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Security Limited Additional Mechanisms for PKIX and SMIME self-signed certificate 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 The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the Limited Additional Mechanisms for PKIX and SMIME Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction An X.509 certificate 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 , 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 () 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 exist to convey trust anchors, but self-signed certificates remain widely used. Additionally, some TLS 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.

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 when, and only when, they appear in all capitals, as shown here.

Constructing Unsigned Certificates This document defines the id-alg-unsigned object identifier (OID) under the OID arc defined in : 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 cannot meaningfully be applied. However, the application may have pre-existing requirements derived from and , 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 ()
• issuerUniqueID ()
• authority key identifier ()
• issuer alternative name ()

The issuer field is not optional, and both and 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-alg-unsigned and value a zero-length UTF8String. This placeholder name, in the string representation of , is: 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 . Senders SHOULD omit the authority key identifier and issuer alternative name extensions. requires CA certificates to include authority key identifier, but includes an exception for self-signed certificates used when distributing a public key. This document updates 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 ()
• basic constraints ()

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.

Consuming Unsigned Certificates X.509 signatures of type id-alg-unsigned are always invalid. This contrasts with . 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.

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, 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 .

IANA Considerations IANA is requested to create the following entry in the SMI Security for PKIX Module Identifier registry, defined by :

Decimal	Description	References
TBD	id-mod-algUnsigned-2025	[this-RFC]

IANA is requested to add the following entry to the "SMI Security for PKIX Algorithms" registry :

Decimal	Description	References
36	id-alg-unsigned	[this-RFC]

References Normative References This memo profiles the X.509 v3 certificate and X.509 v2 certificate revocation list (CRL) for use in the Internet. An overview of this approach and model is provided as an introduction. The X.509 v3 certificate format is described in detail, with additional information regarding the format and semantics of Internet name forms. Standard certificate extensions are described and two Internet-specific extensions are defined. A set of required certificate extensions is specified. The X.509 v2 CRL format is described in detail along with standard and Internet-specific extensions. An algorithm for X.509 certification path validation is described. An ASN.1 module and examples are provided in the appendices. [STANDARDS-TRACK] In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. When the Curdle Security Working Group was chartered, a range of object identifiers was donated by DigiCert, Inc. for the purpose of registering the Edwards Elliptic Curve key agreement and signature algorithms. This donated set of OIDs allowed for shorter values than would be possible using the existing S/MIME or PKIX arcs. This document describes the donated range and the identifiers that were assigned from that range, transfers control of that range to IANA, and establishes IANA allocation policies for any future assignments within that range. When the Public-Key Infrastructure using X.509 (PKIX) Working Group was chartered, an object identifier arc was allocated by IANA for use by that working group. This document describes the object identifiers that were assigned in that arc, returns control of that arc to IANA, and establishes IANA allocation policies for any future assignments within that arc. Informative References ITU-T This document provides guidance and recommendations to developers building X.509 public-key certification paths within their applications. By following the guidance and recommendations defined in this document, an application developer is more likely to develop a robust X.509 certificate-enabled application that can build valid certification paths across a wide range of PKI environments. This memo provides information for the Internet community. This document describes a structure for representing trust anchor information. A trust anchor is an authoritative entity represented by a public key and associated data. The public key is used to verify digital signatures, and the associated data is used to constrain the types of information or actions for which the trust anchor is authoritative. The structures defined in this document are intended to satisfy the format-related requirements defined in Trust Anchor Management Requirements. [STANDARDS-TRACK] This document specifies version 1.3 of the Transport Layer Security (TLS) protocol. TLS allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery. This document updates RFCs 5705 and 6066, and obsoletes RFCs 5077, 5246, and 6961. This document also specifies new requirements for TLS 1.2 implementations. This specification defines the string format for representing names, which is designed to give a clean representation of commonly used distinguished names, while being able to represent any distinguished name. [STANDARDS-TRACK]

ASN.1 Module

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 . Thanks to Russ Housley for additional input.