Online Certificate Status Protocol (OCSP) Nonce Extension
Defines secure length limits for the nonce extension to prevent replay attacks.
Sahni, M., Ed., "Online Certificate Status Protocol (OCSP) Nonce Extension", RFC 8954, DOI 10.17487/RFC8954, November 2020, <https://www.rfc-editor.org/info/rfc8954>.
@misc{rfc8954,
series = {Request for Comments},
number = 8954,
howpublished = {RFC 8954},
publisher = {RFC Editor},
doi = {10.17487/RFC8954},
url = {https://www.rfc-editor.org/info/rfc8954},
author = {M. Sahni},
title = {{Online Certificate Status Protocol (OCSP) Nonce Extension}},
pagetotal = 6,
year = 2020,
month = nov,
} RFC text
Contents
Internet Engineering Task Force (IETF) M. Sahni, Ed.
Request for Comments: 8954 Palo Alto Networks
Updates: 6960 November 2020
Category: Standards Track
ISSN: 2070-1721
Online Certificate Status Protocol (OCSP) Nonce Extension
Abstract
This document specifies the updated format of the Nonce extension in
the Online Certificate Status Protocol (OCSP) request and response
messages. OCSP is used to check the status of a certificate, and the
Nonce extension is used to cryptographically bind an OCSP response
message to a particular OCSP request message. This document updates
RFC 6960.
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 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8954.
Copyright Notice
Copyright (c) 2020 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
(https://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.
Table of Contents
1. Introduction
1.1. Terminology
2. OCSP Extensions
2.1. Nonce Extension
3. Security Considerations
3.1. Replay Attack
3.2. Nonce Collision
4. IANA Considerations
5. Changes to Appendix B of RFC 6960
5.1. Changes to Appendix B.1 OCSP in ASN.1 - 1998 Syntax
5.2. Changes to Appendix B.2 OCSP in ASN.1 - 2008 Syntax
6. References
6.1. Normative References
6.2. Informative References
Author's Address
1. Introduction
This document updates the usage and format of the Nonce extension in
OCSP request and response messages. This extension was previously
defined in Section 4.4.1 of [RFC6960]. [RFC6960] does not mention
any minimum or maximum length of the nonce in the Nonce extension.
Lacking limits on the length of the nonce in the Nonce extension,
OCSP responders that follow [RFC6960] may be vulnerable to various
attacks, like Denial-of-Service attacks [RFC4732] or chosen-prefix
attacks (to get a desired signature), and possible evasions using the
Nonce extension data. This document specifies a lower limit of 1 and
an upper limit of 32 for the length of the nonce in the Nonce
extension. This document updates [RFC6960].
1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. OCSP Extensions
The message formats for OCSP requests and responses are defined in
[RFC6960]. [RFC6960] also defines the standard extensions for OCSP
messages based on the extension model employed in X.509 version 3
certificates (see [RFC5280]). This document only specifies the new
format for the Nonce extension and does not change the specifications
of any of the other standard extensions defined in [RFC6960].
2.1. Nonce Extension
This section replaces the entirety of Section 4.4.1 of [RFC6960],
which describes the OCSP Nonce extension.
The nonce cryptographically binds a request and a response to prevent
replay attacks. The nonce is included as one of the
requestExtensions in requests; in responses, it would be included as
one of the responseExtensions. In both the request and the response,
the nonce will be identified by the object identifier id-pkix-ocsp-
nonce, while the extnValue is the value of the nonce. If the Nonce
extension is present, then the length of the nonce MUST be at least 1
octet and can be up to 32 octets.
A server MUST reject any OCSP request that has a nonce in the Nonce
extension with a length of either 0 octets or more than 32 octets
with the malformedRequest OCSPResponseStatus, as described in
Section 4.2.1 of [RFC6960].
The value of the nonce MUST be generated using a cryptographically
strong pseudorandom number generator (see [RFC4086]). The minimum
nonce length of 1 octet is defined to provide backward compatibility
with older clients that follow [RFC6960]. Newer OCSP clients that
support this document MUST use a length of 32 octets for the nonce in
the Nonce extension. OCSP responders MUST accept lengths of at least
16 octets and MAY choose to ignore the Nonce extension for requests
where the length of the nonce is less than 16 octets.
id-pkix-ocsp OBJECT IDENTIFIER ::= { id-ad-ocsp }
id-pkix-ocsp-nonce OBJECT IDENTIFIER ::= { id-pkix-ocsp 2 }
Nonce ::= OCTET STRING(SIZE(1..32))
3. Security Considerations
The security considerations of OCSP, in general, are described in
[RFC6960]. During the interval in which the previous OCSP response
for a certificate is not expired but the responder has a changed
status for that certificate, a copy of that OCSP response can be used
to indicate that the status of the certificate is still valid.
Including a client's nonce value in the OCSP response makes sure that
the response is the latest response from the server and not an old
copy.
3.1. Replay Attack
The Nonce extension is used to avoid replay attacks. Since the OCSP
responder may choose not to send the Nonce extension in the OCSP
response even if the client has sent the Nonce extension in the
request [RFC5019], an on-path attacker can intercept the OCSP request
and respond with an earlier response from the server without the
Nonce extension. This can be mitigated by configuring the server to
use a short time interval between the thisUpdate and nextUpdate
fields in the OCSP response.
3.2. Nonce Collision
If the value of the nonce used by a client in the OCSP request is
predictable, then an attacker may prefetch responses with the
predicted nonce and can replay them, thus defeating the purpose of
using the nonce. Therefore, the value of the Nonce extension in the
OCSP request MUST contain cryptographically strong randomness and
MUST be freshly generated at the time of the creation of the OCSP
request. Also, if the length of the nonce is too small (e.g., 1
octet), then an on-path attacker can prefetch responses with all the
possible values of the nonce and replay a matching nonce.
4. IANA Considerations
This document has no IANA actions.
5. Changes to Appendix B of RFC 6960
This section updates the ASN.1 definitions of the OCSP Nonce
extension in Appendices B.1 and B.2 of [RFC6960]. Appendix B.1
defines OCSP using ASN.1 - 1998 Syntax; Appendix B.2 defines OCSP
using ASN.1 - 2008 Syntax.
5.1. Changes to Appendix B.1 OCSP in ASN.1 - 1998 Syntax
OLD Syntax:
The definition of OCSP Nonce extension is not provided in
Appendix B.1 of [RFC6960] for the ASN.1 - 1998 Syntax.
NEW Syntax:
Nonce ::= OCTET STRING(SIZE(1..32))
5.2. Changes to Appendix B.2 OCSP in ASN.1 - 2008 Syntax
OLD Syntax:
re-ocsp-nonce EXTENSION ::= { SYNTAX OCTET STRING IDENTIFIED
BY id-pkix-ocsp-nonce }
NEW Syntax:
re-ocsp-nonce EXTENSION ::= { SYNTAX OCTET STRING(SIZE(1..32))
IDENTIFIED BY id-pkix-ocsp-nonce }
6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC5280] 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, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.
[RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A.,
Galperin, S., and C. Adams, "X.509 Internet Public Key
Infrastructure Online Certificate Status Protocol - OCSP",
RFC 6960, DOI 10.17487/RFC6960, June 2013,
<https://www.rfc-editor.org/info/rfc6960>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
6.2. Informative References
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<https://www.rfc-editor.org/info/rfc4086>.
[RFC4732] Handley, M., Ed., Rescorla, E., Ed., and IAB, "Internet
Denial-of-Service Considerations", RFC 4732,
DOI 10.17487/RFC4732, December 2006,
<https://www.rfc-editor.org/info/rfc4732>.
[RFC5019] Deacon, A. and R. Hurst, "The Lightweight Online
Certificate Status Protocol (OCSP) Profile for High-Volume
Environments", RFC 5019, DOI 10.17487/RFC5019, September
2007, <https://www.rfc-editor.org/info/rfc5019>.
Author's Address
Mohit Sahni (editor)
Palo Alto Networks
3000 Tannery Way
Santa Clara, CA 95054
United States of America
Email: msahni@paloaltonetworks.com
AI ELI5
On this page
§1OCSP and Nonce Binding
The Online Certificate Status Protocol (OCSP) is used to check whether a digital certificate is still valid or has been revoked. Clients make OCSP requests to a responder server, which returns a signed response stating the certificate status.
The problem is that an attacker could capture an old OCSP response and replay it, tricking clients into thinking a revoked certificate is still valid. To prevent this, clients include a nonce (a random number) in the request. The responder includes the same nonce in the response, which proves the response is fresh and not a replay of an old answer. However, the original RFC 6960 did not specify how long the nonce should be, leaving implementations vulnerable to attacks.
OCSP is like asking a traffic cop whether your driver's license is suspended. You write down a random number on your question, and if the cop repeats that number in the answer, you know it is a fresh answer and not an old one someone copied.
§2Nonce Extension Format and Length Limits
The Nonce extension appears as requestExtensions in OCSP requests and as responseExtensions in OCSP responses, identified by the object identifier id-pkix-ocsp-nonce. The nonce itself is an OCTET STRING.
This update specifies strict length limits: if the Nonce extension is present, the nonce MUST be at least 1 octet and at most 32 octets. Servers MUST reject any OCSP request with a nonce of 0 octets or more than 32 octets, returning a malformedRequest status.
The nonce value MUST be generated using a cryptographically strong pseudorandom number generator. Newer OCSP clients that support this update use a nonce length of 32 octets. OCSP responders MUST accept nonces of at least 16 octets and MAY ignore nonces shorter than 16 octets.
The random number must be at least 1 byte and at most 32 bytes long. Modern programs use all 32 bytes to make the random number hard to guess.
§3Replay Attacks and Nonce Randomness
The primary security concern is the replay attack. During the interval when a previous OCSP response is valid but the responder has changed its status for the certificate, an attacker could copy that old response and use it to falsely claim the certificate is still valid. By including the client's nonce in the response, the server proves it is a fresh answer for that specific request, not a replayed old response.
However, attackers could bypass the nonce protection in two ways. First, if the nonce value is predictable, an attacker could prefetch responses with predicted nonces and then replay the matching response. Second, if the nonce is too short, an attacker could exhaust all possible values (for example, with a 1-octet nonce there are only 256 possibilities) and prefetch responses for all of them.
To prevent this, the nonce MUST be generated using strong randomness and MUST be freshly generated for each OCSP request. The 32-octet minimum for newer clients makes exhaustive prefetching computationally infeasible.
Attackers could try to guess or trick the random number. Using 32 bytes of real randomness makes it almost impossible to guess in advance.
§4Responder Configuration and Backward Compatibility
The minimum nonce length of 1 octet is defined for backward compatibility with older clients that follow RFC 6960, which had no length specification. However, these short nonces offer weak security.
The update does not strictly require responders to send the Nonce extension in responses even when clients request it; as noted in RFC 5019, responders MAY omit the extension. To mitigate replay attacks when the responder does not echo the nonce, administrators can configure servers to use short validity intervals between the thisUpdate and nextUpdate fields in OCSP responses, so old responses expire quickly.
Old programs can still use short random numbers, but new ones should use longer ones. Even if the nonce is not echoed back, responses can expire quickly to prevent old ones from being reused.