EdDSA and Ed25519 for Transport Layer Security (TLS)
draft-josefsson-tls-eddsa-01

Abstract

This document introduce the public-key signature algorithm EdDSA for use in Transport Layer Security (TLS). With the previous NamedCurve and ECPointFormat assignments for the Curve25519 ECDHE key exchange mechanism, this enables use of Ed25519 in TLS. New Cipher Suites for EdDSA together with AES-GCM and ChaCha20-Poly1305 are introduced here. This is intended to work with any version of TLS and Datagram TLS.

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1. Introduction

TLS [RFC5246] and DTLS [RFC6347] support different key exchange algorithms and authentication mechanisms. In ECC in TLS [RFC4492], key exchange and authentication using ECC is specified, where the NamedCurve and ECPointFormat registries and associated TLS extensions are introduced.

In [I-D.josefsson-tls-curve25519] support for ECDHE key exchange with the Curve25519 curve is added. That document introduces a new NamedCurve value for Curve25519, and a new ECPointFormat value to correspond to the public-key encoding.

This document describes how to use EdDSA and Ed25519 [I-D.josefsson-eddsa-ed25519] as a new authentication mechanism in TLS, reusing the NamedCurve and ECPointFormat values already introduced for Curve25519, and finally specifying new Cipher Suites for Ed25519 with AES-GCM [RFC5288] and ChaCha20-Poly1305 [I-D.mavrogiannopoulos-chacha-tls].

This document is a self-contained alternative to draft-josefsson-tls-eddsa2. This document specify new cipher suites for EdDSA, whereas draft-josefsson-tls-eddsa2 reuse the ECDSA cipher suites for EdDSA. It is an open issue which approach is to be prefered.

1.1. Requirements Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

2. The ECDHE_EDDSA Key Exchange Algorithm

Negotiation of the authentication mechanism is signalled by sending a SignatureAlgorithm value. Here we extend this enumeration for EdDSA.

   enum {
      eddsa(4)
   } SignatureAlgorithm;

EdDSA is suitable for use with TLS [RFC5246] and DTLS [RFC6347].

The new key exchange mechanism ECDHE_EDDSA provides forward secrecy. The key exchange mechanism works just like ECDHE_ECDSA but with ECDSA replaced with EDDSA. Currently the only applicable curve is Curve25519.

The HashAlgorithm value to specify for EdDSA MUST be "none" as the EdDSA signature algorithm does not hash the input before signing.

3. Cipher Suites

The following Cipher Suite values are registered, using the ChaCha20/Poly1305 authenticated encryption with additional data (AEAD) cipher described in [I-D.mavrogiannopoulos-chacha-tls] and the AES Galois Counter Mode (GCM) cipher. The AES-GCM cipher suites use the AEAD algorithms AEAD_AES_128_GCM and AEAD_AES_256_GCM described in [RFC5116]. GCM is used as described in [RFC5288], but see also [RFC5289].

CipherSuite TLS_ECDHE_EDDSA_WITH_CHACHA20_POLY1305  = { 0xCC, 0xB0 }
CipherSuite TLS_ECDHE_EDDSA_WITH_AES_128_GCM_SHA256 = { 0xCC, 0xB1 }
CipherSuite TLS_ECDHE_EDDSA_WITH_AES_256_GCM_SHA384 = { 0xCC, 0xB2 }

The cipher suites are suitable for TLS [RFC5246] and DTLS [RFC6347].

4. IANA Considerations

EdDSA should be registered in the Transport Layer Security (TLS) Parameters [IANA-TLS] registry under "SignatureAlgorithm" as follows.

The follow cipher suites should be registered in the TLS Parameters registry under "TLS Cipher Suite Registry" as follows. They should all be marked as DTLS-OK.

CipherSuite TLS_ECDHE_EDDSA_WITH_CHACHA20_POLY1305  = { 0xCC, 0xB0 }
CipherSuite TLS_ECDHE_EDDSA_WITH_AES_128_GCM_SHA256 = { 0xCC, 0xB1 }
CipherSuite TLS_ECDHE_EDDSA_WITH_AES_256_GCM_SHA384 = { 0xCC, 0xB2 }

5. Security Considerations

The security considerations of TLS [RFC5246], DTLS [RFC6347], ECC in TLS [RFC4492] Curve25519 in TLS [I-D.josefsson-tls-curve25519], EdDSA and Ed25519 [I-D.josefsson-eddsa-ed25519], ChaCha20-Poly1305 [I-D.mavrogiannopoulos-chacha-tls], AES-GCM [RFC5116] an AES-GCM in TLS [RFC5288] are inherited.

As with all cryptographic algorithms, the reader should stay informed about new research insights into the security of the algorithms involved.

While discussed in the EdDSA/Ed25519 specification and papers, we would like to stress the significance of secure implementation of EdDSA/Ed25519. For example, implementations ought to be constant-time to avoid certain attacks.

6. Acknowledgements

Thanks to Klaus Hartke and Nicolas Williams for fixes to the document.

7. References

7.1. Normative References

7.2. Informative References

Author's Address

Table of Contents

• 1. Introduction
• 1.1. Requirements Terminology
• 2. The ECDHE_EDDSA Key Exchange Algorithm
• 3. Cipher Suites
• 4. IANA Considerations
• 5. Security Considerations
• 6. Acknowledgements
• 7. References
• 7.1. Normative References
• 7.2. Informative References
• Author's Address