Limited Additional Mechanisms for PKIX and SMIME
Internet Engineering Task Force (IETF) D. Benjamin
Internet-Draft
Request for Comments: 9925 Google LLC
Updates: 5280 (if approved) 5 September 2025
Intended status: February 2026
Category: Standards Track
Expires: 9 March 2026
ISSN: 2070-1721
Unsigned X.509 Certificates
draft-ietf-lamps-x509-alg-none-10
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.
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 9 March 2026.
https://www.rfc-editor.org/info/rfc9925.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 4 Requirements Language
3. Constructing Unsigned Certificates . . . . . . . . . . . . . 4
3.1. Signature . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Issuer . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.3. Extensions . . . . . . . . . . . . . . . . . . . . . . . 5
4. Consuming Unsigned Certificates . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
6.1. Module Identifier . . . . . . . . . . . . . . . . . . . . 7
6.2. Algorithm . . . . . . . . . . . . . . . . . . . . . . . . 7
6.3. Relative Distinguished Name Attribute . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1. Normative References . . . . . . . . . . . . . . . . . . 8
7.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. ASN.1 Module . . . . . . . . . . . . . . . . . . . . 10
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 12
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, 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
[RFC5914]
[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. 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. (e.g., a KEM 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. Conventions and Definitions 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 self-signed or self-issued.
Senders MAY alternatively use a short placeholder issuer consisting
of a single relative distinguished name, with a single attribute of
type id-rdna-unsigned and value a zero-length UTF8String. id-rdna-
unsigned is defined as follows:
id-rdna-unsigned OBJECT IDENTIFIER ::= {1 3 6 1 5 5 7 TBD1 TBD2} 25 1}
This placeholder name, in the string representation of [RFC4514], is:
1.3.6.1.5.5.7.TBD1.TBD2=#0C00
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 includes 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 extensions 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, 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, ignored but more efficiently.
In other contexts, an application may require modifications, modifications or limit
itself to particular forms of unsigned certificate. certificates. For example, an
application might check self-signedness to classify locally- 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, 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
[I-D.ietf-jose-deprecate-none-rsa15]. [JOSE].
The signature in a self-signed certificate is self-derived and thus
of limited use to convey trust. However, some applications might use
it as an integrity check to guard against accidental storage
corruption, etc. 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. out-of-band.
6. IANA Considerations
6.1. Module Identifier
IANA is requested to add has added the following entry in the "SMI Security for PKIX
Module Identifier" registry, defined by [RFC7299]:
+=========+=========================+============+
+=========+=========================+===========+
| Decimal | Description | References Reference |
+=========+=========================+============+
+=========+=========================+===========+
| TBD 122 | id-mod-algUnsigned-2025 | [this-RFC] RFC 9925 |
+---------+-------------------------+------------+
+---------+-------------------------+-----------+
Table 1
6.2. Algorithm
IANA is requested to add has added the following entry to the "SMI Security for PKIX
Algorithms" registry [RFC7299]:
+=========+=================+============+
+=========+=================+===========+
| Decimal | Description | References Reference |
+=========+=================+============+
+=========+=================+===========+
| 36 | id-alg-unsigned | [this-RFC] 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 is requested to add has added the following entry to the "SMI Security for PKIX"
registry [RFC7299]:
+=========+=======================================+============+
+=========+=======================================+===========+
| Decimal | Description | References Reference |
+=========+=======================================+============+
+=========+=======================================+===========+
| TBD1 25 | Relative Distinguished Name Attribute | [this-RFC] RFC 9925 |
+---------+---------------------------------------+------------+
+---------+---------------------------------------+-----------+
Table 3
IANA is requested to create 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.TBD1)".
(1.3.6.1.5.5.7.25)".
The new registry has three columns and is initialized with the
following values:
+=========+==================+============+
+=========+==================+===========+
| Decimal | Description | References Reference |
+=========+==================+============+
+=========+==================+===========+
| TBD2 1 | id-rdna-unsigned | [this-RFC] 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/rfc/rfc2119>.
<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/rfc/rfc5280>.
<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/rfc/rfc5912>.
<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/rfc/rfc8126>.
<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/rfc/rfc8174>. <https://www.rfc-editor.org/info/rfc8174>.
7.2. Informative References
[I-D.ietf-jose-deprecate-none-rsa15]
[JOSE] Madden, N., "JOSE: Deprecate 'none' and 'RSA1_5'", Work in
Progress, Internet-Draft, draft-ietf-jose-deprecate-none-
rsa15-02, 2 April
rsa15-03, 19 September 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-jose-
deprecate-none-rsa15-02>.
deprecate-none-rsa15-03>.
[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>.
<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/rfc/rfc4514>.
<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/rfc/rfc5914>.
<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/rfc/rfc7299>.
<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/rfc/rfc8446>.
<https://www.rfc-editor.org/info/rfc8446>.
[X.509] ITU-T, "Information technology - Open Systems
Interconnection - The Directory: Public-key and attribute
certificate frameworks", ISO/IEC 9594-8:2020 , ITU-T Recommendation X.509, ISO/
IEC 9594-8:2020, October
2019. 2019,
<https://www.itu.int/rec/t-rec-x.509/en>.
Appendix A. ASN.1 Module
SignatureAlgorithmNone
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-algUnsigned-2025(TBD)
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(TBD1) TBD2 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