A Mechanism for X.509 Certificate Discovery

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.¶

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.¶

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."¶

This Internet-Draft will expire on 2 May 2027.¶

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 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

Cryptographic agility and fallback often feature prominently in cryptographic migrations, whether it's migrating algorithms, infrastructures, keys, or some other aspect. Discovery is a key aspect of agility -- in this case meaning that an application holding an X.509 certificate for a peer can easily learn about and retrieve alternative versions of the certificate they are holding, a space that is under-served by existing X.509 extensions. To address these limitations, this document proposes a novel approach to certificate discovery utilizing the Subject Information Access extension within X.509 certificates.¶

The primary objective of this approach is to enable efficient multi-certificate handling in protocols, offering several key benefits. First, it enhances cryptographic agility by facilitating smooth transitions between different algorithms or X.509 certificate types. This is particularly valuable in scenarios where subscribers need to upgrade their cryptographic algorithms or adopt new certificate types while maintaining backward compatibility with existing systems.¶

Second, the proposed method improves operational availability by introducing redundancy in certificate usage. It enables the automatic discovery and use of Referenced Certificates that can serve as fallbacks, ensuring seamless continuity of services even in the event of expiration or revocation of the Current Certificate, or disruptions in the Certification Authority (CA) infrastructure.¶

Finally, the approach accommodates multi-key/certificate usage, allowing for a CA to indicate that two (or more) certificates are to be used together.¶

The proposed method is designed to maximize compatibility with existing systems, including legacy implementations. It leverages the subjectInfoAccess extension, which is already established in X.509 certificates, and does not require modifications to the referring certificates. This ensures ease of adoption and avoids disruptions to current certificate management practices.¶

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. Certificate Discovery Access Method

This document specifies the new certDiscovery access method for X.509 Subject Information Access (SIA) extension defined in [RFC5280].¶

The syntax of subject information access extension syntax is repeated here for convenience:¶

SubjectInfoAccessSyntax  ::=
         SEQUENCE SIZE (1..MAX) OF AccessDescription

AccessDescription  ::=  SEQUENCE {
         accessMethod          OBJECT IDENTIFIER,
         accessLocation        GeneralName  }

This document defines a new SubjectInfoAccessSyntax access method id-ad-certDiscovery which is an OBJECT IDENTIFIER that indicates the accessMethod is for carrying a certificate discovery description of a Referenced Certificate.¶

id-ad-certDiscovery OBJECT IDENTIFIER ::= { id-ad TBD }

The 'accessLocation' is a GeneralName otherName type as defined in [RFC5280]. Recall that the otherName type is defined as AnotherName:¶

AnotherName ::= SEQUENCE {
     type-id    OBJECT IDENTIFIER,
     value      [0] EXPLICIT ANY DEFINED BY type-id }

This document defines the RelatedCertificateDescriptor type and its corresponding identifier as follows:¶

-- Other Name OID Arc --
id-on OBJECT IDENTIFIER ::= { id-pkix 8 }

-- Certificate Discovery Access Descriptor --
id-on-relatedCertificateDescriptor OBJECT IDENTIFIER ::= { id-on TBD }

on-RelatedCertificateDescriptor OTHER-NAME ::= {
      RelatedCertificateDescriptor IDENTIFIED BY id-on-relatedCertificateDescriptor
   }

When the accessMethod has a value of id-ad-certDiscovery, then the accessLocation MUST contain an otherName whose type-id is id-on-relatedCertificateDescriptor and the value is RelatedCertificateDescriptor.¶

RelatedCertificateDescriptor is defined as follows:¶

RelatedCertificateDescriptor ::= SEQUENCE {
   method CertDiscoveryMethod,
   signatureAlgorithm [0] AlgorithmIdentifier OPTIONAL,
   publicKeyAlgorithm [1] AlgorithmIdentifier OPTIONAL,
   certHash [2] RelatedCertificate OPTIONAL
}

A certificate MAY have any number of RelatedCertificateDescriptor SIA AccessDescriptions.¶

Each component of the RelatedCertificateDescriptor is defined below.¶

CertDiscoveryMethod ::= CHOICE {
   byUri IA5String,
   byInclusion Certificate,
   byLocalPolicy NULL,
   byOther [0] INSTANCE OF OTHER-DISCOVERY-METHOD
}

RelatedCertificate ::= SEQUENCE {
  hashAlgorithm DigestAlgorithmIdentifier,
  hashValue     OCTET STRING }

4. Certificate Discovery Self Location Access Method

This document defines a new SubjectInfoAccessSyntax access method id-ad-certDiscoverySelfLocation which is an OBJECT IDENTIFIER that indicates the accessMethod is for carrying a certificate discovery self-reference. In other words, a certificate MAY carry its own URI as a way to further enhance discoverability and validate that the correct certificate was retrieved.¶

id-ad-certDiscoverySelfLocation OBJECT IDENTIFIER ::= { id-ad TBD4 }

The SubjectInfoAccessSyntax AccessDescription accessLocation MUST carry a GeneralName uniformResourceIdentifier.¶

This access method indicates the URI [!RFC3986] where the Current Certificate is located. Applications which have retrieved this certificate using a byUri method SHOULD extract this value from the retrieved certificate confirm that the correct certificate was retrieved.¶

A certificate MAY have any number of certDiscoverySelfLocation SIA AccessDescriptions.¶

5. Expired, Revoked, or Untrusted Current Certificate

In the case that the Current Certificate is unusable, for example because it is expired, revoked, does not chain to a trusted root, or for any other reason, the application MAY still follow the RelatedCertificateDescriptor to see if this yields a valid and usable certificate.¶

This provides great flexibility in error-recovery scenarios. In addition, this will allow for more pro-active migration to new infrastructures, knowing that applications will be able to follow RelatedCertificateDescriptors to their preferred version of the certificate.¶

Since this is merely a mechanism for discovering the existence of the Referenced Certificate, doing so from an untrusted certificate does not pose security issues.¶

6. Security Considerations

Retrieval of the Referenced Certificate is not sufficient to consider the Referenced Certificate trustworthy. The certification path validation algorithm as defined in section 6 of [RFC5280] MUST be performed for the Referenced Certificate. A reference from a RelatedCertificateDescriptor, even in the presence of a certHash does not imply any form of endorsement or hierarchical relationship between the two certificates unless the mechanism in this draft is augmented with additional X.509v3 extensions or policies that specify those semantics.¶

The use of the self-reference can be used to provide a subject binding between the Current and Referenced Certificates even if their other data such as subject name or public key do not match. However, the procedure for validating subject equivalence MUST be defined by policy and be consistent with the policies of the issuing CA. As a result, validation of subject equivalence is out of scope of this document.¶

The Referenced Certificate may also have a certDiscovery access method. In order to avoid cyclic loops or infinite chaining, leading to denial-of-service scenarios, the validator SHOULD employ a maximum depth or cycle-detection algorithm.¶

The same security considerations for caIssuers access method outlined in [RFC5280] applies to the certDiscovery access method. In order to avoid recursive certificate validations which involve online revocation checking, untrusted transport protocols (such as plaintext HTTP) are commonly used for serving certificate files. While the use of such protocols avoids issues with recursive certification path validations and associated online revocation checking, it also enables an attacker to tamper with data and perform substitution attacks. Applications retrieving certificates using the mechanism specified in this document MUST treat downloaded certificate data as untrusted and perform requisite checks to ensure that the downloaded data is not malicious.¶

Typically, it is unsafe to use any content from a certificate that is expired, revoked, or whose signature cannot be validated. The RelatedCertificateDescriptor extension is an exception to this rule since it is intended to bridge across these exact types of failure scenarios and is merely making the application aware of the existence of a Referenced Certificate. Once retrieved, the Referenced Certificate SHOULD be validated independently as if it had been provided as the Current Certificate. However, applications MAY refuse to retrieve URIs from an untrusted, or, to a lesser extent, revoked certificate. In most cases, a RelatedCertificateDescriptor URI asserted by the CA at issuance time will remain valid for the lifetime of the certificate, unless an incorrect URI is the reason for revocation, or the certificate was forged by a malicious actor for the explicit purpose of getting a victim application to fetch the malicious URI.¶

This specification does not provide a mechanism for identifying the purpose of the cross-reference between certificates. The implied purpose is fallback redundancy. If the purpose is security-related, for example a pair of certificates are only to be used together in a two-certificate protocol, or their revocation status is intertwined as is the case when a signing certificate signs a Certificate Signing Request (CSR) for an encryption certificate, then the CA SHOULD add an additional extension or policy OID to the certificates to further constrain verifier handling.¶

7. IANA Considerations

8. 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>.

[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>.

[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>.

Acknowledgments

TODO acknowledge.¶

Appendix A. ASN.1 Module

The following ASN.1 module provides the complete definition of the Certificate Discovery access descriptor.¶

CertDiscovery { iso(1) identified-organization(3) dod(6) internet(1)
   security(5) mechanisms(5) pkix(7) id-mod(0) id-mod-CertDiscovery(TBD) }

   DEFINITIONS EXPLICIT TAGS ::=

   BEGIN

   -- EXPORTS ALL --

   IMPORTS
    OTHER-NAME, AlgorithmIdentifier, Certificate
    FROM PKIX1Implicit-2009
      { iso(1) identified-organization(3) dod(6) internet(1) security(5)
      mechanisms(5) pkix(7) id-mod(0) id-mod-pkix1-implicit-02(59) }

    id-pkix, id-ad
    FROM PKIX1Explicit-2009
      { iso(1) identified-organization(3) dod(6) internet(1) security(5)
      mechanisms(5) pkix(7) id-mod(0) id-mod-pkix1-explicit-02(51) }

    RelatedCertificate
    FROM RelatedCertificate
      { iso(1) identified-organization(3) dod(6)
       internet(1) security(5) mechanisms(5) pkix(7) id-mod(0)
     id-mod-related-cert-2023(115)} ;

   id-ad-certDiscovery OBJECT IDENTIFIER ::= { id-ad TBD2 }

   -- Other Name OID Arc --

   id-on OBJECT IDENTIFIER ::= { id-pkix 8 }

   -- Certificate Discovery Access Descriptor --

   id-on-relatedCertificateDescriptor OBJECT IDENTIFIER ::= { id-on TBD3 }

   -- Always encode as a GeneralName uniform resource identifier (URI)
   id-ad-certDiscoverySelfLocation OBJECT IDENTIFIER ::= { id-ad TBD4 }

   on-RelatedCertificateDescriptor OTHER-NAME ::= {
      RelatedCertificateDescriptor IDENTIFIED BY id-on-relatedCertificateDescriptor
   }

   RelatedCertificateDescriptor ::= SEQUENCE {
     method CertDiscoveryMethod,
     signatureAlgorithm [0] AlgorithmIdentifier OPTIONAL,
     publicKeyAlgorithm [1] AlgorithmIdentifier OPTIONAL,
     certHash [2] RelatedCertificate OPTIONAL
   }

   -- RelatedCertificate is defined in RFC 9763

   CertDiscoveryMethod ::= CHOICE {
     byUri IA5String,
     byInclusion Certificate,
     byLocalPolicy NULL,
     byOther [0] INSTANCE OF OTHER-DISCOVERY-METHOD
   }

   OTHER-DISCOVERY-METHOD ::= TYPE-IDENTIFIER

   END

Authors' Addresses