Unsigned X.509 certs: Unsigned X.509 Certificates

Internet Engineering Task Force (IETF)                       D. Benjamin
Request for Comments: 9925                                    Google LLC
Updates: 5280                                              February 2026
Category: Standards Track
ISSN: 2070-1721


                      Unsigned X.509 Certificates

Abstract

   This document defines a placeholder X.509 signature algorithm that
   may be used in contexts where the consumer of the certificate is not
   expected to verify the signature.  As part of this, it updates RFC
   5280.

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/rfc9925.

Copyright Notice

   Copyright (c) 2026 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
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   in the Revised BSD License.

Table of Contents

   1.  Introduction
   2.  Requirements Language
   3.  Constructing Unsigned Certificates
     3.1.  Signature
     3.2.  Issuer
     3.3.  Extensions
   4.  Consuming Unsigned Certificates
   5.  Security Considerations
   6.  IANA Considerations
     6.1.  Module Identifier
     6.2.  Algorithm
     6.3.  Relative Distinguished Name Attribute
   7.  References
     7.1.  Normative References
     7.2.  Informative References
   Appendix A.  ASN.1 Module
   Acknowledgements
   Author's Address

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 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, sometimes referred
   to as a 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
   [RFC5914], exist to convey trust anchors, but self-signed
   certificates remain widely used.

   Additionally, some TLS [RFC8446] server deployments use self-signed
   end entity 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 the subject is an end entity, and the end entity's key is not a
      signing key (e.g., a Key Encapsulation Mechanism (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.  Requirements Language

   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 section describes how a sender constructs an unsigned
   certificate.

3.1.  Signature

   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-unsigned,
   defined below:

     id-alg-unsigned OBJECT IDENTIFIER ::= {1 3 6 1 5 5 7 6 36}

   The parameters for id-alg-unsigned MUST be omitted.  The
   Certificate's signatureValue field MUST be a BIT STRING of length
   zero.

3.2.  Issuer

   An unsigned certificate takes the place of a self-signed certificate
   in scenarios where the application only requires subject information.
   It has no issuer, so some requirements in the profile defined in
   [RFC5280] cannot meaningfully be applied.  However, the application
   may have pre-existing requirements derived from [X.509] and
   [RFC5280], so senders MAY construct the certificate as if it were a
   self-signed certificate, if needed for interoperability.

   In particular, the following fields describe a certificate's issuer:

   *  issuer (Section 4.1.2.4 of [RFC5280])

   *  issuerUniqueID (Section 4.1.2.8 of [RFC5280])

   The issuer field is not optional, and both [X.509] and
   Section 4.1.2.4 of [RFC5280] forbid empty issuers, so such a value
   may not be interoperable with existing applications.

   If the subject is not empty, senders MAY set the issuer to the
   subject, similar to how they would construct a self-signed
   certificate.  This may be useful in applications that, for example,
   expect trust anchors to have a matching issuer and subject.  This is,
   however, a placeholder value.  The unsigned certificate is not
   considered self-signed or self-issued.

   Senders MAY alternatively use a short placeholder issuer consisting
   of a single relative distinguished name that has a single attribute
   with a type of id-rdna-unsigned and value of a zero-length
   UTF8String. id-rdna-unsigned is defined as follows:

     id-rdna-unsigned OBJECT IDENTIFIER ::= {1 3 6 1 5 5 7 25 1}

   This placeholder name, in the string representation of [RFC4514], is:

     1.3.6.1.5.5.7.25.1=#0C00

   Senders MUST omit the issuerUniqueID field, as it is optional, not
   applicable, and already forbidden by Section 4.1.2.8 of [RFC5280].

3.3.  Extensions

   Some X.509 extensions also describe the certificate issuer and thus
   are not meaningful for an unsigned certificate:

   *  authority key identifier (Section 4.2.1.1 of [RFC5280])

   *  issuer alternative name (Section 4.2.1.7 of [RFC5280])

   Senders SHOULD omit the authority key identifier and issuer
   alternative name extensions.  Section 4.2.1.1 of [RFC5280] requires
   certificates to include the authority key identifier, but it permits
   an exception for self-signed certificates used when distributing a
   public key.  This document updates [RFC5280] to also permit omitting
   the authority key identifier in unsigned certificates.

   Some extensions reflect whether the subject is a CA or an end entity:

   *  key usage (Section 4.2.1.3 of [RFC5280])

   *  basic constraints (Section 4.2.1.9 of [RFC5280])

   Senders SHOULD fill in these values to reflect the subject.  That is:

   *  If the subject is a CA, it SHOULD assert the keyCertSign key usage
      bit and SHOULD include a basic constraints extension that sets the
      cA boolean to TRUE.

   *  If the subject is an end entity, it SHOULD NOT assert the
      keyCertSign key usage bit, and it SHOULD either omit the basic
      constraints extension or set the cA boolean to FALSE.  Unlike a
      self-signed certificate, an unsigned certificate does not issue
      itself, so there is no need to accommodate a self-signature in
      either extension.

4.  Consuming Unsigned Certificates

   X.509 signatures of type id-alg-unsigned are always invalid:

   *  When processing X.509 certificates without verifying signatures,
      receivers MAY accept id-alg-unsigned.

   *  When verifying X.509 signatures, receivers MUST reject id-alg-
      unsigned.

   In particular, X.509 validators MUST NOT accept id-alg-unsigned in
   the place of a signature in the certification path.

   It is expected that most unmodified X.509 applications will already
   be compliant with this guidance.  X.509 applications are thus
   RECOMMENDED to satisfy these requirements by ignoring this document
   and instead treating id-alg-unsigned as the same as an unrecognized
   signature algorithm.  An unmodified X.509 validator will be unable to
   verify the signature (Step (a.1) of Section 6.1.3 of [RFC5280]) and
   thus reject the certification path.  Conversely, in contexts where an
   X.509 application was ignoring the self-signature, id-alg-unsigned
   will also be ignored but more efficiently.

   In other contexts, an application may require modifications or limit
   itself to particular forms of unsigned certificates.  For example, an
   application might check self-signedness to classify locally
   configured certificates as trust anchors or untrusted intermediates.
   Such an application may need to modify its configuration model or
   user interface before using an unsigned certificate as a trust
   anchor.

5.  Security Considerations

   It is best practice to limit cryptographic keys to a single purpose
   each.  If a key is reused across contexts, applications risk cross-
   protocol attacks when the two uses collide.  However, in applications
   that use self-signed end entity certificates, the subject's key is
   necessarily used in two ways: the X.509 self-signature and the end
   entity protocol.  Unsigned certificates fix this key reuse by
   removing the X.509 self-signature.

   If an application accepts id-alg-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 treat id-
   alg-unsigned the same as any other previously unrecognized signature
   algorithm.  Non-compliant applications risk vulnerabilities analogous
   to those described in [JWT] and Section 1.1 of [JOSE].

   The signature in a self-signed certificate is self-derived and thus
   of limited use to convey trust.  However, some applications might,
   for example, use it as an integrity check to guard against accidental
   storage corruption.  An unsigned certificate does not provide any
   integrity check.  Applications checking self-signature for integrity
   SHOULD instead use some other mechanism, such as an external hash
   that is verified out-of-band.

6.  IANA Considerations

6.1.  Module Identifier

   IANA has added the following entry in the "SMI Security for PKIX
   Module Identifier" registry, defined by [RFC7299]:

             +=========+=========================+===========+
             | Decimal | Description             | Reference |
             +=========+=========================+===========+
             | 122     | id-mod-algUnsigned-2025 | RFC 9925  |
             +---------+-------------------------+-----------+

                                  Table 1

6.2.  Algorithm

   IANA has added the following entry to the "SMI Security for PKIX
   Algorithms" registry [RFC7299]:

                 +=========+=================+===========+
                 | Decimal | Description     | Reference |
                 +=========+=================+===========+
                 | 36      | id-alg-unsigned | RFC 9925  |
                 +---------+-----------------+-----------+

                                  Table 2

6.3.  Relative Distinguished Name Attribute

   To allocate id-rdna-unsigned, this document introduces a new PKIX OID
   arc for relative distinguished name attributes:

   IANA has added the following entry to the "SMI Security for PKIX"
   registry [RFC7299]:

      +=========+=======================================+===========+
      | Decimal | Description                           | Reference |
      +=========+=======================================+===========+
      | 25      | Relative Distinguished Name Attribute | RFC 9925  |
      +---------+---------------------------------------+-----------+

                                  Table 3

   IANA has created the "SMI Security for PKIX Relative Distinguished
   Name Attribute" registry within the "Structure of Management
   Information (SMI) Numbers (MIB Module Registrations)" registry group.

   The new registry's description is
   "iso.org.dod.internet.security.mechanisms.pkix.rdna
   (1.3.6.1.5.5.7.25)".

   The new registry has three columns and is initialized with the
   following values:

                +=========+==================+===========+
                | Decimal | Description      | Reference |
                +=========+==================+===========+
                | 1       | id-rdna-unsigned | RFC 9925  |
                +---------+------------------+-----------+

                                 Table 4

   Future updates to this table are to be made according to the
   Specification Required policy as defined in [RFC8126].

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/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>.

   [RFC5912]  Hoffman, P. and J. Schaad, "New ASN.1 Modules for the
              Public Key Infrastructure Using X.509 (PKIX)", RFC 5912,
              DOI 10.17487/RFC5912, June 2010,
              <https://www.rfc-editor.org/info/rfc5912>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [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>.

7.2.  Informative References

   [JOSE]     Madden, N., "JOSE: Deprecate 'none' and 'RSA1_5'", Work in
              Progress, Internet-Draft, draft-ietf-jose-deprecate-none-
              rsa15-03, 19 September 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-jose-
              deprecate-none-rsa15-03>.

   [JWT]      Sanderson, J., "How Many Days Has It Been Since a JWT
              alg:none Vulnerability?",
              <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/info/rfc4158>.

   [RFC4514]  Zeilenga, K., Ed., "Lightweight Directory Access Protocol
              (LDAP): String Representation of Distinguished Names",
              RFC 4514, DOI 10.17487/RFC4514, June 2006,
              <https://www.rfc-editor.org/info/rfc4514>.

   [RFC5914]  Housley, R., Ashmore, S., and C. Wallace, "Trust Anchor
              Format", RFC 5914, DOI 10.17487/RFC5914, June 2010,
              <https://www.rfc-editor.org/info/rfc5914>.

   [RFC7299]  Housley, R., "Object Identifier Registry for the PKIX
              Working Group", RFC 7299, DOI 10.17487/RFC7299, July 2014,
              <https://www.rfc-editor.org/info/rfc7299>.

   [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/info/rfc8446>.

   [X.509]    ITU-T, "Information technology - Open Systems
              Interconnection - The Directory: Public-key and attribute
              certificate frameworks", ITU-T Recommendation X.509, ISO/
              IEC 9594-8:2020, October 2019,
              <https://www.itu.int/rec/t-rec-x.509/en>.

Appendix A.  ASN.1 Module

   This ASN.1 module uses the conventions established by [RFC5912].

   SignatureAlgorithmNone
     { iso(1) identified-organization(3) dod(6) internet(1)
       security(5) mechanisms(5) pkix(7) id-mod(0)
       id-mod-algUnsigned-2025(122) }

   DEFINITIONS IMPLICIT TAGS ::=
   BEGIN

   IMPORTS
     SIGNATURE-ALGORITHM
     FROM AlgorithmInformation-2009  -- in [RFC5912]
       { iso(1) identified-organization(3) dod(6) internet(1)
         security(5) mechanisms(5) pkix(7) id-mod(0)
         id-mod-algorithmInformation-02(58) }
     ATTRIBUTE
     FROM PKIX-CommonTypes-2009 -- in [RFC5912]
       { iso(1) identified-organization(3) dod(6) internet(1)
         security(5) mechanisms(5) pkix(7) id-mod(0)
         id-mod-pkixCommon-02(57) } ;

   -- Unsigned Signature Algorithm

   id-alg-unsigned OBJECT IDENTIFIER ::= { iso(1)
      identified-organization(3) dod(6) internet(1) security(5)
      mechanisms(5) pkix(7) alg(6) 36 }

   sa-unsigned SIGNATURE-ALGORITHM ::= {
      IDENTIFIER id-alg-unsigned
      PARAMS ARE absent
   }

   id-rdna-unsigned OBJECT IDENTIFIER ::= { iso(1)
      identified-organization(3) dod(6) internet(1) security(5)
      mechanisms(5) pkix(7) rdna(25) 1 }

   at-unsigned ATTRIBUTE ::= {
      TYPE UTF8String (SIZE (0))
      IDENTIFIED BY id-rdna-unsigned
   }

   END

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