Unsigned X.509 Certificates

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Table of Contents

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, 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] 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 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 section describes how to construct an unsigned certificate.¶

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

  id-rdna-unsigned OBJECT IDENTIFIER ::= {1 3 6 1 5 5 7 TBD1 TBD2}

1.3.6.1.5.5.7.TBD1.TBD2=#0C00

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. 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 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 certificate. 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 [I-D.ietf-jose-deprecate-none-rsa15].¶

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.¶

6. IANA Considerations

7. References

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) }

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 }

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.¶

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