EdDSA for OpenPGP
draft-koch-eddsa-for-openpgp-00

Abstract

This specification extends OpenPGP with the EdDSA public key algorithm and describes the use of curve Ed25519.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on February 20, 2015.

Copyright Notice

Copyright (c) 2014 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://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.

Table of Contents

• 1. Introduction
• 2. Supported Curves
• 3. Point Format
• 4. Encoding of Public and Private Keys
• 5. Message Encoding
• 6. Curve OID
• 7. Security Considerations
• 8. IANA Considerations
• 9. Acknowledgments
• 10. Normative References
• Appendix A. Test vectors
• A.1. Sample key
• A.2. Sample signature
• Author's Address

1. Introduction

The OpenPGP specification in [RFC4880] defines the RSA, Elgamal, and DSA public key algorithms. [RFC6637] adds support for Elliptic Curve Cryptography and specifies the ECDSA and ECDH algorithms. Due to patent reasons no point compression was defined.

This document specifies how to use the EdDSA public key signature algorithm [ED25519] with the OpenPGP standard. It defines a new signature algorithm named EdDSA and specifies how to use the Ed25519 curve with EdDSA. This algorithm uses a custom point compression method.

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. Supported Curves

This document references the Curve "Ed25519" which is the Edwards form of "Curve25519" and specified in the same paper as the "EdDSA" algorithm ([ED25519]).

Other curves may be used by using a specific OID for the curve and its EdDSA parameters.

The following public key algorithm IDs are added to expand section 9.1 of [RFC4880], "Public-Key Algorithms":

Compliant applications MUST support EdDSA with the curve Ed25519. Applications MAY support other curves as long as a dedicated OID for that curve is used.

3. Point Format

The EdDSA algorithm defines a specific point compression format. To indicate the use of this compression format and to make sure that the key can be represented in the Multiprecision Internet (MPI) format of [RFC4880] the octet string specifying the point is prefixed with the octet 0x40. This encoding is an extension of the encoding given in [RFC6637] which uses 0x04 to indicate an uncompressed point.

For example, the length of a public key for the curve Ed25519 is 263 bit: 7 bit to represent the 0x40 prefix octet and 32 octets for the native value of the public key.

4. Encoding of Public and Private Keys

The following algorithm specific packets are added to Section 5.5.2 of [RFC4880], "Public-Key Packet Formats", to support EdDSA.

Algorithm-Specific Fields for EdDSA keys:

• a variable length field containing a curve OID, formatted as follows:
  • a one-octet size of the following field; values 0 and 0xFF are reserved for future extensions,
  • octets representing a curve OID, defined in Section 6.

• MPI of an EC point representing a public key Q as described under Point Format above.

The following algorithm specific packets are added to Section 5.5.3 of [RFC4880], "Secret-Key Packet Formats", to support EdDSA.

Algorithm-Specific Fields for EdDSA keys:

• an MPI of an integer representing the secret key, which is a scalar of the public EC point.

The version 4 packet format MUST be used.

5. Message Encoding

Section 5.2.3 of [RFC4880], "Version 4 Signature Packet Format" specifies formats. To support EdDSA no change is required, the MPIs representing the R and S value are encoded as MPIs in the same way as done for the DSA and ECDSA algorithms; in particular the Algorithm-Specific Fields for an EdDSA signature are:

 - MPI of EdDSA value r.

 - MPI of EdDSA value s.

Note that the compressed version of R and S as specified for EdDSA ([ED25519]) is used.

The version 3 signature format MUST NOT be used with EdDSA.

Although that algorithm allows arbitrary data as input, its use with OpenPGP requires that a digest of the message is used as input. See section 5.2.4 of [RFC4880], "Computing Signatures" for details. Truncation of the resulting digest is never applied; the resulting digest value is used verbatim as input to the EdDSA algorithm.

6. Curve OID

The EdDSA key parameter curve OID is an array of octets that defines a named curve. The table below specifies the exact sequence of bytes for each named curve referenced in this document:

See [RFC6637] for a description of the OID encoding given in the second and third columns.

7. Security Considerations

The security considerations of [RFC4880] apply accordingly.

The use of EdDSA with the Ed25519 curve is believed to be as strong as other curves of the same size. However, a proper implementation of this algorithm avoids most security problems due to wrong usage. The algorithm does not require a unique random number for each signature created by the same key.

8. IANA Considerations

IANA is requested to assign an algorithm number from the OpenPGP Public-Key Algorithms range, or the "namespace" in the terminology of [RFC5226], that was created by [RFC4880]. See section 2.

[Notes to RFC-Editor: Please remove the table above on publication. It is desirable not to reuse old or reserved algorithms because some existing tools might print a wrong description. A higher number is also an indication for a newer algorithm. As of now 22 is the next free number.]

9. Acknowledgments

The author would like to acknowledge the help of the individuals who kindly voiced their opinions on the IETF OpenPGP and GnuPG mailing lists, in particular, the help of Andrey Jivsov, Jon Callas, and NIIBE Yutaka.

10. Normative References

Appendix A. Test vectors

To help implementing this specification a non-normative example is given. This example assumes that the algorithm id for EdDSA will be 22.

A.1. Sample key

The secret key used for this example is:

D: 1a8b1ff05ded48e18bf50166c664ab023ea70003d78d9e41f5758a91d850f8d2

Note that this is the raw secret key as used as input to the EdDSA signing operation. The key was created on 2014-08-19 14:28:27 and thus the fingerprint of the OpenPGP key is:

   C959 BDBA FA32 A2F8 9A15  3B67 8CFD E121 9796 5A9A

The algorithm specific input parameters without the MPI length headers are:

oid: 2b06010401da470f01

q: 403f098994bdd916ed4053197934e4a87c80733a1280d62f8010992e43ee3b2406

The entire public key packet is thus

   98 33 04 53 f3 5f 0b 16  09 2b 06 01 04 01 da 47
   0f 01 01 07 40 3f 09 89  94 bd d9 16 ed 40 53 19
   79 34 e4 a8 7c 80 73 3a  12 80 d6 2f 80 10 99 2e
   43 ee 3b 24 06

A.2. Sample signature

The signature is created using the sample key over the input data "OpenPGP" on 2015-09-16 12:24:53 and thus the input to the hash function is

m: 4f70656e504750040016080006050255f95f9504ff0000000c

using the SHA-256 hash algorithm yields this digest

d: f6220a3f757814f4c2176ffbb68b00249cd4ccdc059c4b34ad871f30b1740280

which is fed into the EdDSA signature function and yields this signature:

r: 56f90cca98e2102637bd983fdb16c131dfd27ed82bf4dde5606e0d756aed3366

s: d09c4fa11527f038e0f57f2201d82f2ea2c9033265fa6ceb489e854bae61b404

Note that the MPI encoding rules require that the value of S needs to be prefixed with a 0x00 octet. The entire signature packet is thus

   88 5e 04 00 16 08 00 06  05 02 55 f9 5f 95 00 0a
   09 10 8c fd e1 21 97 96  5a 9a f6 22 01 00 56 f9
   0c ca 98 e2 10 26 37 bd  98 3f db 16 c1 31 df d2
   7e d8 2b f4 dd e5 60 6e  0d 75 6a ed 33 66 01 00
   d0 9c 4f a1 15 27 f0 38  e0 f5 7f 22 01 d8 2f 2e
   a2 c9 03 32 65 fa 6c eb  48 9e 85 4b ae 61 b4 04

Author's Address