INTERNET-DRAFT D. Kuegler Intended Status: informational (BSI) Expires: November 4, 2010 May 3, 2010 Password Authenticated Connection Establishment with IKEv2 draft-kuegler-ipsecme-pace-ikev2-00.txt Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." 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Kuegler Expires November 4, 2010 [Page 1] INTERNET DRAFT PACE May 3, 2010 Abstract This document provides an adaptation of PACE (Password Authenticated Connection Establishment) to the setting of IKEv2 to allow for a password-based authentication mode. Table of Contents 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1 Security Criteria . . . . . . . . . . . . . . . . . . . . . 3 1.2 Intellectual Property Criteria . . . . . . . . . . . . . . 4 1.3 MISC Criteria . . . . . . . . . . . . . . . . . . . . . . . 4 1.4 Terminology . . . . . . . . . . . . . . . . . . . . . . . . 4 2 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3 Protocol Sequence . . . . . . . . . . . . . . . . . . . . . . . 5 3.1 The IKE_SA_INIT Exchange . . . . . . . . . . . . . . . . . 6 3.2 The IKE_PACE Exchange . . . . . . . . . . . . . . . . . . . 7 3.3 The IKE_PACE_AUTH Exchange . . . . . . . . . . . . . . . . 7 4 Mapping . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 4.1 Diffie Hellman . . . . . . . . . . . . . . . . . . . . . . 8 4.2 Elliptic Curve Diffie Hellman . . . . . . . . . . . . . . . 8 4.3 Validation . . . . . . . . . . . . . . . . . . . . . . . . 8 5 Security Considerations . . . . . . . . . . . . . . . . . . . . 8 6 IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 9 7 References . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 7.1 Normative References . . . . . . . . . . . . . . . . . . . 9 7.2 Informative References . . . . . . . . . . . . . . . . . . 9 Author's Addresses . . . . . . . . . . . . . . . . . . . . . . . 10 Kuegler Expires November 4, 2010 [Page 2] INTERNET DRAFT PACE May 3, 2010 1 Introduction This is a preliminary draft describing the possible integration of the password-based authentication protocol PACE [TR03110] in IKEv2. PACE establishes an mutually authenticated (and encrypted) channel between two parties based on weak (short) passwords. PACE provides strong session keys that are independent of the strength of the password. Compared to other protocols aiming at similar goals, PACE has several advantages. PACE was designed to be free of patents, and to allow for a high level of flexibility with respect to cryptographic algorithms, e.g. it can be implemented based on standard Diffie Hellman as well as Elliptic Curve Diffie Hellman without any restrictions on the mathematic group to be used other than the requirement that the group is cryptographically secure. The protocol itself is also proven to be cryptographically secure [PACEsec]. The integration aims at keeping as much as possible of IKEv2 unchanged, i.e. the mechanisms used to establish session keys as provided by IKEv2 are completely maintained. NOTE: Due to the adaptations of the original protocol [TR03110], the proof [PACEsec] requires some modifications, that will be provided once the details of the integration are fixed. To support the selection of a password-based protocol for inclusion in IKEv2, a number of criteria are provided in [I-D.harkins-ipsecme- pake-criteria]. In the following sections, those criteria are applied to the PACE protocol. 1.1 Security Criteria SEC1: PACE is a zero knowledge protocol. SEC2: The protocol supports perfect forward secrecy and is resistant to replay attacks. SEC3: The identity protection provided by IKEv2 remains unchanged. SEC4: Any cryptographically secure Diffie-Hellman group can be used. SEC5: The protocol is proven secure in the Bellare-Pointcheval- Rogaway model. SEC6: Strong session keys are generated. SEC7: A transform of the password can be used instead of the password itself. Kuegler Expires November 4, 2010 [Page 3] INTERNET DRAFT PACE May 3, 2010 1.2 Intellectual Property Criteria IPR1: The first draft of [TR03110] was published on May 21, 2007. IPR2: BSI has developed PACE aiming to be free of patents. BSI has not applied for a patent on PACE. IPR3: The protocol itself is believed to be free of IPR. 1.3 MISC Criteria MISC1: One additional exchange is required. MISC2: The protocol requires the following operations per entity: o one key derivation from the password, o one symmetric encryption or decryption, o one multi-exponentiation for the mapping, o one exponentiation for the key pair generation, o one exponentiation for the shared secret calculation, and o two symmetric authentications (generation & verification). MISC3: The performance is independent of the type/size of password. MISC4: Internationalization of character-based passwords is supported. MISC5: The protocol uses the same group as negotiated for IKEv2. MISC6: The protocol fits into the request/response nature of IKE. MISC7: The password-based symmetric encryption must be additionally negotiated. MISC8: Neither trusted third parties nor clock synchronization are required. MISC9: Only general cryptographic primitives are required. MISC10: Any secure variant of Diffie Hellman (e.g. standard or Elliptic Curve) can be used. MISC11: The protocol can be implemented easily based on existing cryptographic primitives. 1.4 Terminology 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 RFC 2119 [RFC2119]. 2 Overview The following notation is used in this draft: E() Symmetric encryption D() Symmetric decryption KA() Key agreement Map() Mapping function Pwd Shared Password Kuegler Expires November 4, 2010 [Page 4] INTERNET DRAFT PACE May 3, 2010 KPwd Symmetric key derived from a password Pwd G Static group generator GE Ephemeral group generator CIPH Enciphered nonce PKEi Ephemeral public key of the initiator SKEi Ephemeral secret key of the initiator PKEr Ephemeral public key of the responder SKEr Ephemeral secret key of the responder AUTH Authentication token At a high level the following steps are performed by the initiator and the responder. They result in exchanges IKE_PACE and IKE_PACE_AUTH as described in Section 3 that are performed directly after IKE_SA_INIT and are partially replacing IKE_AUTH. 1. The initiator randomly and uniformly chooses a nonce, encrypts the nonce, and sends the ciphertext CIPH to the responder. 2. The responder recovers the plaintext with the help of the shared password Pwd. 3. The initiator and the responder perform the following steps: a) A mapping function Map() is used to derive an ephemeral generator GE = Map(G,s) from the exchanged nonce s and the generator G of the used group. b) They perform an anonymous Diffie-Hellman key agreement based on the ephemeral generator and compute the shared secret PACESharedSecret = KA(SKEi, PKEi, GE) = KA(SKEr, PKEr, GE). During the Diffie-Hellman key agreement, each party SHOULD check that the two public keys PKEi and PKEr differ. c) They generate, exchange, and verify the authentication token AUTH using the shared secret PACESharedSecret. 3 Protocol Sequence The protocol consists of three exchanges, IKE_SA_INIT, IKE_PACE, and IKE_PACE_AUTH as follows: Kuegler Expires November 4, 2010 [Page 5] INTERNET DRAFT PACE May 3, 2010 Initiator Responder --------- --------- IKE_SA_INIT: HDR, SAi1, KEi, Ni, N(PACE_SUPPORTED) -> <- HDR, SAr1, KEr, Nr, N(PACE_SUPPORTED) IKE_PACE: HDR, SK{IDi, [IDr,], SAi2, TSi, TSr, CIPH, PKEi} -> <- HDR, SK{IDr, PKEr} IKE_PACE_AUTH: HDR, SK{AUTH} -> <- HDR, SK{AUTH, SAr2, TSi, TSr} 3.1 The IKE_SA_INIT Exchange Within this exchange the initiator and the responder negotiate the use of PACE by exchanging a PACE_SUPPORTED notifications. If PACE is supported the algorithms negotiated in SAi1 and SAr1 are also used for the execution of PACE, i.e. the key agreement protocol (standard Diffie Hellman or Elliptic Curve Diffie Hellman), the group to be used, and the authentication algorithm. In addition, a new transform type is used to negotiate the password- based encryption of the nonce. This transform includes the cipher and it's mode of operation as well as the bit length of the nonce to be used. NOTE: The password-based encryption MUST NOT introduce any redundancy that allows for excluding possible plaintexts with a brute-force attack on the password. An example for a suitable password based encryption is a block cipher in ECB mode using key KPwd = prf+(Pwd, "PACE Password") which is derived from the shared password Pwd. In this case the size of the nonce SHALL be the block size of the cipher. Kuegler Expires November 4, 2010 [Page 6] INTERNET DRAFT PACE May 3, 2010 3.2 The IKE_PACE Exchange The initiator selects a nonce s as binary bit string. The nonce MUST be chosen randomly and uniformly, the length of the nonce SHALL be according to the negotiated value. The nonce is encrypted to CIPH = E(Pwd, s) using the negotiated password based-encryption using the password Pwd. NOTE: The is no other requirement on the generation of the nonce other than the fact that it MUST be random and uniformly distributed. The initiator maps the nonce to an ephemeral generator of the group as described in Section 4, chooses randomly and uniformly an ephemeral key pair (SKEi,PKEi) based on the ephemeral generator and finally generates the payloads CIPH containing the encrypted nonce and PKEi containing the ephemeral public key. The responder decrypts the received encrypted nonce s = D(Pwd, CIPH), performs the mapping and randomly and uniformly chooses an ephemeral key pair (SKEr,PKEr) based on the ephemeral generator. The responder generates the PKEr payload containing the ephemeral public key. 3.3 The IKE_PACE_AUTH Exchange The initiator and the responder calculate the shared secret PACESharedSecret, derive the authentication key, and calculate the authentication token AUTH. The initiator calculates: AUTH = prf(prf+(PACESharedSecret, "PACE Authentication i" | IDr | IDi), PKEr) The responder calculates: AUTH = prf(prf+(PACESharedSecret, "PACE Authentication r" | IDi | IDr), PKEi) The authentication token are then exchanged and verified by the other party. 4 Mapping The mapping is based on a second anonymous Diffe-Hellman key agreement protocol to create a shared secret which is used together Kuegler Expires November 4, 2010 [Page 7] INTERNET DRAFT PACE May 3, 2010 with the exchanged nonce to calculate a common secret generator of the group. While in [TR03110] the generation of the shared secret is part of the mapping, in the setting of IKEv2 a shared secret SASharedSecret has already been generated as part of the IKE_SA_INIT step. This shared secret SHALL be reused. Let G, and GE be the generator of the group, and the calculated ephemeral generator, respectively. 4.1 Diffie Hellman The function Map:G->GE is defined as GE = G^s * SASharedSecret. Note that the protocol will fail if G^s = 1/SASharedSecret. If s is chosen randomly, this event occurs with negligible probability. Implementations that detect such a failure SHOULD choose s again. 4.2 Elliptic Curve Diffie Hellman The function Map:G->GE is defined as GE = s*G + SASharedSecret. Note that the protocol will fail if s*G = -SharedSecret. If s is chosen randomly, this event occurs with negligible probability. Implementations that detect such a failure SHOULD choose s again. 4.3 Validation Implementations MUST verify that the shared secrets SASharedSecret and PACESharedSecret are elements of the group generated by G to prevent small subgroup attacks. It is RECOMMENDED to use the public key validation method (or an Elliptic Curve equivalent) described in Section 2.1.5 of [RFC2631]. For Elliptic Curves compatible cofactor multiplication [TR03111] MAY be used instead of public key validation. In this case implementations MUST check that PACESharedSecret is not the point at infinity. Any failure in the validation SHALL be interpreted as an attack. 5 Security Considerations Kuegler Expires November 4, 2010 [Page 8] INTERNET DRAFT PACE May 3, 2010 TODO: PACE is cryptographically proven secure in [PACEsec]. The application of PACE in IKEv2 however is however a slightly different setting that requires a modification of the proof. A new proof will be provided once the details of the integration are fixed. 6 IANA Considerations TBD: One notification (N(PACE_SUPPORTED)), one transform type (Password-based Encryption), two payloads (CIPH, PKEi/PKEr) and an Auth Method Identifier. 7 References 7.1 Normative References [RFC2119] S. Bradner, "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2361] E. Rescorla, "Diffie-Hellman key agreement method", RFC 2631, 1999 [I-D.harkins-ipsecme-pake-criteria] D. Harkins, "Password-Based Authentication in IKEv2: Selection Criteria and Considerations", draft-harkins- ipsecme-pake-criteria-00.txt, April 2010 7.2 Informative References [TR03110] BSI TR-03110, "Advanced Security Mechanisms for Machine Readable Travel Documents - Extended Access Control (EAC), Password Authenticated Connection Establishment (PACE), and Restricted Identification (RI), Version 2.03, 2010 [TR03111] BSI TR-03111, "Elliptic Curve Cryptography", Version 1.11, 2009 [PACEsec] J. Bender, M. Fishlin, D. Kuegler, "Security Analysis of the PACE Key-Agreement Protocol", Information Security Conference (ISC) 2009, Lecture Notes in Computer Science, Volume 5735, pp. 33-48, Springer-Verlag, 2009. Full version available at http://eprint.iacr.org/2009/624 Kuegler Expires November 4, 2010 [Page 9] INTERNET DRAFT PACE May 3, 2010 Author's Addresses Dennis Kuegler Bundesamt fuer Sicherheit in der Informationstechnik (BSI) Postfach 200363 53133 Bonn Germany EMail: dennis.kuegler@bsi.bund.de Kuegler Expires November 4, 2010 [Page 10]