Internet Engineering Task Force (IETF) L. Velvindron
Request for Comments: 9155 cyberstorm.mu
Updates: 5246 K. Moriarty
Category: Standards Track CIS
ISSN: 2070-1721 A. Ghedini
Cloudflare Inc.
December 2021
Deprecating MD5 and SHA-1 Signature Hashes in TLS 1.2 and DTLS 1.2
Abstract
The MD5 and SHA-1 hashing algorithms are increasingly vulnerable to
attack, and this document deprecates their use in TLS 1.2 and DTLS
1.2 digital signatures. However, this document does not deprecate
SHA-1 with Hashed Message Authentication Code (HMAC), as used in
record protection. This document updates RFC 5246.
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/rfc9155.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction
1.1. Requirements Language
2. Signature Algorithms
3. Certificate Request
4. Server Key Exchange
5. Certificate Verify
6. IANA Considerations
7. Security Considerations
8. References
8.1. Normative References
8.2. Informative References
Acknowledgements
Authors' Addresses
1. Introduction
The usage of MD5 and SHA-1 for signature hashing in (D)TLS 1.2 is
specified in [RFC5246]. MD5 and SHA-1 have been proven to be
insecure, subject to collision attacks [Wang]. In 2011, [RFC6151]
detailed the security considerations, including collision attacks for
MD5. NIST formally deprecated use of SHA-1 in 2011
[NISTSP800-131A-R2] and disallowed its use for digital signatures at
the end of 2013, based on both the attack described in [Wang] and the
potential for brute-force attack. In 2016, researchers from the
National Institute for Research in Digital Science and Technology
(INRIA) identified a new class of transcript collision attacks on TLS
(and other protocols) that relies on efficient collision-finding
algorithms on the underlying hash constructions
[Transcript-Collision]. Further, in 2017, researchers from Google
and Centrum Wiskunde & Informatica (CWI) Amsterdam [SHA-1-Collision]
proved SHA-1 collision attacks were practical. This document updates
[RFC5246] in such a way that MD5 and SHA-1 MUST NOT be used for
digital signatures. However, this document does not deprecate SHA-1
with HMAC, as used in record protection. Note that the CA/Browser
Forum (CABF) has also deprecated use of SHA-1 for use in certificate
signatures [CABF].
1.1. Requirements Language
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.
2. Signature Algorithms
Clients MUST include the signature_algorithms extension. Clients
MUST NOT include MD5 and SHA-1 in this extension.
3. Certificate Request
Servers SHOULD NOT include MD5 and SHA-1 in CertificateRequest
messages.
4. Server Key Exchange
Servers MUST NOT include MD5 and SHA-1 in ServerKeyExchange messages.
If the client receives a ServerKeyExchange message indicating MD5 or
SHA-1, then it MUST abort the connection with an illegal_parameter
alert.
5. Certificate Verify
Clients MUST NOT include MD5 and SHA-1 in CertificateVerify messages.
If a server receives a CertificateVerify message with MD5 or SHA-1,
it MUST abort the connection with an illegal_parameter alert.
6. IANA Considerations
IANA has updated the "TLS SignatureScheme" registry by changing the
recommended status of SHA-1-based signature schemes to "N" (not
recommended), as defined by [RFC8447]. The following entries have
been updated; other entries in the registry remain the same.
+========+================+=============+=====================+
| Value | Description | Recommended | Reference |
+========+================+=============+=====================+
| 0x0201 | rsa_pkcs1_sha1 | N | [RFC8446] [RFC9155] |
+--------+----------------+-------------+---------------------+
| 0x0203 | ecdsa_sha1 | N | [RFC8446] [RFC9155] |
+--------+----------------+-------------+---------------------+
Table 1
IANA has also updated the reference for the "TLS SignatureAlgorithm"
and "TLS HashAlgorithm" registries to refer to this document in
addition to RFCs 5246 and 8447.
7. Security Considerations
Concerns with (D)TLS 1.2 implementations falling back to SHA-1 is an
issue. This document updates the TLS 1.2 specification [RFC5246] to
deprecate support for MD5 and SHA-1 for digital signatures. However,
this document does not deprecate SHA-1 with HMAC, as used in record
protection.
8. References
8.1. 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>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<https://www.rfc-editor.org/info/rfc5246>.
[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>.
[RFC8447] Salowey, J. and S. Turner, "IANA Registry Updates for TLS
and DTLS", RFC 8447, DOI 10.17487/RFC8447, August 2018,
<https://www.rfc-editor.org/info/rfc8447>.
8.2. Informative References
[CABF] CA/Browser Forum, "Ballot 118 -- SHA-1 Sunset (passed)",
October 2014, <https://cabforum.org/2014/10/16/ballot-118-
sha-1-sunset/>.
[NISTSP800-131A-R2]
Barker, E. and A. Roginsky, "Transitioning the Use of
Cryptographic Algorithms and Key Lengths", NIST Special
Publication 800-131A, Revision 2,
DOI 10.6028/NIST.SP.800-131Ar2, March 2019,
<https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
NIST.SP.800-131Ar2.pdf>.
[RFC6151] Turner, S. and L. Chen, "Updated Security Considerations
for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
RFC 6151, DOI 10.17487/RFC6151, March 2011,
<https://www.rfc-editor.org/info/rfc6151>.
[SHA-1-Collision]
Stevens, M., Bursztein, E., Karpman, P., Albertini, A.,
and Y. Markov, "The First Collision for Full SHA-1", 2017,
<https://eprint.iacr.org/2017/190>.
[Transcript-Collision]
Bhargavan, K. and G. Leurent, "Transcript Collision
Attacks: Breaking Authentication in TLS, IKE, and SSH",
DOI 10.14722/ndss.2016.23418, February 2016,
<https://hal.inria.fr/hal-01244855/document>.
[Wang] Wang, X., Yin, Y., and H. Yu, "Finding Collisions in the
Full SHA-1", DOI 10.1007/11535218_2, 2005,
<https://www.iacr.org/archive/
crypto2005/36210017/36210017.pdf>.
Acknowledgements
The authors would like to thank Hubert Kario for his help in writing
the initial draft version of this document. We are also grateful to
Daniel Migault, Martin Thomson, Sean Turner, Christopher Wood, and
David Cooper for their feedback.
Authors' Addresses
Loganaden Velvindron
cyberstorm.mu
Rose Hill
Mauritius
Phone: +230 59762817
Email: logan@cyberstorm.mu
Kathleen Moriarty
Center for Internet Security
East Greenbush, NY
United States of America
Email: Kathleen.Moriarty.ietf@gmail.com
Alessandro Ghedini
Cloudflare Inc.
Email: alessandro@cloudflare.com
