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Security Transport Layer Security This document allocates code points for the use of RSASSA-PKCS1-v1_5 with client certificates in TLS 1.3. This removes an obstacle for some deployments to migrate to TLS 1.3. 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 Transport Layer Security Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction TLS 1.3 removed support for RSASSA-PKCS1-v1_5 in CertificateVerify messages in favor of RSASSA-PSS. While RSASSA-PSS is a long-established signature algorithm, some legacy hardware cryptographic devices lack support for it. While uncommon in TLS servers, these devices are sometimes used by TLS clients for client certificates. For example, Trusted Platform Modules (TPMs) are ubiquitous hardware cryptographic devices that are often used to protect TLS client certificate private keys. However, a large number of TPMs are unable to produce RSASSA-PSS signatures compatible with TLS 1.3. TPM specifications prior to 2.0 did not define RSASSA-PSS support (see Section 5.8.1 of ). TPM 2.0 includes RSASSA-PSS, but only those TPM 2.0 devices compatible with FIPS 186-4 can be relied upon to use the salt length matching the digest length, as required for compatibility with TLS 1.3 (see Appendix B.7 of ). TLS connections that rely on such devices cannot migrate to TLS 1.3. Staying on TLS 1.2 leaks the client certificate to network attackers and additionally prevents such deployments from protecting traffic against retroactive decryption by an attacker with a quantum computer . Additionally, TLS negotiates protocol versions before client certificates. Clients send ClientHellos without knowing whether the server will request to authenticate with legacy keys. Conversely, servers respond with a TLS version and CertificateRequest without knowing if the client will then respond with a legacy key. If the client and server, respectively, offer and negotiate TLS 1.3, the connection will fail due to the legacy key, when it previously succeeded at TLS 1.2. To recover from this failure, one side must globally disable TLS 1.3 or the client must implement an external fallback. Disabling TLS 1.3 impacts connections that would otherwise be unaffected by this issue, while external fallbacks break TLS's security analysis and may introduce vulnerabilities . This document allocates code points to use these legacy keys with client certificates in TLS 1.3. This reduces the pressure on implementations to select one of these problematic mitigations and unblocks TLS 1.3 deployment.

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.

PKCS#1 v1.5 SignatureScheme Types The following SignatureScheme values are defined for use with TLS 1.3. The above code points indicate a signature algorithm using RSASSA-PKCS1-v1_5 with the corresponding hash algorithm as defined in . They are only defined for signatures in the client CertificateVerify message and are not defined for use in other contexts. In particular, servers intending to advertise support for RSASSA-PKCS1-v1_5 signatures in the certificates themselves should use the rsa_pkcs1_* constants defined in . Clients MUST NOT advertise these values in the signature_algorithms extension of the ClientHello. They MUST NOT accept these values in the server CertificateVerify message. Servers that wish to support clients authenticating with legacy RSASSA-PKCS1-v1_5-only keys MAY send these values in the signature_algorithms extension of the CertificateRequest message and accept them in the client CertificateVerify message. Servers MUST NOT accept these code points if not offered in the CertificateRequest message. Clients with such legacy keys MAY negotiate the use of these signature algorithms if offered by the server. Clients SHOULD NOT negotiate them with keys that support RSASSA-PSS, though this may not be practical to determine in all applications. For example, attempting to test a key for support might display a message to the user or have other side effects. TLS implementations SHOULD disable these code points by default. See .

Security Considerations The considerations in do not apply to server keys, so these new code points are forbidden for use with server certificates. RSASSA-PSS continues to be required for TLS 1.3 servers using RSA keys. This minimizes the impact to only those cases necessary to unblock TLS 1.3 deployment. When implemented incorrectly, RSASSA-PKCS1-v1_5 admits signature forgeries . Implementations producing or verifying signatures with these algorithms MUST implement RSASSA-PKCS1-v1_5 as specified in section 8.2 of . In particular, clients MUST include the mandatory NULL parameter in the DigestInfo structure and produce a valid DER encoding. Servers MUST reject signatures which do not meet these requirements.

IANA Considerations IANA is requested to create the following entries in the TLS SignatureScheme registry, defined in . The "Recommended" column should be set to "N", and the "Reference" column should be set to this document.

Value	Description
0x0420	rsa_pkcs1_sha256_legacy
0x0520	rsa_pkcs1_sha384_legacy
0x0620	rsa_pkcs1_sha512_legacy

References Normative References 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. This document provides recommendations for the implementation of public-key cryptography based on the RSA algorithm, covering cryptographic primitives, encryption schemes, signature schemes with appendix, and ASN.1 syntax for representing keys and for identifying the schemes. This document represents a republication of PKCS #1 v2.2 from RSA Laboratories' Public-Key Cryptography Standards (PKCS) series. By publishing this RFC, change control is transferred to the IETF. This document also obsoletes RFC 3447. 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. ITU-T Trusted Computing Group Trusted Computing Group 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. National Institute of Standards and Technology (U.S.) Informative References IEEE University of Waterloo Cisco Systems University of Haifa and Meta Hybrid key exchange refers to using multiple key exchange algorithms simultaneously and combining the result with the goal of providing security even if a way is found to defeat the encryption for all but one of the component algorithms. It is motivated by transition to post-quantum cryptography. This document provides a construction for hybrid key exchange in the Transport Layer Security (TLS) protocol version 1.3.

Acknowledgements Thanks to Rifaat Shekh-Yusef, Martin Thomson, and Paul Wouters for providing feedback on this document.