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
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   This Internet-Draft will expire on 5 June 2026.

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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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