Legacy RSASSA-PKCS1-v1_5 codepoints for TLS 1.3 (2024)

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Table of Contents

1. Introduction

TLS 1.3 [RFC8446] removed support for RSASSA-PKCS1-v1_5 [RFC8017] inCertificateVerify messages in favor of RSASSA-PSS. While RSASSA-PSS is along-established signature algorithm, some legacy hardware cryptographic deviceslack support for it. While uncommon in TLS servers, these devices are sometimesused by TLS clients for client certificates.¶

For example, Trusted Platform Modules (TPMs) are ubiquitous hardwarecryptographic devices that are often used to protect TLS client certificateprivate keys. However, a large number of TPMs are unable to produce RSASSA-PSSsignatures compatible with TLS 1.3. TPM specifications prior to 2.0 did notdefine RSASSA-PSS support (see Section 5.8.1 of [TPM12]). TPM 2.0includes RSASSA-PSS, but only those TPM 2.0 devices compatible with FIPS 186-4can be relied upon to use the salt length matching the digest length, asrequired for compatibility with TLS 1.3 (see Appendix B.7 of [TPM2]).¶

TLS connections that rely on such devices cannot migrate to TLS 1.3. Staying onTLS 1.2 leaks the client certificate to network attackers and additionallyprevents such deployments from protecting traffic against retroactivedecryption by an attacker with a quantum computer.¶

Moreover, TLS negotiates the protocol version before client certificates, soclients and servers cannot smoothly transition unaffected connections toTLS 1.3. As a result, this issue is not limited to individual connectionsthat use affected devices. It prevents entire deployments from migrating toTLS 1.3. See Section 4 for further discussion.¶

This document allocates code points to use these legacy keys with clientcertificates in TLS 1.3.¶

2. Conventions and Definitions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD","SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in thisdocument are to be interpreted as described in BCP 14 [RFC2119] [RFC8174]when, and only when, they appear in all capitals, as shown here.¶

3. PKCS#1 v1.5 SignatureScheme Types

The following SignatureScheme values are defined for use with TLS 1.3.¶

 enum { rsa_pkcs1_sha256_legacy(0x0420), rsa_pkcs1_sha384_legacy(0x0520), rsa_pkcs1_sha512_legacy(0x0620), } SignatureScheme;

The above code points indicate a signature algorithm using RSASSA-PKCS1-v1_5[RFC8017] with the corresponding hash algorithm as defined in[SHS]. They are only defined for signatures inthe client CertificateVerify message and are not defined for use in othercontexts. In particular, servers intending to advertise support forRSASSA-PKCS1-v1_5 signatures in the certificates themselves should use thersa_pkcs1_* constants defined in [RFC8446].¶

Clients MUST NOT advertise these values in the signature_algorithms extensionof the ClientHello. They MUST NOT accept these values in the serverCertificateVerify message.¶

Servers that wish to support clients authenticating with legacyRSASSA-PKCS1-v1_5-only keys MAY send these values in thesignature_algorithms extension of the CertificateRequest message and acceptthem in the client CertificateVerify message. Servers MUST NOT accept these codepoints if not offered in the CertificateRequest message.¶

Clients with such legacy keys MAY negotiate the use of these signaturealgorithms if offered by the server. Clients SHOULD NOT negotiate them withkeys that support RSASSA-PSS.¶

TLS implementations SHOULD disable these code points by default.¶

4. Security Considerations

Prior to this document, legacy RSA keys would prevent client certificatedeployments from adopting TLS 1.3. The new code points allow such deploymentsto upgrade without replacing the keys. TLS 1.3 fixes a privacy flaw[PRIVACY] with client certificates, soupgrading is a particular benefit to these deployments. TLS 1.3 is also aprequisite for post-quantum key exchanges [I-D.ietf-tls-hybrid-design],necessary for deployments to protect traffic against retroactive decryption byan attacker with a quantum computer.¶

Additionally, TLS negotiates protocol versions before client certificates.Clients send ClientHellos without knowing whether the server will request toauthenticate with legacy keys. Conversely, servers respond with a TLSversion and CertificateRequest without knowing if the client will thenrespond with a legacy key. If the client and server, respectively, offer andnegotiate TLS 1.3, the connection will fail due to the legacy key, when itpreviously succeeded at TLS 1.2.¶

To recover from this failure, one side must globally disable TLS 1.3 or theclient must implement an external fallback. Disabling TLS 1.3 impactsconnections that would otherwise be unaffected by this issue, while externalfallbacks break TLS's security analysis and may introduce vulnerabilities[POODLE]. The new code points reduce the pressure on implementations to selectone of these problematic mitigations and unblocks TLS 1.3 deployment.¶

At the same time, the new code points also reduce the pressure onimplementations to migrate to RSASSA-PSS. The above considerations do not applyto server keys, so these new code points are forbidden for use with servercertificates. RSASSA-PSS continues to be required for TLS 1.3 servers using RSAkeys. This minimizes the impact to only those cases necessary to unblock TLS1.3 deployment.¶

Finally, when implemented incorrectly, RSASSA-PKCS1-v1_5 admits signatureforgeries [MFSA201473]. Implementations producing or verifying signatureswith these algorithms MUST implement RSASSA-PKCS1-v1_5 as specified in section8.2 of [RFC8017]. In particular, clients MUST include the mandatory NULLparameter in the DigestInfo structure and produce a valid DER [X690]encoding. Servers MUST reject signatures which do not meet these requirements.¶

5. IANA Considerations

IANA is requested to create the following entries in theTLS SignatureScheme registry, defined in [RFC8446]. The "Recommended" columnshould be set to "N", and the "Reference" column should be set to this document.¶

Authors' Addresses