



Internet Engineering Task Force (IETF)                       D. Benjamin
Request for Comments: 9963                                    Google LLC
Category: Standards Track                                       A. Popov
ISSN: 2070-1721                                          Microsoft Corp.
                                                              April 2026


            Legacy RSASSA-PKCS1-v1_5 Code Points for TLS 1.3

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.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9963.

Copyright Notice

   Copyright (c) 2026 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   in the Revised BSD License.

Table of Contents

   1.  Introduction
   2.  Conventions and Definitions
   3.  PKCS #1 v1.5 SignatureScheme Types
   4.  Security Considerations
   5.  IANA Considerations
   6.  References
     6.1.  Normative References
     6.2.  Informative References
   Acknowledgements
   Authors' Addresses

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]).

   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 [RFC9954].

   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
   CertificateVerify message and are not defined for use in other
   contexts.  In particular, servers that intend 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 the use of these signature algorithms 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 result in 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 in which it is
   necessary to unblock deployment of TLS 1.3.

   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 has created the following entries in the "TLS SignatureScheme"
   registry.  The "Recommended" column has been set to "N", and the
   "Reference" column refers to this document.

                   +========+=========================+
                   | 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/info/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/info/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/info/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/info/rfc8446>.

   [SHS]      NIST, "Secure Hash Standard", NIST FIPS 180-4,
              DOI 10.6028/NIST.FIPS.180-4, August 2015,
              <https://nvlpubs.nist.gov/nistpubs/FIPS/
              NIST.FIPS.180-4.pdf>.

   [TPM12]    Trusted Computing Group, "TPM Main, Part 2 - Structures of
              the TPM", Level 2, Version 1.2, Revision 116, 1 March
              2011, <https://trustedcomputinggroup.org/wp-
              content/uploads/TPM-Main-Part-2-TPM-
              Structures_v1.2_rev116_01032011.pdf>.

   [TPM2]     Trusted Computing Group, "Trusted Platform Module Library,
              Part 1: Architecture", Family 2.0, Level 00, Revision
              01.59, 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)", ITU-T Recommendation X.690, ISO/IEC 8825-1:2021,
              February 2021, <https://www.itu.int/rec/T-REC-X.690>.

6.2.  Informative References

   [MFSA201473]
              Delignat-Lavaud, A., "Mozilla Foundation Security Advisory
              2014-73: RSA Signature Forgery in NSS", 24 September 2014,
              <https://www.mozilla.org/en-US/security/advisories/
              mfsa2014-73/>.

   [POODLE]   Moeller, B., "This POODLE bites: exploiting the SSL 3.0
              fallback", Google Security Blog, 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", 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>.

   [RFC9954]  Stebila, D., Fluhrer, S., and S. Gueron, "Hybrid Key
              Exchange in TLS 1.3", RFC 9954, DOI 10.17487/RFC9954,
              April 2026, <https://www.rfc-editor.org/info/rfc9954>.

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


   Andrei Popov
   Microsoft Corp.
   Email: andreipo@microsoft.com
