Limited Additional Mechanisms for PKIX and SMIME D. Benjamin Internet-Draft Google LLC Updates: 5280 (if approved) 28 May 2025 Intended status: Standards Track Expires: 29 November 2025 Unsigned X.509 Certificates draft-ietf-lamps-x509-alg-none-05 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. 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 https://mailarchive.ietf.org/arch/browse/spasm/. Subscribe at 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. 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." Benjamin Expires 29 November 2025 [Page 1] Internet-Draft Unsigned X.509 Certificates May 2025 This Internet-Draft will expire on 29 November 2025. Copyright Notice Copyright (c) 2025 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 . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3 3. Constructing Unsigned Certificates . . . . . . . . . . . . . 4 4. Consuming Unsigned Certificates . . . . . . . . . . . . . . . 5 5. Security Considerations . . . . . . . . . . . . . . . . . . . 6 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 6.1. Module Identifier . . . . . . . . . . . . . . . . . . . . 6 6.2. Algorithm . . . . . . . . . . . . . . . . . . . . . . . . 6 6.3. Relative Distinguished Name Attribute . . . . . . . . . . 7 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 7.1. Normative References . . . . . . . . . . . . . . . . . . 7 7.2. Informative References . . . . . . . . . . . . . . . . . 8 Appendix A. ASN.1 Module . . . . . . . . . . . . . . . . . . . . 9 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 10 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10 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. Benjamin Expires 29 November 2025 [Page 2] Internet-Draft Unsigned X.509 Certificates May 2025 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. Benjamin Expires 29 November 2025 [Page 3] Internet-Draft Unsigned X.509 Certificates May 2025 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. Benjamin Expires 29 November 2025 [Page 4] Internet-Draft Unsigned X.509 Certificates May 2025 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. Benjamin Expires 29 November 2025 [Page 5] Internet-Draft Unsigned X.509 Certificates May 2025 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 6.1. Module Identifier IANA is requested to add the following entry in the "SMI Security for PKIX Module Identifier" registry, defined by [RFC7299]: +=========+=========================+============+ | Decimal | Description | References | +=========+=========================+============+ | TBD | id-mod-algUnsigned-2025 | [this-RFC] | +---------+-------------------------+------------+ Table 1 6.2. Algorithm IANA is requested to add the following entry to the "SMI Security for PKIX Algorithms" registry [RFC7299]: +=========+=================+============+ | Decimal | Description | References | +=========+=================+============+ | 36 | id-alg-unsigned | [this-RFC] | +---------+-----------------+------------+ Table 2 Benjamin Expires 29 November 2025 [Page 6] Internet-Draft Unsigned X.509 Certificates May 2025 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 the following entry to the "SMI Security for PKIX" registry [RFC7299]: +=========+=======================================+============+ | Decimal | Description | References | +=========+=======================================+============+ | TBD1 | Relative Distinguished Name Attribute | [this-RFC] | +---------+---------------------------------------+------------+ Table 3 IANA is requested to create the "SMI Security for PKIX Relative Distinguished Name Attribute" registry within the "Structure of Management Information (SMI) Numbers (MIB Module Registrations)" group. The new registry's description is "iso.org.dod.internet.security.mechanisms.pkix.rdna (1.3.6.1.5.5.7.TBD1)". The new registry has three columns and is initialized with the following values: +=========+==================+============+ | Decimal | Description | References | +=========+==================+============+ | TBD2 | id-rdna-unsigned | [this-RFC] | +---------+------------------+------------+ Table 4 Future updates to this table are to be made according to the Specification Required policy as defined in [RFC5226]. 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, . Benjamin Expires 29 November 2025 [Page 7] Internet-Draft Unsigned X.509 Certificates May 2025 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", RFC 5226, DOI 10.17487/RFC5226, May 2008, . [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, . [RFC7299] Housley, R., "Object Identifier Registry for the PKIX Working Group", RFC 7299, DOI 10.17487/RFC7299, July 2014, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . 7.2. Informative References [JWT] Sanderson, J., "How Many Days Has It Been Since a JWT alg:none Vulnerability?", 9 October 2024, . [RFC2253] Wahl, M., Kille, S., and T. Howes, "Lightweight Directory Access Protocol (v3): UTF-8 String Representation of Distinguished Names", RFC 2253, DOI 10.17487/RFC2253, December 1997, . [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, . [RFC5914] Housley, R., Ashmore, S., and C. Wallace, "Trust Anchor Format", RFC 5914, DOI 10.17487/RFC5914, June 2010, . [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, . [X.509] ITU-T, "Information technology - Open Systems Interconnection – The Directory: Public-key and attribute certificate frameworks", ISO/IEC 9594-8:2020 , October 2019. Benjamin Expires 29 November 2025 [Page 8] Internet-Draft Unsigned X.509 Certificates May 2025 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 Benjamin Expires 29 November 2025 [Page 9] Internet-Draft Unsigned X.509 Certificates May 2025 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 Benjamin Expires 29 November 2025 [Page 10]