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 document defines the id-alg-unsigned and id-rdna-unsigned object identifiers (OIDs) under the OID arc defined in [RFC7299]:¶

  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}

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

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])¶

• authority key identifier (Section 4.2.1.1 of [RFC5280])¶

• issuer alternative name (Section 4.2.1.7 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.¶

Senders MAY 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. This placeholder name, in the string representation of [RFC2253], is:¶

1.3.6.1.5.5.7.TBD1.TBD2=

Alternatively, if the subject is not empty, senders MAY use the subject field, as in a self-signed certificate. This may be useful in applications that, for example, expect trust anchors to have matching issuer and subject.¶

Senders MUST omit the issuerUniqueID field, as it is optional, not applicable, and already forbidden by Section 4.1.2.8 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 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. In particular, an unsigned end entity certificate does not issue itself, so 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.¶

4. Consuming Unsigned Certificates

X.509 signatures of type id-alg-unsigned are always invalid. This contrasts with [JWT]. 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.¶

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-unsigned 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-unsigned will also be ignored, but more efficiently.¶

In other contexts, applications may require modifications. For example, an application that uses self-signedness in interpreting its local configuration may need to modify its configuration model or user interface before using an unsigned certificate as a trust anchor.¶

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-unsigned avoids a single key being used for both X.509 and the end-entity protocol, eliminating this risk.¶

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 not treat id-alg-unsigned differently from any other previously unrecognized signature algorithm. Non-compliant applications that instead accept id-alg-unsigned as a valid signature risk of vulnerabilities analogous to [JWT].¶

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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