RFC 9925

Unsigned X.509 Certificates

📅 published February 2026, from ietf

Unsigned X.509 Certificates 🎀

okay bestie, here's the whole vibe up front 💅

RFC: 9925 📜
Authors: D. Benjamin, Google LLC 💖
Category: Standards Track 🌈
Date: February 2026 🌸
Updates: RFC 5280 ✨
Obsoletes: none listed 🙈
Series: Internet Engineering Task Force (IETF), ISSN 2070-1721 🦋

Abstract ✨

okay so this doc defines a placeholder X.509 signature algorithm 🦋 that may be used in situations where the certificate's consumer isn't expected to verify the signature 🫶. As part of that, 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. 🌷

Info about the current status of this document, any errata, and how to send feedback lives at https://www.rfc-editor.org/info/rfc9925. 🔗

Copyright Notice ©

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 Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described 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 ✨

okay so an X.509 certificate [RFC5280] links two entities in the PKI 🔗: info about a subject and a proof from an issuer. If you look at the PKI as a graph with entities as nodes, as in [RFC4158], a certificate is the edge between the subject and the issuer 🕸️.

Sometimes an application just needs standalone subject info, not a whole certificate. In the graph model, that means it needs a node, not an edge 🧚. For example, certification path validation (Section 6 of [RFC5280]) starts at a trust anchor, which people sometimes call a root certification authority (root CA) 👑. The application trusts this trust anchor info out-of-band and doesn't need an issuer's signature 🤝.

X.509 doesn't 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 are still super popular 💜.

Also, some TLS [RFC8446] server deployments use self-signed end entity certificates when they don't plan to present a CA-issued identity 🙈. Instead, they expect the relying party to authenticate the certificate out-of-band, e.g., via a known fingerprint 🫶.

These self-signatures usually have no security value, aren't checked by the receiver, and only exist as placeholders to meet the syntactic requirements of an X.509 certificate 💀.

Using 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's 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 🙅.

So this document defines a profile for unsigned X.509 certificates 🦋. You can use them when the certificate is just a container for subject info, 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 part is all about how a sender builds 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 app only needs subject info 🎀. It has no issuer, so some requirements in the profile defined in [RFC5280] can't really be applied 🙃. However, the application might have pre-existing requirements 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, these 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 an empty 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'd build a self-signed certificate 🪞. This can be useful in apps 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 made of a single relative distinguished name that has a single attribute with a type of id-rdna-unsigned and a value of a zero-length UTF8String. Here's how id-rdna-unsigned is defined:

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

In the string representation of [RFC4514], this placeholder name looks like:

1.3.6.1.5.5.7.25.1=#0C00

Senders MUST omit the issuerUniqueID field, since it's 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, so they aren't 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 makes an exception for self-signed certificates used when distributing a public key 🔑. This document updates [RFC5280] so unsigned certificates can also leave out the authority key identifier ✨.

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 means:

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 doesn't issue itself, so there's 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 place of a signature in the certification path 🙅‍♀️.

Most unmodified X.509 apps are expected to already follow this guidance 🎀. So X.509 applications are RECOMMENDED to satisfy these requirements by ignoring this document and instead treating id-alg-unsigned the same as an unrecognized signature algorithm 💅. An unmodified X.509 validator won't be able to verify the signature (Step (a.1) of Section 6.1.3 of [RFC5280]) and will therefore reject the certification path 🙅. On the flip side, in contexts where an X.509 application was already ignoring the self-signature, id-alg-unsigned will also be ignored, just more efficiently ⚡.

In other contexts, an application may need changes or may limit itself to particular forms of unsigned certificates 🛠️. For example, an application might check self-signedness to sort locally configured certificates into trust anchors or untrusted intermediates 🗂️. That kind of application may need to update its configuration model or user interface before using an unsigned certificate as a trust anchor 🫶.

5. Security Considerations 🔐

It's best practice to limit each cryptographic key to a single purpose 🔑. If a key gets reused across contexts, apps 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 ✨.

bestie, this is the dramatic part 😭 If an application accepts id-alg-unsigned as part of a certification path, or in any other context where it's necessary to verify the X.509 signature, the signature check would be bypassed!

So 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 similar to those described in [JWT] and Section 1.1 of [JOSE] 💀.

The signature in a self-signed certificate is self-derived, so it's of limited use for conveying trust 🙃. However, some applications might, for example, use it as an integrity check to guard against accidental storage corruption 🧺. An unsigned certificate doesn't provide any integrity check at all 😭. Applications that check self-signatures for integrity SHOULD instead use some other mechanism, such as an external hash that's verified out-of-band 🔒.

6. IANA Considerations 📚

6.1. Module Identifier

okay bestie, IANA gets to add a shiny new entry to the "SMI Security for PKIX Module Identifier" registry (the one [RFC7299] defines) 🦋✨ and it's giving: decimal 122, called id-mod-algUnsigned-2025, pointing right back at RFC 9925 💅

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

                                Table 1

6.2. Algorithm

next up, IANA adds an entry to the "SMI Security for PKIX Algorithms" registry [RFC7299] 🌸 decimal 36 is the one, called id-alg-unsigned, and it also points at RFC 9925. so cute 🫶

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

                                Table 2

6.3. Relative Distinguished Name Attribute

okay this one is a whole glow-up 🎀 to allocate id-rdna-unsigned, this document introduces a brand new PKIX OID arc for relative distinguished name attributes. so IANA adds a new entry to the "SMI Security for PKIX" registry [RFC7299] with decimal 25 for the Relative Distinguished Name Attribute, also pointing at RFC 9925 💜

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

                                Table 3

IANA also creates a brand new "SMI Security for PKIX Relative Distinguished Name Attribute" registry inside the "Structure of Management Information (SMI) Numbers (MIB Module Registrations)" registry group 🧚 its 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 starts out with just one value in it, so it's very minimalist and iconic 🍓

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

                               Table 4

and any future changes to this table go through the Specification Required policy from [RFC8126], so no random drive-bys, bestie 👑

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 is the ASN.1 module, and it uses the conventions set out in [RFC5912]. no notes, just vibes 🧾✨

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

big thanks to Bob Beck, Nick Harper, and Sophie Schmieg 🫶 for reviewing an early version of this doc. thanks to Alex Gaynor for hooking us up with a link to cite for [JWT] 💖 and to Russ Housley for extra input 🌟

Author's Address

David Benjamin Google LLC Email: davidben@google.com 💌

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