Internet-Draft Deprecating FFDH July 2021
Bartle, et al. Expires 31 January 2022 [Page]
Workgroup:
Network Working Group
Internet-Draft:
draft-bartle-tls-deprecate-ffdh-00
Published:
Intended Status:
Standards Track
Expires:
Authors:
C. Bartle
Apple, Inc.
N. Aviram
F. Valsorda

Deprecating FFDH Ciphersuites in TLS

Abstract

This document deprecates the use of finite field Diffie Hellman cipher suites and discourages the use of elliptic curve Diffie Hellman cipher suites, both of which have known vulnerabilities or improper security properties when implemented incorrectly.

Discussion Venues

This note is to be removed before publishing as an RFC.

Discussion of this document takes place on the Transport Layer Security Working Group mailing list (tls@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/tls/.

Source for this draft and an issue tracker can be found at https://github.com/cbartle891/draft-deprecate-ffdhe.

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 31 January 2022.

Table of Contents

1. Introduction

TLS supports a variety of key exchange algorithms, including those based on finite field and elliptic curve Diffie Hellman (DH) groups. Each of these also come in ephemeral and non-ephemeral varieties. Non-ephemeral DH algorithms use static DH public keys included in the authenticating peer's certificate; see [RFC4492] for discussion. In contrast, ephemeral DH algorithms use ephemeral DH public keys sent in the handshake and authenticated by the peer's certificate. Ephemeral and non-ephemeral finite field DH algorithms are called DHE and DH, respectively, and ephemeral and non-ephemeral elliptic curve DH algorithms are called ECDHE and ECDH, respectively [RFC4492].

In general, non-ephemeral cipher suites are not recommended due to their lack of forward secrecy. However, as demonstrated by the [Raccoon] attack on finite-field DH, public key reuse, either via non-ephemeral cipher suites or reused keys with ephemeral cipher suites, can lead to timing side channels that may leak connection secrets. For elliptic curve DH, invalid curve attacks broadly follow the same pattern, where a long-lived secret is extracted using side channels [ICA], further demonstrating the security risk of reusing public keys. While both side channels can be avoided in implementations, experience shows that in practice, implementations may fail to thwart such attacks due to the complexity of the required mitigations.

Given these problems, this document updates [RFC4346], [RFC5246], [RFC4162], [RFC6347], [RFC5932], [RFC5288], [RFC6209], [RFC6367], [RFC8422], [RFC5289], and [RFC5469] to deprecate cipher suites with key reuse, prohibiting and discouraging their use.

1.1. Requirements

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. Non-Ephemeral Diffie Hellman

Clients MUST NOT offer non-ephemeral DH cipher suites in TLS 1.2 connections. (Note that TLS 1.0 and 1.1 are deprecated by [RFC8996].) This includes all cipher suites listed in the following table.

Table 1
Ciphersuite Reference
TLS_DH_DSS_EXPORT_WITH_DES40_CBC_SHA [RFC4346]
TLS_DH_DSS_WITH_DES_CBC_SHA [RFC5469]
TLS_DH_DSS_WITH_3DES_EDE_CBC_SHA [RFC5246]
TLS_DH_RSA_EXPORT_WITH_DES40_CBC_SHA [RFC4346]
TLS_DH_RSA_WITH_DES_CBC_SHA [RFC5469]
TLS_DH_RSA_WITH_3DES_EDE_CBC_SHA [RFC5246]
TLS_DH_anon_EXPORT_WITH_RC4_40_MD5 [RFC4346][RFC6347]
TLS_DH_anon_WITH_RC4_128_MD5 [RFC5246][RFC6347]
TLS_DH_anon_EXPORT_WITH_DES40_CBC_SHA [RFC4346]
TLS_DH_anon_WITH_DES_CBC_SHA [RFC5469]
TLS_DH_anon_WITH_3DES_EDE_CBC_SHA [RFC5246]
TLS_DH_DSS_WITH_AES_128_CBC_SHA [RFC5246]
TLS_DH_RSA_WITH_AES_128_CBC_SHA [RFC5246]
TLS_DH_anon_WITH_AES_128_CBC_SHA [RFC5246]
TLS_DH_DSS_WITH_AES_256_CBC_SHA [RFC5246]
TLS_DH_RSA_WITH_AES_256_CBC_SHA [RFC5246]
TLS_DH_anon_WITH_AES_256_CBC_SHA [RFC5246]
TLS_DH_DSS_WITH_AES_128_CBC_SHA256 [RFC5246]
TLS_DH_RSA_WITH_AES_128_CBC_SHA256 [RFC5246]
TLS_DH_DSS_WITH_CAMELLIA_128_CBC_SHA [RFC5932]
TLS_DH_RSA_WITH_CAMELLIA_128_CBC_SHA [RFC5932]
TLS_DH_anon_WITH_CAMELLIA_128_CBC_SHA [RFC5932]
TLS_DH_DSS_WITH_AES_256_CBC_SHA256 [RFC5246]
TLS_DH_RSA_WITH_AES_256_CBC_SHA256 [RFC5246]
TLS_DH_anon_WITH_AES_128_CBC_SHA256 [RFC5246]
TLS_DH_anon_WITH_AES_256_CBC_SHA256 [RFC5246]
TLS_DH_DSS_WITH_CAMELLIA_256_CBC_SHA [RFC5932]
TLS_DH_RSA_WITH_CAMELLIA_256_CBC_SHA [RFC5932]
TLS_DH_anon_WITH_CAMELLIA_256_CBC_SHA [RFC5932]
TLS_DH_DSS_WITH_SEED_CBC_SHA [RFC4162]
TLS_DH_RSA_WITH_SEED_CBC_SHA [RFC4162]
TLS_DH_anon_WITH_SEED_CBC_SHA [RFC4162]
TLS_DH_RSA_WITH_AES_128_GCM_SHA256 [RFC5288]
TLS_DH_RSA_WITH_AES_256_GCM_SHA384 [RFC5288]
TLS_DH_DSS_WITH_AES_128_GCM_SHA256 [RFC5288]
TLS_DH_DSS_WITH_AES_256_GCM_SHA384 [RFC5288]
TLS_DH_anon_WITH_AES_128_GCM_SHA256 [RFC5288]
TLS_DH_anon_WITH_AES_256_GCM_SHA384 [RFC5288]
TLS_DH_DSS_WITH_CAMELLIA_128_CBC_SHA256 [RFC5932]
TLS_DH_RSA_WITH_CAMELLIA_128_CBC_SHA256 [RFC5932]
TLS_DH_anon_WITH_CAMELLIA_128_CBC_SHA256 [RFC5932]
TLS_DH_DSS_WITH_CAMELLIA_256_CBC_SHA256 [RFC5932]
TLS_DH_RSA_WITH_CAMELLIA_256_CBC_SHA256 [RFC5932]
TLS_DH_anon_WITH_CAMELLIA_256_CBC_SHA256 [RFC5932]
TLS_DH_DSS_WITH_ARIA_128_CBC_SHA256 [RFC6209]
TLS_DH_DSS_WITH_ARIA_256_CBC_SHA384 [RFC6209]
TLS_DH_RSA_WITH_ARIA_128_CBC_SHA256 [RFC6209]
TLS_DH_RSA_WITH_ARIA_256_CBC_SHA384 [RFC6209]
TLS_DH_anon_WITH_ARIA_128_CBC_SHA256 [RFC6209]
TLS_DH_anon_WITH_ARIA_256_CBC_SHA384 [RFC6209]
TLS_DH_RSA_WITH_ARIA_128_GCM_SHA256 [RFC6209]
TLS_DH_RSA_WITH_ARIA_256_GCM_SHA384 [RFC6209]
TLS_DH_DSS_WITH_ARIA_128_GCM_SHA256 [RFC6209]
TLS_DH_DSS_WITH_ARIA_256_GCM_SHA384 [RFC6209]
TLS_DH_anon_WITH_ARIA_128_GCM_SHA256 [RFC6209]
TLS_DH_anon_WITH_ARIA_256_GCM_SHA384 [RFC6209]
TLS_DH_RSA_WITH_CAMELLIA_128_GCM_SHA256 [RFC6367]
TLS_DH_RSA_WITH_CAMELLIA_256_GCM_SHA384 [RFC6367]
TLS_DH_DSS_WITH_CAMELLIA_128_GCM_SHA256 [RFC6367]
TLS_DH_DSS_WITH_CAMELLIA_256_GCM_SHA384 [RFC6367]
TLS_DH_anon_WITH_CAMELLIA_128_GCM_SHA256 [RFC6367]
TLS_DH_anon_WITH_CAMELLIA_256_GCM_SHA384 [RFC6367]

Clients SHOULD NOT offer non-ephemeral ECDH cipher suites in TLS 1.2 connections. (Note that TLS 1.0 and 1.1 are deprecated by [RFC8996].) This includes all cipher suites listed in the following table.

Table 2
Ciphersuite Reference
TLS_ECDH_ECDSA_WITH_NULL_SHA [RFC8422]
TLS_ECDH_ECDSA_WITH_RC4_128_SHA [RFC8422][RFC6347]
TLS_ECDH_ECDSA_WITH_3DES_EDE_CBC_SHA [RFC8422]
TLS_ECDH_ECDSA_WITH_AES_128_CBC_SHA [RFC8422]
TLS_ECDH_ECDSA_WITH_AES_256_CBC_SHA [RFC8422]
TLS_ECDH_RSA_WITH_NULL_SHA [RFC8422]
TLS_ECDH_RSA_WITH_RC4_128_SHA [RFC8422][RFC6347]
TLS_ECDH_RSA_WITH_3DES_EDE_CBC_SHA [RFC8422]
TLS_ECDH_RSA_WITH_AES_128_CBC_SHA [RFC8422]
TLS_ECDH_RSA_WITH_AES_256_CBC_SHA [RFC8422]
TLS_ECDH_anon_WITH_NULL_SHA [RFC8422]
TLS_ECDH_anon_WITH_RC4_128_SHA [RFC8422][RFC6347]
TLS_ECDH_anon_WITH_3DES_EDE_CBC_SHA [RFC8422]
TLS_ECDH_anon_WITH_AES_128_CBC_SHA [RFC8422]
TLS_ECDH_anon_WITH_AES_256_CBC_SHA [RFC8422]
TLS_ECDH_ECDSA_WITH_AES_128_CBC_SHA256 [RFC5289]
TLS_ECDH_ECDSA_WITH_AES_256_CBC_SHA384 [RFC5289]
TLS_ECDH_RSA_WITH_AES_128_CBC_SHA256 [RFC5289]
TLS_ECDH_RSA_WITH_AES_256_CBC_SHA384 [RFC5289]
TLS_ECDH_ECDSA_WITH_AES_128_GCM_SHA256 [RFC5289]
TLS_ECDH_ECDSA_WITH_AES_256_GCM_SHA384 [RFC5289]
TLS_ECDH_RSA_WITH_AES_128_GCM_SHA256 [RFC5289]
TLS_ECDH_RSA_WITH_AES_256_GCM_SHA384 [RFC5289]
TLS_ECDH_ECDSA_WITH_ARIA_128_CBC_SHA256 [RFC6209]
TLS_ECDH_ECDSA_WITH_ARIA_256_CBC_SHA384 [RFC6209]
TLS_ECDH_RSA_WITH_ARIA_128_CBC_SHA256 [RFC6209]
TLS_ECDH_RSA_WITH_ARIA_256_CBC_SHA384 [RFC6209]
TLS_ECDH_ECDSA_WITH_ARIA_128_GCM_SHA256 [RFC6209]
TLS_ECDH_ECDSA_WITH_ARIA_256_GCM_SHA384 [RFC6209]
TLS_ECDH_RSA_WITH_ARIA_128_GCM_SHA256 [RFC6209]
TLS_ECDH_RSA_WITH_ARIA_256_GCM_SHA384 [RFC6209]
TLS_ECDH_ECDSA_WITH_CAMELLIA_128_CBC_SHA256 [RFC6367]
TLS_ECDH_ECDSA_WITH_CAMELLIA_256_CBC_SHA384 [RFC6367]
TLS_ECDH_RSA_WITH_CAMELLIA_128_CBC_SHA256 [RFC6367]
TLS_ECDH_RSA_WITH_CAMELLIA_256_CBC_SHA384 [RFC6367]
TLS_ECDH_ECDSA_WITH_CAMELLIA_128_GCM_SHA256 [RFC6367]
TLS_ECDH_ECDSA_WITH_CAMELLIA_256_GCM_SHA384 [RFC6367]
TLS_ECDH_RSA_WITH_CAMELLIA_128_GCM_SHA256 [RFC6367]
TLS_ECDH_RSA_WITH_CAMELLIA_256_GCM_SHA384 [RFC6367]

3. Ephemeral Diffie Hellman

Clients and servers MUST NOT reuse ephemeral DHE or ECDHE public keys across TLS connections for all existing (and future) TLS versions. Doing so invalidates forward secrecy properties of these connections. In the case of DHE (finite field DH) cipher suites, such reuse may also lead to vulnerabilities such as those used in the [Raccoon] attack. See Section 5 for related discussion.

4. IANA Considerations

This document makes no requests to IANA. All cipher suites listed in Section 2 are already marked as not recommended in the "TLS Cipher Suites" registry.

5. Security Considerations

Non-ephemeral finite field DH cipher suites (TLS_DH_*), as well as ephemeral key reuse for finite field DH cipher suites, are prohibited due to the [Raccoon] attack. Both are already considered bad practice since they do not provide forward secrecy. However, Raccoon revealed that timing side channels in processing TLS premaster secrets may be exploited to reveal the encrypted premaster secret.

For non-ephemeral elliptic curve DH cipher suites, invalid curve attacks similarly exploit side channels to extract the secret from a long-lived public key. These attacks have been shown to be practical against real-world TLS implementations [ICA]. Therefore, this document discourages the reuse of elliptic curve DH public keys.

6. Acknowledgments

This document was inspired by discussion on the TLS WG mailing list and a suggestion by Filippo Valsorda following the release of the [Raccoon] attack. Thanks to Christopher A. Wood for writing up the initial draft of this document.

7. References

7.1. Normative References

[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/info/rfc2119>.
[RFC4162]
Lee, H.J., Yoon, J.H., and J.I. Lee, "Addition of SEED Cipher Suites to Transport Layer Security (TLS)", RFC 4162, DOI 10.17487/RFC4162, , <https://www.rfc-editor.org/info/rfc4162>.
[RFC4346]
Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.1", RFC 4346, DOI 10.17487/RFC4346, , <https://www.rfc-editor.org/info/rfc4346>.
[RFC5246]
Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/RFC5246, , <https://www.rfc-editor.org/info/rfc5246>.
[RFC5288]
Salowey, J., Choudhury, A., and D. McGrew, "AES Galois Counter Mode (GCM) Cipher Suites for TLS", RFC 5288, DOI 10.17487/RFC5288, , <https://www.rfc-editor.org/info/rfc5288>.
[RFC5289]
Rescorla, E., "TLS Elliptic Curve Cipher Suites with SHA-256/384 and AES Galois Counter Mode (GCM)", RFC 5289, DOI 10.17487/RFC5289, , <https://www.rfc-editor.org/info/rfc5289>.
[RFC5469]
Eronen, P., Ed., "DES and IDEA Cipher Suites for Transport Layer Security (TLS)", RFC 5469, DOI 10.17487/RFC5469, , <https://www.rfc-editor.org/info/rfc5469>.
[RFC5932]
Kato, A., Kanda, M., and S. Kanno, "Camellia Cipher Suites for TLS", RFC 5932, DOI 10.17487/RFC5932, , <https://www.rfc-editor.org/info/rfc5932>.
[RFC6209]
Kim, W., Lee, J., Park, J., and D. Kwon, "Addition of the ARIA Cipher Suites to Transport Layer Security (TLS)", RFC 6209, DOI 10.17487/RFC6209, , <https://www.rfc-editor.org/info/rfc6209>.
[RFC6347]
Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, , <https://www.rfc-editor.org/info/rfc6347>.
[RFC6367]
Kanno, S. and M. Kanda, "Addition of the Camellia Cipher Suites to Transport Layer Security (TLS)", RFC 6367, DOI 10.17487/RFC6367, , <https://www.rfc-editor.org/info/rfc6367>.
[RFC8174]
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <https://www.rfc-editor.org/info/rfc8174>.
[RFC8422]
Nir, Y., Josefsson, S., and M. Pegourie-Gonnard, "Elliptic Curve Cryptography (ECC) Cipher Suites for Transport Layer Security (TLS) Versions 1.2 and Earlier", RFC 8422, DOI 10.17487/RFC8422, , <https://www.rfc-editor.org/info/rfc8422>.
[RFC8996]
Moriarty, K. and S. Farrell, "Deprecating TLS 1.0 and TLS 1.1", BCP 195, RFC 8996, DOI 10.17487/RFC8996, , <https://www.rfc-editor.org/info/rfc8996>.

7.2. Informative References

[ICA]
Jager, T., Schwenk, J., and J. Somorovsky, "Practical invalid curve attacks on TLS-ECDH", , <https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.704.7932&rep=rep1&type=pdf>.
[Raccoon]
Merget, R., Brinkmann, M., Aviram, N., Somorovsky, J., Mittmann, J., and J. Schwenk, "Raccoon Attack: Finding and Exploiting Most-Significant-Bit-Oracles in TLS-DH(E)", , <https://raccoon-attack.com/RacoonAttack.pdf>.
[RFC4492]
Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B. Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites for Transport Layer Security (TLS)", RFC 4492, DOI 10.17487/RFC4492, , <https://www.rfc-editor.org/info/rfc4492>.

Authors' Addresses

Carrick Bartle
Apple, Inc.
Nimrod Aviram
Filippo Valsorda