Anonymous ECDH Ciphersuites with Modern Ciphers and Cipher Modes for Transport Layer Security (TLS)
draft-williams-tls-anon-ecdh-modern-cipher-01

Abstract

This document requests the registration and allocation of codepoints for new Transport Layer Security (TLS) ciphersuites with modern ciphers and cipher modes.

Status of This Memo

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This Internet-Draft will expire on September 14, 2014.

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Table of Contents

• 1. Introduction and Motivation
• 2. Security Considerations
• 3. IANA Considerations
• 4. Normative References
• Author's Address

1. Introduction and Motivation

The Transport Layer Security (TLS) [RFC5246] protocol supports a mode where key exchange is done without authenticating either the client nor the server to each other. This is done with ciphersuites using “anonymous” key agreement algorithms.

TLS ciphersuites are distinct sets of key agreement, server authentication, data encryption and integrity protection ciphers (and cipher modes), and pseudo-random functions (PRF). Each set that one might desire to use must be registered in the IANA TLS ciphersuite registry.

In recent years new, more modern ciphersuites have been added, but none with support for Elliptic Curve Diffie-Hellman (ECDH) [RFC4492] key agreement algorithms. This is problematic because ECDH is more efficient (both, in terms of compute and network bandwidth resources), and is generally thought to be more secure than the alternative. Thus implementations that want anonymous connections must trade-off security and performance in key agreement for security and performance in data encryption and integrity protection.

Note that there are good reasons to use anonymous ciphersuites, such as:

• to protect against passive attackers even when there's no way to authenticate the peer;
• to protect the identity of the server and/or the identity of the server as requested by the client in the server name indication (SNI) TLS extension -- here the client initiates a renegotiation with the protection of the outer, unauthenticated connection.

This is not an exhaustive list.

This document requests the allocation -and registration- of ciphersuite codepoints for at least some of the missing ciphersuites, specifically, the sets of ciphersuites resulting from the cartesian product of:

• ECDH for key agreement, with ephemeral keys
• no server authentication
• Advanced Encryption Standard (AES) [AES] for data encryption with the following key sizes:
  • 128-bit keys
  • 256-bit keys

• Two block cipher modes for authenticated encryption with additional data (AEAD):
  • Counter with CBC-MAC Mode (CCM) [RFC3610] [CCM]
  • Galois Counter Mode [GCM]

• Two hash functions for the TLS PRF:
  • SHA256 [RFC4634]
  • SHA384 [RFC4634]

filtered such that block cipher key lengths are matched to PRF hash functions as follows:

• 128-bit key-length ciphers should use SHA256 for the PRF
• 256-bit key-length ciphers should use SHA384 for the PRF

That's four new ciphersuites, see Section 3.

2. Security Considerations

There are no new security considerations here beyond those that are described in each of the documents normatively referenced here.

3. IANA Considerations

Pursuant to the TLS ciphersuite registry's allocation policy (Standards Action or Specification Required [RFC2434]), upon IESG Standards Action publishing this document on the Proposed Standards track, or acceptance by the RFC-Editor of this document for publication on the Informational track, the IANA should assign ciphersuite codepoints to the following ciphersuites, and add them to the TLS ciphersuite registry:

TLS_ECDH_anon_WITH_AES_128_GCM_SHA256

This is anonymous key agreement with ephemeral ECDH keys, with AES-128 in GCM mode, and SHA256 as the hash function for the TLS PRF.

TLS_ECDH_anon_WITH_AES_128_CCM_SHA256

This is anonymous key agreement with ephemeral ECDH keys, with AES-128 in CCM mode, and SHA256 as the hash function for the TLS PRF.

TLS_ECDH_anon_WITH_AES_256_GCM_SHA384

This is anonymous key agreement with ephemeral ECDH keys, with AES-256 in GCM mode, and SHA384 as the hash function for the TLS PRF.

TLS_ECDH_anon_WITH_AES_256_CCM_SHA384

This is anonymous key agreement with ephemeral ECDH keys, with AES-256 in CCM mode, and SHA384 as the hash function for the TLS PRF.

4. Normative References

Author's Address