Legacy RSASSA-PKCS1-v1_5 codepoints for TLS 1.3
draft-ietf-tls-tls13-pkcs1-07

Transport Layer Security D. Benjamin
Internet-Draft Google LLC
Intended status: Standards Track A. Popov
Expires: 5 June 2026 Microsoft Corp.
 2 December 2025
 Legacy RSASSA-PKCS1-v1_5 codepoints for TLS 1.3
 draft-ietf-tls-tls13-pkcs1-07
Abstract
 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
 This note is to be removed before publishing as an RFC.
 The latest revision of this draft can be found at
 https://tlswg.github.io/tls13-pkcs1/draft-ietf-tls-tls13-pkcs1.html.
 Status information for this document may be found at
 https://datatracker.ietf.org/doc/draft-ietf-tls-tls13-pkcs1/.
 Discussion of this document takes place on the Transport Layer
 Security Working Group mailing list (mailto:tls@ietf.org), which is
 archived at https://mailarchive.ietf.org/arch/browse/tls/. Subscribe
 at https://www.ietf.org/mailman/listinfo/tls/.
 Source for this draft and an issue tracker can be found at
 https://github.com/tlswg/tls13-pkcs1.
Status of This Memo
 This Internet-Draft is submitted in full conformance with the
 provisions of BCP 78 and BCP 79.
 Internet-Drafts are working documents of the Internet Engineering
 Task Force (IETF). Note that other groups may also distribute
 working documents as Internet-Drafts. The list of current Internet-
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 document authors. All rights reserved.
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 Provisions Relating to IETF Documents (https://trustee.ietf.org/
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Table of Contents
 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
 2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
 3. PKCS#1 v1.5 SignatureScheme Types . . . . . . . . . . . . . . 3
 4. Security Considerations . . . . . . . . . . . . . . . . . . . 4
 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
 6.1. Normative References . . . . . . . . . . . . . . . . . . 5
 6.2. Informative References . . . . . . . . . . . . . . . . . 6
 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 6
 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
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 US 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]).
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 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
 [PRIVACY] and additionally prevents such deployments from protecting
 traffic against retroactive decryption by an attacker with a quantum
 computer [I-D.ietf-tls-hybrid-design].
 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].
 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.
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
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 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, 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
 Section 4.
4. Security Considerations
 The considerations in Section 1 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 [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. The "Recommended" column should be set to
 "N", and the "Reference" column should be set to this document.
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 +========+=========================+
 | Value | Description |
 +========+=========================+
 | 0x0420 | rsa_pkcs1_sha256_legacy |
 +--------+-------------------------+
 | 0x0520 | rsa_pkcs1_sha384_legacy |
 +--------+-------------------------+
 | 0x0620 | rsa_pkcs1_sha512_legacy |
 +--------+-------------------------+
 Table 1
6. References
6.1. Normative References
 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
 Requirement Levels", BCP 14, RFC 2119,
 DOI 10.17487/RFC2119, March 1997,
 <https://www.rfc-editor.org/rfc/rfc2119>.
 [RFC8017] Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A. Rusch,
 "PKCS #1: RSA Cryptography Specifications Version 2.2",
 RFC 8017, DOI 10.17487/RFC8017, November 2016,
 <https://www.rfc-editor.org/rfc/rfc8017>.
 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
 May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
 [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
 Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
 <https://www.rfc-editor.org/rfc/rfc8446>.
 [SHS] "Secure hash standard", National Institute of Standards
 and Technology (U.S.), DOI 10.6028/nist.fips.180-4, 2015,
 <https://doi.org/10.6028/nist.fips.180-4>.
 [TPM12] Trusted Computing Group, "TPM Main Specification Level 2
 Version 1.2, Revision 116, Part 2 - Structures of the
 TPM", 1 March 2011, <https://trustedcomputinggroup.org/wp-
 content/uploads/TPM-Main-Part-2-TPM-
 Structures_v1.2_rev116_01032011.pdf>.
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 [TPM2] Trusted Computing Group, "Trusted Platform Module Library
 Specification, Family 2.0, Level 00, Revision 01.59, Part
 1: Architecture", 8 November 2019,
 <https://trustedcomputinggroup.org/wp-content/uploads/
 TCG_TPM2_r1p59_Part1_Architecture_pub.pdf>.
 [X690] ITU-T, "Information technology - ASN.1 encoding Rules:
 Specification of Basic Encoding Rules (BER), Canonical
 Encoding Rules (CER) and Distinguished Encoding Rules
 (DER)", ISO/IEC 8825-1:2002, 2002.
6.2. Informative References
 [I-D.ietf-tls-hybrid-design]
 Stebila, D., Fluhrer, S., and S. Gueron, "Hybrid key
 exchange in TLS 1.3", Work in Progress, Internet-Draft,
 draft-ietf-tls-hybrid-design-16, 7 September 2025,
 <https://datatracker.ietf.org/doc/html/draft-ietf-tls-
 hybrid-design-16>.
 [MFSA201473]
 Delignat-Lavaud, A., "RSA Signature Forgery in NSS", 23
 September 2014, <https://www.mozilla.org/en-
 US/security/advisories/mfsa2014-73/>.
 [POODLE] Moeller, B., "This POODLE bites: exploiting the SSL 3.0
 fallback", 14 October 2014,
 <https://security.googleblog.com/2014/10/this-poodle-
 bites-exploiting-ssl-30.html>.
 [PRIVACY] Wachs, M., Scheitle, Q., and G. Carle, "Push away your
 privacy: Precise user tracking based on TLS client
 certificate authentication", IEEE, 2017 Network Traffic
 Measurement and Analysis Conference (TMA) pp. 1-9,
 DOI 10.23919/tma.2017.8002897, June 2017,
 <https://doi.org/10.23919/tma.2017.8002897>.
Acknowledgements
 Thanks to Rifaat Shekh-Yusef, Martin Thomson, and Paul Wouters for
 providing feedback on this document.
Authors' Addresses
 David Benjamin
 Google LLC
 Email: davidben@google.com
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 Andrei Popov
 Microsoft Corp.
 Email: andreipo@microsoft.com
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