Legacy RSASSA-PKCS1-v1_5 codepoints for TLS 1.3

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Table of Contents

1. Introduction

TLS 1.3 [RFC8446] removed support for RSASSA-PKCS1-v1_5 [RFC8017] 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 [TPM12]). 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 [TPM2]).¶

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.¶

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

This document allocates code points to use these legacy keys with client certificates 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 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.¶

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 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 [RFC8446].¶

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.¶

TLS implementations SHOULD disable these code points by default.¶

4. Security Considerations

Prior to this document, legacy RSA keys would prevent client certificate deployments from adopting TLS 1.3. The new code points allow such deployments to upgrade without replacing the keys. TLS 1.3 fixes a privacy flaw [PRIVACY] with client certificates, so upgrading is a particular benefit to these deployments. TLS 1.3 is also a prequisite for post-quantum key exchanges [I-D.ietf-tls-hybrid-design], necessary for deployments to protect 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 [POODLE]. The new code points reduce the pressure on implementations to select one of these problematic mitigations and unblocks TLS 1.3 deployment.¶

At the same time, the new code points also reduce the pressure on implementations to migrate to RSASSA-PSS. The above considerations 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.¶

Finally, when implemented incorrectly, RSASSA-PKCS1-v1_5 admits signature forgeries [MFSA201473]. Implementations producing or verifying signatures with these algorithms MUST implement RSASSA-PKCS1-v1_5 as specified in section 8.2 of [RFC8017]. In particular, clients MUST include the mandatory NULL parameter 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 the TLS SignatureScheme registry, defined in [RFC8446]. The "Recommended" column should be set to "N", and the "Reference" column should be set to this document.¶

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