Internet Engineering Task Force (IETF)
Request for Comments: 8420
Category: Standards Track
ISSN: 2070-1721

Y. Nir
Dell EMC
August 2018

Using the Edwards-Curve Digital Signature Algorithm (EdDSA) in the

Internet Key Exchange Protocol Version 2 (IKEv2)

Abstract

This document describes the use of the Edwards-curve Digital Signature Algorithm (EdDSA) in the Internet Key Exchange Protocol Version 2 (IKEv2).

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc8420.

Copyright Notice

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 the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

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

1. Introduction

The Internet Key Exchange Protocol Version 2 [RFC7296] can use arbitrary signature algorithms as described in [RFC7427]. [RFC7427] 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 prehashing of the message. Unlike other methods, these accept messages of arbitrary size, so no prehashing is required. These methods are called Ed25519 and Ed448; they use the Edwards 25519 and the Edwards 448 ("Goldilocks") curves, respectively. Although that document also defines prehashed versions of these algorithms, those versions are not recommended for protocols where there is minimal burden in buffering the entire message so as to make it practical to make two passes over the message. This is true of IKEv2. See Section 8.5 of [RFC8032] 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 of RFC 8032. [RFC8410] 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 to indicate 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", "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. The "Identity" Hash Identifier

This document defines a new value called "Identity" (5) in the "IKEv2 Hash Algorithms" 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 messages of arbitrary size 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 prehashed 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 input of arbitrary size. 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 prehash 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 document as reference.

5. 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>.
   
   [RFC7296]  Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
              Kivinen, "Internet Key Exchange Protocol Version 2
              (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
              2014, <https://www.rfc-editor.org/info/rfc7296>.
   
   [RFC7427]  Kivinen, T. and J. Snyder, "Signature Authentication in
              the Internet Key Exchange Version 2 (IKEv2)", RFC 7427,
              DOI 10.17487/RFC7427, January 2015,
              <https://www.rfc-editor.org/info/rfc7427>.
   
   [RFC8032]  Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
              Signature Algorithm (EdDSA)", RFC 8032,
              DOI 10.17487/RFC8032, January 2017,
              <https://www.rfc-editor.org/info/rfc8032>.
   
   [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>.
   
   [RFC8410]  Josefsson, S. and J. Schaad, "Algorithm Identifiers for
              Ed25519, Ed448, X25519, and X448 for Use in the Internet
              X.509 Public Key Infrastructure", RFC 8410,
              DOI 10.17487/RFC8410, August 2018,
              <https://www.rfc-editor.org/info/rfc8410>.

Appendix A. ASN.1 Objects

[RFC8410] is the normative reference for the ASN.1 objects for Ed25519 and Ed448. 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 }