Using Edwards-curve Digital Signature Algorithm (EdDSA) in the Internet Key Exchange (IKEv2)
draft-ietf-ipsecme-eddsa-04

Abstract

This document describes the use of the Edwards-curve digital signature algorithm in the IKEv2 protocol.

Status of This Memo

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Copyright Notice

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Table of Contents

• 1. Introduction
• 1.1. Conventions Used in This Document
• 2. The "Identity" Hash Identifier
• 3. Security Considerations
• 4. IANA Considerations
• 5. Normative References
• Appendix A. ASN.1 Objects
• A.1. ASN.1 Object for Ed25519
• A.2. ASN.1 Object for Ed448
• Author's Address

1. Introduction

The Internet Key Exchange protocol [RFC7296] can use arbitrary signature algorithms as described in [RFC7427]. The latter RFC defines the SIGNATURE_HASH_ALGORITHMS notification where each side of the IKE negotiation lists its supported hash algorithms. This assumes that all signature schemes involve a hashing phase followed by a signature phase. This made sense because most signature algorithms either cannot sign messages bigger than their key or truncate messages bigger than their key.

EdDSA ([RFC8032]) defines signature methods that do not require pre-hashing of the message. Unlike other methods, these accept arbitrary-sized messages, so no pre-hashing is required. These methods are called Ed25519 and Ed448, which respectively use the Edwards 25519 and the Edwards 448 ("Goldilocks") curves. Although that document also defines pre-hashed versions of these algorithm, those versions are not recommended for protocols where the entire to-be-signed message is available at once. See section 8.5 or RFC 8032 for that recommendation.

EdDSA defines the binary format of the signatures that should be used in the "Signature Value" field of the Authentication Data Format in section 3. The CURDLE PKIX document ([I.D-curdle-pkix]) defines the object identifiers (OIDs) for these signature methods. For convenience, these OIDs are repeated in Appendix A.

In order to signal within IKE that no hashing needs to be done, we define a new value in the SIGNATURE_HASH_ALGORITHMS notification, one that indicates that no hashing is performed.

1.1. Conventions Used in This Document

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 "Identity" Hash Identifier

This document defines a new value called "Identity" (value is 5) in the hash algorithm registry for use in the SIGNATURE_HASH_ALGORITHMS notification. Inserting this new value into the notification indicates that the receiver supports at least one signature algorithm that accepts arbitrary-sized messages such as Ed25519 and Ed448.

Ed25519 and Ed448 are only defined with the Identity hash, and MUST NOT be sent to a receiver that has not indicated support for the "Identity" hash.

The pre-hashed versions of Ed25519 and Ed448 (Ed25519ph and Ed448ph respectively) MUST NOT be used in IKE.

3. Security Considerations

The new "Identity" value is needed only for signature algorithms that accept an arbitrary-sized input. It MUST NOT be used if none of the supported and configured algorithms have this property. On the other hand there is no good reason to pre-hash the inputs where the signature algorithm has that property. For this reason implementations MUST have the "Identity" value in the SIGNATURE_HASH_ALGORITHMS notification when EdDSA is supported and configured. Implementations SHOULD NOT have other hash algorithms in the notification if all supported and configured signature algorithms have this property.

4. IANA Considerations

IANA has assigned the value 5 for the algorithm with the name "Identity" in the "IKEv2 Hash Algorithms" registry with this draft as reference.

Upon publication of this document IANA is requested to update the entry with this document as reference.

5. Normative References

Appendix A. ASN.1 Objects

The normative reference for the ASN.1 objects for Ed25519 and Ed448 is in [I.D-curdle-pkix]. They are repeated below for convenience.

A.1. ASN.1 Object for Ed25519

id-Ed25519 OBJECT IDENTIFIER ::= { 1.3.101.112 }

Parameters are absent. Length is 7 bytes.

Binary encoding: 3005 0603 2B65 70

A.2. ASN.1 Object for Ed448

id-Ed448 OBJECT IDENTIFIER ::= { 1.3.101.113 }

Parameters are absent. Length is 7 bytes.

Binary encoding: 3005 0603 2B65 71

Author's Address