-Baker Standards Track PAGE 3
RFC 6277 OCSP Algorithm Agility June 2011
o Rules for signature algorithm selection that maximize the
probability of successful operation in the case that no supported
preferred algorithm(s) are specified.
3. Updates to Mandatory and Optional Cryptographic Algorithms
Section 4.3 ("Mandatory and Optional Cryptographic Algorithms") of
RFC 2560 [RFC 2560] is updated as follows:
OLD: Clients that request OCSP services SHALL be capable of
processing responses signed used DSA keys identified by the DSA
sig-alg-oid specified in section 7.2.2 of [RFC 2459]. Clients
SHOULD also be capable of processing RSA signatures as specified
in section 7.2.1 of [RFC 2459]. OCSP responders SHALL support
the SHA1 hashing algorithm.
NEW: Clients that request OCSP services SHALL be capable of
processing responses signed using RSA with SHA-1 (identified by
sha1WithRSAEncryption OID specified in [RFC 3279]) and RSA with
SHA-256 (identified by sha256WithRSAEncryption OID specified in
[RFC 4055]). Clients SHOULD also be capable of processing
responses signed using DSA keys (identified by the id-dsa-with-
sha1 OID specified in [RFC 3279]). Clients MAY support other
algorithms.
4. Client Indication of Preferred Signature Algorithms
A client MAY declare a preferred set of algorithms in a request by
including a preferred signature algorithms extension in
requestExtensions of the OCSPRequest [RFC 2560].
id-pkix-ocsp-pref-sig-algs OBJECT IDENTIFIER ::= { id-pkix-ocsp 8 }
PreferredSignatureAlgorithms ::= SEQUENCE OF
PreferredSignatureAlgorithm
PreferredSignatureAlgorithm ::= SEQUENCE {
sigIdentifier AlgorithmIdentifier,
pubKeyAlgIdentifier SMIMECapability OPTIONAL
}
The syntax of AlgorithmIdentifier is defined in Section 4.1.1.2 of
RFC 5280 [RFC 5280]. The syntax of SMIMECapability is defined in RFC
5751 [RFC 5751].
sigIdentifier specifies the signature algorithm the client prefers,
e.g., algorithm=ecdsa-with-sha256. Parameters are absent for most
common signature algorithms.
Santesson & Hallam-Baker Standards Track PAGE 4
RFC 6277 OCSP Algorithm Agility June 2011
pubKeyAlgIdentifier specifies the subject public key algorithm
identifier the client prefers in the server's certificate used to
validate the OCSP response, e.g., algorithm=id-ecPublicKey and
parameters= secp256r1.
pubKeyAlgIdentifier is OPTIONAL and provides means to specify
parameters necessary to distinguish among different usages of a
particular algorithm, e.g., it may be used by the client to specify
what curve it supports for a given elliptic curve algorithm.
The client MUST support each of the specified preferred signature
algorithms, and the client MUST specify the algorithms in the order
of preference, from the most preferred to the least preferred.
Section 5 of this document describes how a server selects an
algorithm for signing OCSP responses to the requesting client.
5. Responder Signature Algorithm Selection
RFC 2560 [RFC 2560] does not specify a mechanism for deciding the
signature algorithm to be used in an OCSP response. As previously
noted, this does not provide a sufficient degree of certainty as to
the algorithm selected to facilitate interoperability.
5.1. Dynamic Response
As long as the selected algorithm meets all security requirements of
the OCSP responder, a responder MAY maximize the potential for
ensuring interoperability by selecting a supported signature
algorithm using the following order of precedence, where the first
method has the highest precedence:
1. Select an algorithm specified as a preferred signing algorithm in
the client request.
2. Select the signing algorithm used to sign a certificate
revocation list (CRL) issued by the certificate issuer to provide
status information for the certificate specified by CertID.
3. Select the signing algorithm used to sign the OCSPRequest.
4. Select a signature algorithm that has been advertised as being
the default signature algorithm for the signing service using an
out-of-band mechanism.
5. Select a mandatory or recommended signing algorithm specified for
the version of the OCSP protocol in use.
Santesson & Hallam-Baker Standards Track PAGE 5
RFC 6277 OCSP Algorithm Agility June 2011
A responder SHOULD always apply the lowest-numbered selection
mechanism that results in the selection of a known and supported
algorithm that meets the responder's criteria for cryptographic
algorithm strength.
5.2. Static Response
For purposes of efficiency, an OCSP responder is permitted to
generate static responses in advance of a request. The case may not
permit the responder to make use of the client request data during
the response generation; however, the responder SHOULD still use the
client request data during the selection of the pre-generated
response to be returned. Responders MAY use the historical client
requests as part of the input to the decisions of what different
algorithms should be used to sign the pre-generated responses.
6. Acknowledgements
The authors acknowledge Santosh Chokhani for the helpful comments
made on earlier drafts, Sean Turner for proposing the syntax for
algorithm identifiers, Jim Schaad for providing and testing the ASN.1
module in Appendix A, and Stephen Kent for valuable review and input.
7. Security Considerations
The mechanism used to choose the response signing algorithm MUST be
considered to be sufficiently secure against cryptanalytic attack for
the intended application.
In most applications, it is sufficient for the signing algorithm to
be at least as secure as the signing algorithm used to sign the
original certificate whose status is being queried. However, this
criteria may not hold in long-term archival applications in which the
status of a certificate is being queried for a date in the distant
past, long after the signing algorithm has ceased to be considered
trustworthy.
7.1. Use of Insecure Algorithms
It is not always possible for a responder to generate a response that
the client is expected to understand and that meets contemporary
standards for cryptographic security. In such cases, an OCSP
responder operator MUST balance the risk of employing a compromised
security solution and the cost of mandating an upgrade, including the
risk that the alternative chosen by end users will offer even less
security or no security.
Santesson & Hallam-Baker Standards Track PAGE 6
RFC 6277 OCSP Algorithm Agility June 2011
In archival applications, it is quite possible that an OCSP responder
might be asked to report the validity of a certificate on a date in
the distant past. Such a certificate might employ a signing method
that is no longer considered acceptably secure. In such
circumstances, the responder MUST NOT generate a signature using a
signing mechanism that is not considered acceptably secure.
A client MUST accept any signing algorithm in a response that it
specified as a preferred signing algorithm in the request.
Therefore, it follows that a client MUST NOT specify a preferred
signing algorithm that is either not supported or not considered
acceptably secure.
7.2. Man-in-the-Middle Downgrade Attack
The mechanism to support client indication of preferred signature
algorithms is not protected against a man-in-the-middle downgrade
attack. This constraint is not considered to be a significant
security concern since the OCSP responder MUST NOT sign OCSP
responses using weak algorithms even if requested by the client. In
addition, the client can reject OCSP responses that do not meet its
own criteria for acceptable cryptographic security no matter what
mechanism is used to determine the signing algorithm of the response.
7.3. Denial-of-Service Attack
Algorithm agility mechanisms defined in this document introduce a
slightly increased attack surface for denial-of-service attacks where
the client request is altered to require algorithms that are not
supported by the server. Denial-of-service considerations from RFC
4732 [RFC 4732] are relevant for this document.
8. References
8.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 2560] 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.
[RFC 3279] Bassham, L., Polk, W., and R. Housley, "Algorithms and
Identifiers for the Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 3279, April 2002.
Santesson & Hallam-Baker Standards Track PAGE 7
RFC 6277 OCSP Algorithm Agility June 2011
[RFC 4055] Schaad, J., Kaliski, B., and R. Housley, "Additional
Algorithms and Identifiers for RSA Cryptography for use in
the Internet X.509 Public Key Infrastructure Certificate
and Certificate Revocation List (CRL) Profile", RFC 4055,
June 2005.
[RFC 5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008.
[RFC 5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Message
Specification", RFC 5751, January 2010.
[RFC 5912] Hoffman, P. and J. Schaad, "New ASN.1 Modules for the
Public Key Infrastructure Using X.509 (PKIX)", RFC 5912,
June 2010.
8.2. Informative References
[RFC 2459] Housley, R., Ford, W., Polk, W., and D. Solo, "Internet
X.509 Public Key Infrastructure Certificate and CRL
Profile", RFC 2459, January 1999.
[RFC 4732] Handley, M., Ed., Rescorla, E., Ed., and IAB, "Internet
Denial-of-Service Considerations", RFC 4732, December
2006.
Santesson & Hallam-Baker Standards Track PAGE 8
RFC 6277 OCSP Algorithm Agility June 2011
Appendix A. ASN.1 Modules
A.1. ASN.1 Module
OCSP-AGILITY-2009 { iso(1) identified-organization(3) dod(6)
internet(1) security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-ocsp-agility-2009-93(66) }
DEFINITIONS EXPLICIT TAGS ::=
BEGIN
EXPORTS ALL; -- export all items from this module
IMPORTS
id-pkix-ocsp
FROM OCSP-2009 -- From OCSP [RFC 2560]
{ iso(1) identified-organization(3) dod(6) internet(1) security(5)
mechanisms(5) pkix(7) id-mod(0) id-mod-ocsp-02(48) }
AlgorithmIdentifier{}, SMIMECapability{}, SIGNATURE-ALGORITHM,
PUBLIC-KEY
FROM AlgorithmInformation-2009 -- From [RFC 5912]
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-algorithmInformation-02(58) }
EXTENSION
FROM PKIX-CommonTypes-2009 -- From [RFC 5912]
{ iso(1) identified-organization(3) dod(6) internet(1) security(5)
mechanisms(5) pkix(7) id-mod(0) id-mod-pkixCommon-02(57)} ;
-- Add re-preferred-signature-algorithms to the set of extensions
-- for TBSRequest.requestExtensions
re-preferred-signature-algorithms EXTENSION ::= {
SYNTAX PreferredSignatureAlgorithms
IDENTIFIED BY id-pkix-ocsp-pref-sig-algs }
id-pkix-ocsp-pref-sig-algs OBJECT IDENTIFIER ::= { id-pkix-ocsp 8 }
PreferredSignatureAlgorithms ::= SEQUENCE OF
PreferredSignatureAlgorithm
PreferredSignatureAlgorithm ::= SEQUENCE {
sigIdentifier AlgorithmIdentifier{SIGNATURE-ALGORITHM, {...}},
pubKeyAlgIdentifier SMIMECapability{PUBLIC-KEY, {...}} OPTIONAL }
END
Santesson & Hallam-Baker Standards Track PAGE 9
RFC 6277 OCSP Algorithm Agility June 2011
A.2. 1988 ASN.1 Module
OCSP-AGILITY-88 { iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-ocsp-agility-2009-88(67) }
DEFINITIONS EXPLICIT TAGS ::=
BEGIN
-- EXPORTS ALL;
IMPORTS
id-pkix-ocsp -- From [RFC 2560]
FROM OCSP
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0) id-mod-ocsp(14)}
AlgorithmIdentifier
FROM PKIX1Explicit88 -- From [RFC 5280]
{ iso(1) identified-organization(3) dod(6) internet(1) security(5)
mechanisms(5) pkix(7) id-mod(0) id-pkix1-explicit(18) };
SMIMECapability
FROM SecureMimeMessageV3dot1 -- From [RFC 5751]
{ iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
smime(16) modules(0) msg-v3dot1(21) }
id-pkix-ocsp-pref-sig-algs OBJECT IDENTIFIER ::= { id-pkix-ocsp 8 }
PreferredSignatureAlgorithms ::= SEQUENCE OF
PreferredSignatureAlgorithm
PreferredSignatureAlgorithm ::= SEQUENCE {
sigIdentifier AlgorithmIdentifier,
pubKeyAlgIdentifier SMIMECapability OPTIONAL
}
END
Santesson & Hallam-Baker Standards Track PAGE 10
RFC 6277 OCSP Algorithm Agility June 2011
Authors' Addresses
Stefan Santesson
3xA Security AB
Sweden
Email: sts@aaa-sec.com
Phillip Hallam-Baker
Default Deny Security
EMail: hallam@gmail.com
Santesson & Hallam-Baker Standards Track PAGE 11
Online Certificate Status Protocol Algorithm Agility
RFC TOTAL SIZE: 21682 bytes
PUBLICATION DATE: Friday, June 10th, 2011
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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