Internet Engineering Task Force L. Velvindron
Internet-Draft cyberstorm.mu
Updates: 5246 7525 (if approved) K. Moriarty
Intended status: Standards Track Dell Technologies
Expires: September 24, 2021 A. Ghedini
Cloudflare Inc.
March 23, 2021
Deprecating MD5 and SHA-1 signature hashes in TLS 1.2
draft-ietf-tls-md5-sha1-deprecate-05
Abstract
The MD5 and SHA-1 hashing algorithms are increasingly vulnerable to
attack and this document deprecates their use in TLS 1.2 digital
signatures. However, this document does not deprecate SHA-1 in HMAC
for record protection. This document updates RFC 5246 and RFC 7525.
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-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 24, 2021.
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
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
Velvindron, et al. Expires September 24, 2021 [Page 1]
Internet-Draft draft-ietf-tls-md5-sha1-deprecate March 2021
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 2
2. Signature Algorithms . . . . . . . . . . . . . . . . . . . . 3
3. Certificate Request . . . . . . . . . . . . . . . . . . . . . 3
4. Server Key Exchange . . . . . . . . . . . . . . . . . . . . . 3
5. Certificate Verify . . . . . . . . . . . . . . . . . . . . . 3
6. Updates to RFC5246 . . . . . . . . . . . . . . . . . . . . . 3
7. Updates to RFC7525 . . . . . . . . . . . . . . . . . . . . . 4
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
9. Security Considerations . . . . . . . . . . . . . . . . . . . 5
10. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 5
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
11.1. Normative References . . . . . . . . . . . . . . . . . . 5
11.2. Informative References . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
The usage of MD5 and SHA-1 for signature hashing in 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 Wang, et. al, attack and the
potential for brute-force attack. In 2016, researchers from INRIA
identified a new class of transcript collision attacks on TLS (and
other protocols) that rely on efficient collision-finding algorithms
on the underlying hash constructions [Transcript-Collision].
Further, in 2017, researchers from Google and CWI Amsterdam
[SHA-1-Collision] proved SHA-1 collision attacks were practical.
This document updates [RFC5246] and [RFC7525] in such a way that MD5
and SHA-1 MUST NOT be used for digital signatures. However, this
document does not deprecate SHA-1 in HMAC for record protection.
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.
Velvindron, et al. Expires September 24, 2021 [Page 2]
Internet-Draft draft-ietf-tls-md5-sha1-deprecate March 2021
2. Signature Algorithms
Clients MUST NOT include MD5 and SHA-1 in the signature_algorithms
extension. If a client does not send a signature_algorithms
extension, then the server MUST abort the handshake and send a
handshake_failure alert, except when digital signatures are not used
(for example, when using PSK ciphers).
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 a client receives a MD5 or SHA-1 signature in a ServerKeyExchange
message it MUST abort the connection with the 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 handshake_failure or
insufficient_security alert.
6. Updates to RFC5246
[RFC5246], The Transport Layer Security (TLS) Protocol Version 1.2,
suggests that implementations can assume support for MD5 and SHA-1 by
their peer. This update changes the suggestion to assume support for
SHA-256 instead, due to MD5 and SHA-1 being deprecated.
In Section 7.4.1.4.1: the text should be revised from:
OLD:
"Note: this is a change from TLS 1.1 where there are no explicit
rules, but as a practical matter one can assume that the peer
supports MD5 and SHA- 1."
NEW:
"Note: This is a change from TLS 1.1 where there are no explicit
rules, but as a practical matter one can assume that the peer
supports SHA-256."
Velvindron, et al. Expires September 24, 2021 [Page 3]
Internet-Draft draft-ietf-tls-md5-sha1-deprecate March 2021
7. Updates to RFC7525
[RFC7525], Recommendations for Secure Use of Transport Layer Security
(TLS) and Datagram Transport Layer Security (DTLS) recommends use of
SHA-256 as a minimum requirement. This update moves the minimum
recommendation to use stronger language deprecating use of both SHA-1
and MD5. The prior text did not explicitly include MD5 or SHA-1; and
this text adds guidance to ensure that these algorithms have been
deprecated.
Section 4.3:
OLD:
When using RSA, servers SHOULD authenticate using certificates with
at least a 2048-bit modulus for the public key. In addition, the use
of the SHA-256 hash algorithm is RECOMMENDED (see [CAB-Baseline] for
more details). Clients SHOULD indicate to servers that they request
SHA-256, by using the "Signature Algorithms" extension defined in TLS
1.2.
NEW:
Servers SHOULD authenticate using certificates with at least a
2048-bit modulus for the public key.
In addition, the use of the SHA-256 hash algorithm is RECOMMENDED;
and SHA-1 or MD5 MUST NOT be used (see [CAB-Baseline] for more
details). Clients MUST indicate to servers that they request SHA-
256, by using the "Signature Algorithms" extension defined in TLS
1.2.
8. IANA Considerations
The document updates the "TLS SignatureScheme" registry to change the
recommended status of SHA-1 based signature schemes to N (not
recommended) as defined by [RFC8447]. The following entries are to
be updated:
+--------+----------------+-------------+--------------------+
| Value | Description | Recommended | Reference |
+--------+----------------+-------------+--------------------+
| 0x0201 | rsa_pkcs1_sha1 | N | [RFC8446] [RFCTBD] |
| 0x0203 | ecdsa_sha1 | N | [RFC8446] [RFCTBD] |
+--------+----------------+-------------+--------------------+
Other entries of the registry remain the same.
Velvindron, et al. Expires September 24, 2021 [Page 4]
Internet-Draft draft-ietf-tls-md5-sha1-deprecate March 2021
9. Security Considerations
Concerns with TLS 1.2 implementations falling back to SHA-1 is an
issue. This document updates the TLS 1.2 specification to deprecate
support for MD5 and SHA-1 for digital signatures. However, this
document does not deprecate SHA-1 in HMAC for record protection.
10. Acknowledgement
The authors would like to thank Hubert Kario for his help in writing
the initial draft. We are also grateful to Daniel Migault, Martin
Thomson and David Cooper for their feedback.
11. References
11.1. 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>.
[RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
2015, <https://www.rfc-editor.org/info/rfc7525>.
[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>.
Velvindron, et al. Expires September 24, 2021 [Page 5]
Internet-Draft draft-ietf-tls-md5-sha1-deprecate March 2021
11.2. Informative References
[CAB-Baseline]
CA/Browser Forum, "Baseline Requirements for the Issuance
and Management of Publicly-Trusted Certificates Version
1.1.6", 2013, <https://www.cabforum.org/documents.html>.
[NISTSP800-131A-R2]
Barker, E. and A. Roginsky, "Transitioning the Use of
Cryptographic Algorithms and Key Lengths", 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", March
2019, <http://shattered.io/static/shattered.pdf>.
[Transcript-Collision]
Bhargavan, K. and G. Leurent, "Transcript Collision
Attacks: Breaking Authentication in TLS, IKE, and SSH",
February 2016, <https://www.mitls.org/downloads/
transcript-collisions.pdf>.
[Wang] Wang, X., Yin, Y., and H. Yu, "Finding Collisions in the
Full SHA-1", 2005.
Authors' Addresses
Loganaden Velvindron
cyberstorm.mu
Rose Hill
MU
Phone: +230 59762817
Email: logan@cyberstorm.mu
Kathleen Moriarty
Dell Technologies
Email: Kathleen.Moriarty.ietf@gmail.com
Velvindron, et al. Expires September 24, 2021 [Page 6]
Internet-Draft draft-ietf-tls-md5-sha1-deprecate March 2021
Alessandro Ghedini
Cloudflare Inc.
Email: alessandro@cloudflare.com
Velvindron, et al. Expires September 24, 2021 [Page 7]