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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. As part of this, it updates RFC 5280. 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 section describes how to construct an unsigned certificate.

Signature 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, defined below: The parameters for id-alg-unsigned MUST be omitted. The Certificate's signatureValue field MUST be a BIT STRING of length zero.

Issuer 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 ()

The issuer field is not optional, and both and forbid empty issuers, so such a value may not be interoperable with existing applications. If the subject is not empty, senders MAY set the issuer to the subject, similar how they would construct a self-signed certificate. This may be useful in applications that, for example, expect trust anchors to have matching issuer and subject. This is, however, a placeholder value. The unsigned certificate is not considered self-signed or self-issued. Senders MAY alternatively 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. id-rdna-unsigned is defined as follows: This placeholder name, in the string representation of , is: Senders MUST omit the issuerUniqueID field, as it is optional, not applicable, and already forbidden by .

Extensions Some X.509 extensions also describe the certificate issuer and thus are not meaningful for an unsigned certificate:

• authority key identifier ()
• issuer alternative name ()

Senders SHOULD omit the authority key identifier and issuer alternative name extensions. 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 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. That is: If the subject is a CA, it SHOULD assert the keyCertSign key usage bit and SHOULD include a basic constraints extensions that sets the cA boolean to TRUE. If the subject is an end entity, 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. Unlike a self-signed certificate, an unsigned certificate does not issue itself, so there is no need to accommodate a self-signature in either extension.

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. X.509 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 be unable to verify the signature (Step (a.1) of ) 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.

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, 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 and . 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 that is verified out of band.

IANA Considerations

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

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

Algorithm 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]

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 :

Decimal	Description	References
TBD1	Relative Distinguished Name Attribute	[this-RFC]

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]

Future updates to this table are to be made according to the Specification Required policy as defined in .

References Normative References The Public Key Infrastructure using X.509 (PKIX) certificate format, and many associated formats, are expressed using ASN.1. The current ASN.1 modules conform to the 1988 version of ASN.1. This document updates those ASN.1 modules to conform to the 2002 version of ASN.1. There are no bits-on-the-wire changes to any of the formats; this is simply a change to the syntax. This document is not an Internet Standards Track specification; it is published for informational purposes. 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. Many protocols make use of points of extensibility that use constants to identify various protocol parameters. To ensure that the values in these fields do not have conflicting uses and to promote interoperability, their allocations are often coordinated by a central record keeper. For IETF protocols, that role is filled by the Internet Assigned Numbers Authority (IANA). To make assignments in a given registry prudently, guidance describing the conditions under which new values should be assigned, as well as when and how modifications to existing values can be made, is needed. This document defines a framework for the documentation of these guidelines by specification authors, in order to assure that the provided guidance for the IANA Considerations is clear and addresses the various issues that are likely in the operation of a registry. This is the third edition of this document; it obsoletes RFC 5226. 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. The X.500 Directory uses distinguished names (DNs) as primary keys to entries in the directory. This document defines the string representation used in the Lightweight Directory Access Protocol (LDAP) to transfer distinguished names. The string representation is designed to give a clean representation of commonly used distinguished names, while being able to represent any distinguished name. [STANDARDS-TRACK] Teya This document updates [RFC7518] to deprecate the JWS algorithm "none" and the JWE algorithm "RSA1_5". These algorithms have known security weaknesses. It also updates the Review Instructions for Designated Experts to establish baseline security requirements that future algorithm registrations should meet. 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.

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.