Network Working GroupInternet Engineering Task Force (IETF) Y. NirInternet-DraftRequest for Comments: 8031 Check PointIntended status:Category: Standards Track S. JosefssonExpires: April 14, 2017ISSN: 2070-1721 SJDOctober 11,November 2016 Curve25519 and Curve448 forIKEv2the Internet Key Exchange Protocol Version 2 (IKEv2) Key Agreementdraft-ietf-ipsecme-safecurves-05Abstract This document describes the use of Curve25519 and Curve448 for ephemeral key exchange in the Internet Key Exchange(IKEv2) protocol.Protocol Version 2 (IKEv2). Status of This Memo ThisInternet-Draftissubmitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documentsan Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF).Note that other groups may also distribute working documents as Internet-Drafts. The listIt represents the consensus ofcurrent Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents validthe IETF community. It has received public review and has been approved fora maximumpublication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 ofsix monthsRFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may beupdated, replaced, or obsoleted by other documentsobtained atany time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on April 14, 2017.http://www.rfc-editor.org/info/rfc8031. 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 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 1.1. Conventions Used in This Document . . . . . . . . . . . . 2 2. Curve25519&and 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.AcknowledgementsReferences . . . . . . . . . . . . . . . . . . . . . . . . . 57.6.1. Normative References . . . . . . . . . . . . . . . . . . 5 6.2. Informative References . . . . . . .5 7.1. Normative References .. . . . . . . . . . 5 Appendix A. Numerical Example for Curve25519 . . . . . . .5 7.2. Informative References. . . 6 Acknowledgements . . . . . . . . . . . . . .5 Appendix A. Numerical Example for Curve25519. . . . . . . . . .67 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7 1. Introduction The "Elliptic Curves for Security" document [RFC7748] describes two ellipticcurves:curves, Curve25519 and Curve448, as well as the X25519 and X448 functions for performing key agreement using Diffie-Hellman operations with these curves. The curves and functions are designed for both performance and security. Elliptic curve Diffie-Hellman [RFC5903] has been specified for the Internet Key Exchange(IKEv2 - [RFC7296])Protocol Version 2 (IKEv2) [RFC7296] for almost ten years. RFC 5903 and its predecessor specified 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 writeconstant- timeconstant-time implementations that are resilient to time-based side-channel attacks. This document defines two such curves for use inIKE.IKEv2. 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&and Curve448 Implementations of Curve25519 and Curve448 in IKEv2 SHALL follow the steps described in this section. 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 anduse of little-endianusing little- endian encoding) are compliant with [RFC7748]. An ephemeral Diffie-Hellman key exchange using Curve25519 or Curve448 is performed as follows:Eacheach party picks a secret key d uniformly at random 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).pub_peer) This shared secret is used directly as the value denoted g^ir insectionSection 2.14 of RFC 7296. It is 32 octets when Curve25519 isused,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 theSASecurity Association (SA) payload of either an IKE_SA_INIT or a CREATE_CHILD_SA exchange. The valueTBA131 is used for the group defined by Curve25519 and the valueTBA232 is used for the group defined by Curve448. 3.1. Key Exchange Payload The diagram for the Key ExchangePayloadpayload fromsectionSection 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 - ForCurve25519Curve25519, the public key is 32 octets, so the Payload Length field will be40, and for Curve44840. For Curve448, the public key is 56 octets, so the Payload Length field will be 64. o The Diffie-Hellman Group Num isTBA131 forCurve25519,Curve25519 orTBA232 for Curve448. o The Key Exchange Data is the 32 or 56 octets as described insectionSection 6 of[RFC7748][RFC7748]. 3.2. Recipient Tests Receiving and handling of incompatible point formats MUST follow the considerations described insectionSection 5 of [RFC7748]. In particular, receiving entities MUST mask the most-significant bit in the final byte for X25519 (but not X448), and implementations MUST acceptnon- canonicalnoncanonical values. 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 crucialimportanceimportance, especially if the implementation chooses to reuse its ephemeral key pair in many key exchanges for performance reasons. Curve25519 is intended for the ~128-bit security level, comparable to the 256-bit random ECPgroupGroups (group 19) defined in RFC 5903, 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 Curve25519 and Curve448 is fully documented and rigid enough so that independent verification can and has been done. This is widely seen as a securityadvantage, sinceadvantage because it prevents the generating party from maliciously manipulating the parameters. Another family of curves available in IKE that were generated in a fully verifiableway,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 ofpseudo-randompseudorandom prime, it is significantly slower than NIST P-256, which is itself slower than Curve25519. 5. IANA Considerations IANAis requested to assignhas assigned two valuesfromfor the names "Curve25519" and "Curve448" in the IKEv2 "Transform Type 4 - Diffie-Hellman Group Transform IDs"registry, with names "Curve25519" and "Curve448"and has listed this document as the reference. The Recipient Tests field should also point to this document:+--------+------------+---------------------+-----------++--------+------------+-----------------------+-----------+ | Number | Name | Recipient Tests | Reference |+--------+------------+---------------------+-----------++--------+------------+-----------------------+-----------+ |TBA131 | Curve25519 |RFCxxxxRFC 8031, Section 3.2 |RFCxxxxRFC 8031 | |TBA232 | Curve448 |RFCxxxxRFC 8031, Section 3.2 |RFCxxxxRFC 8031 |+--------+------------+---------------------+-----------++--------+------------+-----------------------+-----------+ Table 1: New Transform Type 4 Values7.6. References7.1.6.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March1997.1997, <http://www.rfc-editor.org/info/rfc2119>. [RFC7296]Kivinen, T.,Kaufman, C., Hoffman, P., Nir, Y.,and P.Eronen, P., and T. Kivinen, "Internet Key Exchange Protocol Version 2 (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October2014.2014, <http://www.rfc-editor.org/info/rfc7296>. [RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves for Security", RFC 7748, DOI 10.17487/RFC7748, January2016. 7.2.2016, <http://www.rfc-editor.org/info/rfc7748>. 6.2. Informative References [Curve25519] Bernstein, J., "Curve25519: New Diffie-Hellman Speed Records",LNCSPublic Key Cryptography - PKC 2006, Lecture Notes in Computer Science (LNCS), Vol. 3958, pp. 207-228, DOI 10.1007/11745853_14, February 2006, <http://dx.doi.org/10.1007/11745853_14>. [RFC5903] Fu, D. and J. Solinas, "Elliptic Curve Groups modulo a Prime (ECP Groups) for IKE and IKEv2", RFC 5903, DOI 10.17487/RFC5903, June2010.2010, <http://www.rfc-editor.org/info/rfc5903>. [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, DOI 10.17487/RFC6954, July2013.2013, <http://www.rfc-editor.org/info/rfc6954>. Appendix A. Numerical Example for Curve25519 Suppose we have both the initiator and the responder generating private keys by generating 32 random octets. As usual in IKEv2 and its extension, we will denote Initiator values with the suffix _i and responder values with the suffix _r: random_i = 75 1f b4 30 86 55 b4 76 b6 78 9b 73 25 f9 ea 8c dd d1 6a 58 53 3f f6 d9 e6 00 09 46 4a 5f 9d 94 random_r = 0a 54 64 52 53 29 0d 60 dd ad d0 e0 30 ba cd 9e 55 01 ef dc 22 07 55 a1 e9 78 f1 b8 39 a0 56 88 These numbers need to be fixed by unsetting some bits as described insectionSection 5 of RFC 7748. This affects only the first and last octets of each value: fixed_i = 70 1f b4 30 86 55 b4 76 b6 78 9b 73 25 f9 ea 8c dd d1 6a 58 53 3f f6 d9 e6 00 09 46 4a 5f 9d 54 fixed_r = 08 54 64 52 53 29 0d 60 dd ad d0 e0 30 ba cd 9e 55 01 ef dc 22 07 55 a1 e9 78 f1 b8 39 a0 56 48 The actual private keys are considered to be encoded in little-endian format: d_i = 549D5F4A460900E6D9F63F53586AD1DD8CEAF925739B78B676B4558630B41F70 d_r = 4856A039B8F178E9A1550722DCEF01559ECDBA30E0D0ADDD600D295352645408 The public keys are generated from this using the formula in Section 2: pub_i = X25519(d_i, G) = 48 d5 dd d4 06 12 57 ba 16 6f a3 f9 bb db 74 f1 a4 e8 1c 08 93 84 fa 77 f7 90 70 9f 0d fb c7 66 pub_r = X25519(d_r, G) = 0b e7 c1 f5 aa d8 7d 7e 44 86 62 67 32 98 a4 43 47 8b 85 97 45 17 9e af 56 4c 79 c0 ef 6e ee 25 And this is the value of the Key Exchange Data field in thekey exchangeKey Exchange payload described in Section 3.1. The shared value is calculated as in Section 2: SHARED_SECRET = X25519(d_i, pub_r) = X25519(d_r, pub_i) = c7 49 50 60 7a 12 32 7f-32 04 d9 4b 68 25 bf b0 68 b7 f8 31 9a 9e 37 08-ed 3d 43 ce 81 30 c9 506.Acknowledgements Curve25519 was designed by D. J. Bernstein and the parameters for Curve448 ("Goldilocks") were defined by Mike Hamburg. The specification of algorithms, wireformatformat, and other considerations are documented in RFC 7748 by Adam Langley, Mike Hamburg, and Sean Turner. The example in Appendix A was calculated using the master version of OpenSSL, retrieved on August 4th, 2016. 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