Internet DRAFT - draft-ietf-ipsecme-safecurves
draft-ietf-ipsecme-safecurves
Network Working Group Y. Nir
Internet-Draft Check Point
Intended status: Standards Track S. Josefsson
Expires: August 5, 2016 SJD
February 2, 2016
Curve25519 and Curve448 for IKEv2 Key Agreement
draft-ietf-ipsecme-safecurves-01
Abstract
This document describes the use of Curve25519 and Curve448 for
ephemeral key exchange in the Internet Key Exchange (IKEv2) protocol.
Status of This Memo
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This Internet-Draft will expire on August 5, 2016.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Conventions Used in This Document . . . . . . . . . . . . 2
2. Curve25519 & Curve448 . . . . . . . . . . . . . . . . . . . . 2
3. Use and Negotiation in IKEv2 . . . . . . . . . . . . . . . . 3
3.1. Key Exchange Payload . . . . . . . . . . . . . . . . . . 3
3.2. Recipient Tests . . . . . . . . . . . . . . . . . . . . . 4
4. Security Considerations . . . . . . . . . . . . . . . . . . . 4
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 4
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.1. Normative References . . . . . . . . . . . . . . . . . . 5
7.2. Informative References . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 5
1. Introduction
[RFC7748] describes two elliptic curves: Curve25519 and Curve448, as
well as the X25519 and X448 functions for performing key agreement
(Diffie-Hellman) operations with these curves. The curves and
functions are designed for both performance and security.
Almost ten years ago [RFC4753] specified the first elliptic curve
Diffie-Hellman groups for the Internet Key Exchange protocol (IKEv2 -
[RFC7296]). These were the so-called NIST curves. The state of the
art has advanced since then. More modern curves allow faster
implementations while making it much easier to write constant-time
implementations free from time-based side-channel attacks. This
document defines two such curves for use in IKE. See [Curve25519]
for details about the speed and security of the Curve25519 function.
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. Curve25519 & Curve448
All cryptographic computations are done using the X25519 and X448
functions defined in [RFC7748]. All related parameters (for example,
the base point) and the encoding (in particular, pruning the least/
most significant bits and use of little-endian encoding) are
inherited from [RFC7748].
An ephemeral Diffie-Hellman key exchange using Curve25519 or Curve448
goes as follows: Each party picks a secret key d uniformly at random
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and computes the corresponding public key. "X" is used below to
denote either X25519 or X448, and "G" is used to denote the
corresponding base point:
pub_mine = X(d, G)
Parties exchange their public keys (see Section 3.1) and compute a
shared secret:
SHARED_SECRET = X(d, pub_peer).
This shared secret is used directly as the value denoted g^ir in
section 2.14 of RFC 7296. It is 32 octets when Curve25519 is used,
and 56 octets when Curve448 is used.
3. Use and Negotiation in IKEv2
The use of Curve25519 and Curve448 in IKEv2 is negotiated using a
Transform Type 4 (Diffie-Hellman group) in the SA payload of either
an IKE_SA_INIT or a CREATE_CHILD_SA exchange. The value xx is used
for the group defined by Curve25519 and yy is used for the group
defined by Curve448. Both are TBA by IANA.
3.1. Key Exchange Payload
The diagram for the Key Exchange Payload from section 3.4 of RFC 7296
is copied below for convenience:
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Payload |C| RESERVED | Payload Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Diffie-Hellman Group Num | RESERVED |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Key Exchange Data ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
o Payload Length - For Curve25519 the public key is 32 octets, so
the Payload Length field will be 40, and for Curve448 the public
key is 56 octets, so the Payload Length field will be 64.
o The Diffie-Hellman Group Num is xx for Curve25519, or yy for
Curve448 (both TBA by IANA).
o The Key Exchange Data is the 32 or 56 octets as described in
section 6 of [RFC7748]
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3.2. Recipient Tests
This document matches the discussion in [RFC7748] related to
receiving and accepting incompatible point formats. In particular,
receiving entities MUST mask the most-significant bit in the final
byte for X25519 (but not X448), and implementations MUST accept non-
canonical values. See section 5 of [RFC7748] for further discussion.
4. Security Considerations
Curve25519 and Curve448 are designed to facilitate the production of
high-performance constant-time implementations. Implementors are
encouraged to use a constant-time implementation of the functions.
This point is of crucial importance if the implementation chooses to
reuse its supposedly ephemeral key pair for many key exchanges, which
some implementations do in order to improve performance.
Curve25519 is intended for the ~128-bit security level, comparable to
the 256-bit random ECP group (group 19) defined in RFC 4753, also
known as NIST P-256 or secp256r1. Curve448 is intended for the
~224-bit security level.
While the NIST curves are advertised as being chosen verifiably at
random, there is no explanation for the seeds used to generate them.
In contrast, the process used to pick these curves is fully
documented and rigid enough so that independent verification has been
done. This is widely seen as a security advantage, since it prevents
the generating party from maliciously manipulating the parameters.
Another family of curves available in IKE, generated in a fully
verifiable way, is the Brainpool curves [RFC6954]. For example,
brainpoolP256 (group 28) is expected to provide a level of security
comparable to Curve25519 and NIST P-256. However, due to the use of
pseudo-random prime, it is significantly slower than NIST P-256,
which is itself slower than Curve25519.
5. IANA Considerations
IANA is requested to assign two values from the IKEv2 "Transform Type
4 - Diffie-Hellman Group Transform IDs" registry, with names
"Curve25519" and "Curve448" and this document as reference. The
Recipient Tests field should also point to this document.
6. Acknowledgements
Curve25519 was designed by D. J. Bernstein and the parameters for
Curve448 ("Goldilocks") is by Mike Hamburg. The specification of
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algorithms, wire format and other considerations are in RFC 7748 by
Adam Langley, Mike Hamburg, and Sean Turner.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC7296] Kivinen, T., Kaufman, C., Hoffman, P., Nir, Y., and P.
Eronen, "Internet Key Exchange Protocol Version 2
(IKEv2)", RFC 7296, October 2014.
[RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
for Security", RFC 7748, January 2016.
7.2. Informative References
[Curve25519]
Bernstein, J., "Curve25519: New Diffie-Hellman Speed
Records", LNCS 3958, February 2006,
<http://dx.doi.org/10.1007/11745853_14>.
[RFC4753] Fu, D. and J. Solinas, "ECP Groups For IKE and IKEv2",
RFC 4753, January 2007.
[RFC6954] Merkle, J. and M. Lochter, "Using the Elliptic Curve
Cryptography (ECC) Brainpool Curves for the Internet Key
Exchange Protocol Version 2 (IKEv2)", RFC 6954, July 2013.
Authors' Addresses
Yoav Nir
Check Point Software Technologies Ltd.
5 Hasolelim st.
Tel Aviv 6789735
Israel
Email: ynir.ietf@gmail.com
Simon Josefsson
SJD AB
Email: simon@josefsson.org
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