Limited Additional Mechanisms for PKIX and SMIME D. Benjamin
Internet-Draft Google LLC
Intended status: Standards Track 20 March 2025
Expires: 21 September 2025
Unsigned X.509 Certificates
draft-ietf-lamps-x509-alg-none-01
Abstract
This document defines a placeholder X.509 signature algorithm that
may be used in contexts where the consumer of the certificate does
not intend to verify the signature.
About This Document
This note is to be removed before publishing as an RFC.
The latest revision of this draft can be found at
https://davidben.github.io/x509-alg-none/draft-ietf-lamps-x509-alg-
none.html. Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-ietf-lamps-x509-alg-none/.
Discussion of this document takes place on the Limited Additional
Mechanisms for PKIX and SMIME Working Group mailing list
(mailto:spasm@ietf.org), which is archived at
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https://www.ietf.org/mailman/listinfo/spasm/.
Source for this draft and an issue tracker can be found at
https://github.com/davidben/x509-alg-none.
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This Internet-Draft will expire on 21 September 2025.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
3. Constructing Unsigned Certificates . . . . . . . . . . . . . 3
4. Consuming Unsigned Certificates . . . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . 4
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.1. Normative References . . . . . . . . . . . . . . . . . . 5
7.2. Informative References . . . . . . . . . . . . . . . . . 5
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
An X.509 certificate [RFC5280] relates two entities in the PKI:
information about a subject and a proof from an issuer. Viewing the
PKI as a graph of with entities as nodes, as in [RFC4158], a
certificate is an edge between the subject and issuer.
In some contexts, an application needs standalone subject information
instead of a certificate. In the graph model, the application needs
a node, not an edge. For example, certification path validation
(Section 6 of [RFC5280]) begins at a trust anchor, or root
certification authority (root CA). The application trusts this trust
anchor information out-of-band and does not require an issuer's
signature.
X.509 does not define a structure for this scenario. Instead, X.509
trust anchors are often represented with "self-signed" certificates,
where the subject's key signs over itself. Other formats, such as
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[RFC5914] exist to convey trust anchors, but self-signed certificates
remain widely used. Additionally, some TLS [RFC8446] server
deployments use self-signed certificates when they do not intend to
present a CA-issued identity, instead expecting the relying party to
authenticate the certificate out-of-band, e.g. via a known
fingerprint.
These self-signatures typically have no security value, aren't
checked by the receiver, and only serve as placeholders to meet
syntactic requirements of an X.509 certificate.
Computing signatures as placeholders has some drawbacks:
* Post-quantum signature algorithms are large, so including a self-
signature significantly increases the size of the payload.
* If the subject is an end entity, rather than a CA, computing an
X.509 signature risks cross-protocol attacks with the intended use
of the key.
* It is ambiguous whether such a self-signature requires the CA bit
in basic constraints or keyCertSign in key usage. If the key is
intended for a non-X.509 use, asserting those capabilities is an
unnecessary risk.
* If end entity's key is not a signing key (e.g. a KEM key), there
is no valid signature algorithm to use with the key.
This document defines a profile for unsigned X.509 certificates,
which may be used when the certificate is used as a container for
subject information, without any specific issuer.
2. Conventions and Definitions
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.
3. Constructing Unsigned Certificates
This document defines the id-unsigned object identifier (OID) under
the OID arc defined in [RFC8411]:
id-unsigned OBJECT IDENTIFIER ::= {1 3 101 TBD}
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To construct an unsigned X.509 certificate, the sender MUST set the
Certificate's signatureAlgorithm and TBSCertificate's signature
fields each to an AlgorithmIdentifier with algorithm id-unsigned.
The parameters for id-unsigned MUST be present and MUST be encoded as
NULL. The Certificate's signatureValue field MUST be a BIT STRING of
length zero.
4. Consuming Unsigned Certificates
X.509 signatures of type id-unsigned are always invalid. This
contrasts with [JWT]. When processing X.509 certificates without
verifying signatures, receivers MAY accept id-unsigned. When
verifying X.509 signatures, receivers MUST reject id-unsigned. In
particular, X.509 validators MUST NOT accept id-unsigned in the place
of a signature in the certification path.
X.509 applications must already account for unknown signature
algorithms, so applications are RECOMMENDED to satisfy these
requirements by ignoring this document. An unmodified X.509
validator will not recognize id-unsigned and is thus already expected
to reject it in the certification path. Conversely, in contexts
where an X.509 application was ignoring the self-signature, id-
unsigned will also be ignored, but more efficiently.
5. Security Considerations
If an application uses a self-signature when constructing a subject-
only certificate for a non-X.509 key, the X.509 signature payload and
those of the key's intended use may collide. The self-signature
might then be used as part of a cross-protocol attack. Using id-
unsigned avoids a single key being used for both X.509 and the end-
entity protocol, eliminating this risk.
If an application accepts id-unsigned as part of a certification
path, or in any other context where it is necessary to verify the
X.509 signature, the signature check would be bypassed. Thus,
Section 4 prohibits this and recommends that applications not treat
id-unsigned differently from any other previously unrecognized
signature algorithm. Non-compliant applications that instead accept
id-unsigned as a valid signature risk of vulnerabilities analogous to
[JWT].
6. IANA Considerations
IANA is requested to add the following entry to the "SMI Security for
Cryptographic Algorithms" registry [RFC8411]:
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+=========+=============+============+
| Decimal | Description | References |
+=========+=============+============+
| TBD | id-unsigned | [this-RFC] |
+---------+-------------+------------+
Table 1
7. References
7.1. 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/rfc/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/rfc/rfc5280>.
[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/rfc/rfc8174>.
[RFC8411] Schaad, J. and R. Andrews, "IANA Registration for the
Cryptographic Algorithm Object Identifier Range",
RFC 8411, DOI 10.17487/RFC8411, August 2018,
<https://www.rfc-editor.org/rfc/rfc8411>.
7.2. Informative References
[JWT] Sanderson, J., "How Many Days Has It Been Since a JWT
alg:none Vulnerability?", 9 October 2024,
<https://www.howmanydayssinceajwtalgnonevuln.com/>.
[RFC4158] Cooper, M., Dzambasow, Y., Hesse, P., Joseph, S., and R.
Nicholas, "Internet X.509 Public Key Infrastructure:
Certification Path Building", RFC 4158,
DOI 10.17487/RFC4158, September 2005,
<https://www.rfc-editor.org/rfc/rfc4158>.
[RFC5914] Housley, R., Ashmore, S., and C. Wallace, "Trust Anchor
Format", RFC 5914, DOI 10.17487/RFC5914, June 2010,
<https://www.rfc-editor.org/rfc/rfc5914>.
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[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/rfc/rfc8446>.
Acknowledgements
Thanks to Bob Beck, Nick Harper, and Sophie Schmieg for reviewing an
early iteration of this document. Thanks to Alex Gaynor for
providing a link to cite for [JWT]. Thanks to Russ Housley for
additional input.
Author's Address
David Benjamin
Google LLC
Email: davidben@google.com
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