Network Working Group                                        D. Benjamin
Internet-Draft                                                Google LLC
Intended status: Standards Track                        17 December 2024
Expires: 20 June 2025


                      Unsigned X.509 Certificates
                    draft-davidben-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-davidben-x509-alg-
   none.html.  Status information for this document may be found at
   https://datatracker.ietf.org/doc/draft-davidben-x509-alg-none/.

   Source for this draft and an issue tracker can be found at
   https://github.com/davidben/x509-alg-none.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on 20 June 2025.

Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.



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   This document is subject to BCP 78 and the IETF Trust's Legal
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   Please review these documents carefully, as they describe your rights
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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  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   4
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   An X.509 certificate [RFC5280] relates two entities in the PKI:
   information about a subject and a proof from an issuer.  Some
   applications, however, only require subject information.  For
   example, an X.509 trust anchor is described by information about the
   subject (a root certification authority, or root CA).  The relying
   party trusts this information out-of-band and does not require an
   issuer's signature.

   X.509 does not define such a structure.  Instead, X.509 trust anchors
   often use "self-signed" certificates, where the CA's key is used to
   sign the certificate.  Other formats, such as [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:




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   *  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 how to use the id-alg-noSignature OID from
   Appendix C.1 of [RFC5272] with X.509 certificates.

     id-pkix OBJECT IDENTIFIER  ::= { iso(1) identified-organization(3)
         dod(6) internet(1) security(5) mechanisms(5) pkix(7) }

     id-alg-noSignature OBJECT IDENTIFIER ::= {id-pkix id-alg(6) 2}

   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-alg-
   noSignature.  The parameters for id-alg-noSignature MUST be present
   and MUST be encoded as NULL.  The Certificate's signatureValue field
   MUST be a BIT STRING of length zero.








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4.  Consuming Unsigned Certificates

   X.509 signatures of type id-alg-noSignature are always invalid.  This
   contrasts with [JWT].  When processing X.509 certificates without
   verifying signatures, receivers MAY accept id-alg-noSignature.  When
   verifying X.509 signatures, receivers MUST reject id-alg-noSignature.
   In particular, X.509 validators MUST NOT accept id-alg-noSignature 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-alg-noSignature 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-alg-noSignature 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-alg-
   noSignature avoids a single key being used for both X.509 and the
   end-entity protocol, eliminating this risk.

   If an application accepts id-alg-noSignature 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-alg-noSignature differently from any other previously
   unrecognized signature algorithm.  Non-compliant applications that
   instead accept id-alg-noSignature as a valid signature risk of
   vulnerabilities analogous to [JWT].

6.  IANA Considerations

   This document has no IANA actions.

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>.




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   [RFC5272]  Schaad, J. and M. Myers, "Certificate Management over CMS
              (CMC)", RFC 5272, DOI 10.17487/RFC5272, June 2008,
              <https://www.rfc-editor.org/rfc/rfc5272>.

   [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>.

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/>.

   [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>.

   [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
   pointing out that id-alg-noSignature was already defined in
   [RFC5272].

Author's Address

   David Benjamin
   Google LLC
   Email: davidben@google.com










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