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Network Working Group                                            W. Koch
Internet-Draft                                                  g10 Code
Updates: 4880 (if approved)                            February 28, 2016
Intended status: Informational
Expires: August 31, 2016


                           EdDSA for OpenPGP
                    draft-koch-eddsa-for-openpgp-04

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 August 31, 2016.

Copyright Notice

   Copyright (c) 2016 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
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.





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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Supported Curves  . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Point Format  . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Encoding of Public and Private Keys . . . . . . . . . . . . .   3
   5.  Message Encoding  . . . . . . . . . . . . . . . . . . . . . .   4
   6.  Curve OID . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   5
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   5
     10.2.  Informative References . . . . . . . . . . . . . . . . .   6
   Appendix A.  Test vectors . . . . . . . . . . . . . . . . . . . .   6
     A.1.  Sample key  . . . . . . . . . . . . . . . . . . . . . . .   6
     A.2.  Sample signature  . . . . . . . . . . . . . . . . . . . .   7
   Appendix B.  Point compression flag bytes . . . . . . . . . . . .   7
   Appendix C.  Changes since -03  . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

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 [I-D.irtf-cfrg-eddsa] 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.  There are three main advantages of the EdDSA
   algorithm: It does not require the use of a unique random number for
   each signature, there are no padding or truncation issues as with
   ECDSA, and it is more resilient to side-channel attacks.

   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" ([RFC7748]) and originally specified in the same
   paper as the "EdDSA" algorithm ([ED25519]).  For the full
   specification see [I-D.irtf-cfrg-eddsa].




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   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":

                  +-------+-----------------------------+
                  | ID    | Description of Algorithm    |
                  +-------+-----------------------------+
                  | TBD1  | EdDSA public key algorithm  |
                  +-------+-----------------------------+

   Compliant applications MUST support EdDSA with the curve Ed25519.
   Applications MAY support other curves as long as a dedicated OID for
   using that curve with EdDSA 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 Integer (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:

   o  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.

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





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   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:

   o  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
   ([I-D.irtf-cfrg-eddsa]) 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:

   +------------------------+------+------------------------+----------+
   | OID                    | Len  | Encoding in hex format | Name     |
   +------------------------+------+------------------------+----------+
   | 1.3.6.1.4.1.11591.15.1 | 9    | 2B 06 01 04 01 DA 47   | Ed25519  |
   |                        |      | 0F 01                  |          |
   +------------------------+------+------------------------+----------+

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



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7.  Security Considerations

   The security considerations of [RFC4880] apply accordingly.

   Although technically possible the use of EdDSA with digest algorithms
   weaker than SHA-256 (e.g.  SHA-1) is not suggested.

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.

           +-------+-----------------------------+------------+
           | ID    | Algorithm                   | Reference  |
           +-------+-----------------------------+------------+
           | TBD1  | EdDSA public key algorithm  | This doc   |
           +-------+-----------------------------+------------+

   [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.  References

10.1.  Normative References

   [I-D.irtf-cfrg-eddsa]
              Josefsson, S. and I. Liusvaara, "Edwards-curve Digital
              Signature Algorithm (EdDSA)", draft-irtf-cfrg-eddsa-02
              (work in progress), January 2016.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4880]  Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
              Thayer, "OpenPGP Message Format", RFC 4880, November 2007.





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   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [RFC6637]  Jivsov, A., "Elliptic Curve Cryptography (ECC) in
              OpenPGP", RFC 6637, June 2012.

10.2.  Informative References

   [ED25519]  Bernstein, D., Duif, N., Lange, T., Schwabe, P., and B.
              Yang, "High-speed high-security signatures", Journal of
              Cryptographic Engineering Volume 2, Issue 2, pp. 77-89,
              September 2011,
              <http://dx.doi.org/10.1007/s13389-012-0027-1>.

   [RFC7748]  Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
              for Security", RFC 7748, DOI 10.17487/RFC7748, January
              2016, <http://www.rfc-editor.org/info/rfc7748>.

Appendix A.  Test vectors

   For help implementing this specification a non-normative example is
   given.  This example assumes that the algorithm id for EdDSA (TBD1)
   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 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:






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      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 the digest:

   d: f6220a3f757814f4c2176ffbb68b00249cd4ccdc059c4b34ad871f30b1740280

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

   r: 56f90cca98e2102637bd983fdb16c131dfd27ed82bf4dde5606e0d756aed3366

   s: d09c4fa11527f038e0f57f2201d82f2ea2c9033265fa6ceb489e854bae61b404

   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

Appendix B.  Point compression flag bytes

   This specification introduces the new flag byte 0x40 to indicate the
   point compression format.  The value has been chosen so that the high
   bit is not cleared and thus to avoid accidental sign extension.  Two
   other values might also be interesting for other ECC specifications:

     Flag  Description
     ----  -----------
     0x04  Standard flag for uncompressed format
     0x40  Native point format of the curve follows
     0x41  Only X coordinate follows.
     0x42  Only Y coordinate follows.





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Appendix C.  Changes since -03

   o  Changed reference for EdDSA to CFRG draft.

Author's Address

   Werner Koch
   g10 Code

   Email: wk@gnupg.org
   URI:   https://g10code.com








































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