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.
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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 them 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 display 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