Network Working Group D. Kuegler Internet-Draft Bundesamt fuer Sicherheit in der Intended status: Experimental Informationstechnik (BSI) Expires: January 2, 2011 Y. Sheffer Independent July 1, 2010 Password Authenticated Connection Establishment with IKEv2 draft-kuegler-ipsecme-pace-ikev2-01 Abstract IKEv2 does not allow secure peer authentication when using short credential strings, i.e. passwords. Several proposals have been made to integrate password-authentication protocols into IKE. This document provides an adaptation of PACE (Password Authenticated Connection Establishment) to the setting of IKEv2 and demonstrates the advantages of this integration. Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. 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." This Internet-Draft will expire on January 2, 2011. Copyright Notice Copyright (c) 2010 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 Kuegler & Sheffer Expires January 2, 2011 [Page 1] Internet-Draft IKEv2 with PACE July 2010 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 . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Protocol Sequence . . . . . . . . . . . . . . . . . . . . . . 4 3.1. The IKE_SA_INIT Exchange . . . . . . . . . . . . . . . . . 5 3.2. The IKE_PACE Exchange . . . . . . . . . . . . . . . . . . 6 3.3. The IKE_PACE_AUTH Exchange . . . . . . . . . . . . . . . . 6 4. Mapping . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 4.1. Diffie Hellman . . . . . . . . . . . . . . . . . . . . . . 7 4.2. Elliptic Curve Diffie Hellman . . . . . . . . . . . . . . 8 4.3. Validation . . . . . . . . . . . . . . . . . . . . . . . . 8 5. Protocol Details . . . . . . . . . . . . . . . . . . . . . . . 8 5.1. Password Processing . . . . . . . . . . . . . . . . . . . 8 5.2. The PACE_SUPPORTED Notification . . . . . . . . . . . . . 8 5.3. The ENONCE Payload . . . . . . . . . . . . . . . . . . . . 9 5.4. The PKE (PKEi/PKEr) Payloads . . . . . . . . . . . . . . . 9 5.5. PACE and Session Resumption . . . . . . . . . . . . . . . 9 6. Security Considerations . . . . . . . . . . . . . . . . . . . 10 6.1. Credential Security Assumptions . . . . . . . . . . . . . 10 6.2. Vulnerability to Passive and Active Attacks . . . . . . . 10 6.3. Perfect Forward Secrecy . . . . . . . . . . . . . . . . . 10 6.4. Identity Protection . . . . . . . . . . . . . . . . . . . 10 6.5. Denial of Service . . . . . . . . . . . . . . . . . . . . 10 6.6. Choice of Encryption Algorithms . . . . . . . . . . . . . 10 6.7. Security Model and Security Proof . . . . . . . . . . . . 10 6.8. Long-Term Credential Storage . . . . . . . . . . . . . . . 10 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11 8.1. Normative References . . . . . . . . . . . . . . . . . . . 11 8.2. Informative References . . . . . . . . . . . . . . . . . . 11 Appendix A. Protocol Selection Criteria . . . . . . . . . . . . . 12 A.1. Security Criteria . . . . . . . . . . . . . . . . . . . . 12 A.2. Intellectual Property Criteria . . . . . . . . . . . . . . 12 A.3. Miscellaneous Criteria . . . . . . . . . . . . . . . . . . 12 Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 13 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13 Kuegler & Sheffer Expires January 2, 2011 [Page 2] Internet-Draft IKEv2 with PACE July 2010 1. Introduction PACE [TR03110] is a security protocol that establishes a 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. This draft describes the integration of PACE into IKEv2 ([RFC4306] and [I-D.ietf-ipsecme-ikev2bis]) as a new authentication mode, analogous to the existing certificate and PSK authentication modes. 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. 1.1. 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: Kuegler & Sheffer Expires January 2, 2011 [Page 3] Internet-Draft IKEv2 with PACE July 2010 E() Symmetric encryption D() Symmetric decryption KA() Key agreement Map() Mapping function Pwd Shared Password KPwd Symmetric key derived from a password Pwd G Static group generator GE Ephemeral group generator ENONCE Encrypted 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 fully replace IKE_AUTH. 1. The initiator randomly and uniformly chooses a nonce, encrypts the nonce, and sends the ciphertext ENONCE to the responder. 2. The responder recovers the plaintext nonce 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). 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 & Sheffer Expires January 2, 2011 [Page 4] Internet-Draft IKEv2 with PACE July 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, ENONCE, PKEi} -> <- HDR, SK{IDr, PKEr} IKE_PACE_AUTH: HDR, SK{AUTH} -> <- HDR, SK{AUTH, SAr2, TSi, TSr} 3.1. The IKE_SA_INIT Exchange The initiator sends a PACE_SUPPORTED notification to indicate its support of this extension, and its wish to authenticate using a password. If the responder accepts, it responds with the same notification. Otherwise, it omits the notification to indicate a preference for a regular IKE exchange. In the case of anti-DOS cookies (Sec. 2.6 of [RFC4306]), the notification MUST be resent by each peer every time it sends its IKE_SA_INIT message. 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 its 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 Kuegler & Sheffer Expires January 2, 2011 [Page 5] Internet-Draft IKEv2 with PACE July 2010 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. 3.2. The IKE_PACE Exchange This new exchange (number TBD by IANA) is the first part of the PACE authentication of the peers. 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 ENONCE = E(Pwd, s) using the negotiated password based-encryption using the password Pwd. 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 ENONCE containing the encrypted nonce and PKEi containing the ephemeral public key. The responder decrypts the received encrypted nonce s = D(Pwd, ENONCE), 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. During the Diffie-Hellman key agreement, each party MUST check that the two public keys PKEi and PKEr differ. Otherwise, it MUST abort the protocol. The IKE_PACE request is equivalent to the IKE_AUTH request in a normal IKEv2 exchange, i.e. any payload which is valid in an IKE_AUTH request is valid (with the same semantics) in the IKE_PACE request. In particular, certificate-related payloads are allowed, even though their use may not be practical within this mode. 3.3. The IKE_PACE_AUTH Exchange This new exchange (number TBD by IANA) is the second part of the PACE authentication of the peers. The initiator and the responder calculate the shared secret PACESharedSecret, derive the authentication key, and calculate the authentication token AUTH. The initiator calculates: Kuegler & Sheffer Expires January 2, 2011 [Page 6] Internet-Draft IKEv2 with PACE July 2010 AUTHi = prf(prf+(PACESharedSecret, "PACE Authentication i" | IDr | IDi | PKEr), ) See Sec. 2.15 of [RFC4306] (further clarified in Sec. 2.15 of [I-D.ietf-ipsecme-ikev2bis]) for the definition of signed octets. The responder calculates: AUTHr = prf(prf+(PACESharedSecret, "PACE Authentication r" | IDi | IDr | PKEi), ) The authentication tokens are then exchanged and each of them MUST be verified by the other party. The behavior when this verification fails is unchanged from [RFC4306]. Provided authentication was successful, the IKE_PACE_AUTH response is equivalent to the IKE_AUTH response in a normal IKEv2 exchange, i.e. any payload which is valid in an IKE_AUTH response is valid (with the same semantics) in the IKE_PACE_AUTH response. Following authentication, all temporary values MUST be deleted by the peers, including in particular s, the ephemeral generator and public keys, and PACESharedSecret. 4. Mapping The mapping is based on a second anonymous Diffie-Hellman key agreement protocol to create a shared secret which is used together 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. Using the notation of [RFC4306], SASharedSecret = g^ir Let G and GE be the generator of the negotiated DH 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. In implementations that detect such a failure, the initiator SHOULD choose s again. Kuegler & Sheffer Expires January 2, 2011 [Page 7] Internet-Draft IKEv2 with PACE July 2010 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. In implementations that detect such a failure, the initiator 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. Protocol Details 5.1. Password Processing The input password string SHOULD be processed according to the rules of the [RFC4013] profile of [RFC3454]. A password SHOULD be considered a "stored string" per [RFC3454] and unassigned code points are therefore prohibited. The output is the binary representation of the processed UTF-8 character string. Prohibited output and unassigned codepoints encountered in SASLprep preprocessing SHOULD cause a preprocessing failure and the output SHOULD NOT be used. 5.2. The PACE_SUPPORTED Notification This protocol defines a new PACE_SUPPORTED notification, with type number TBD by IANA. This is an empty notification: The Protocol ID and SPI size fields are set to zero, and there is no additional data associated with this notification. Kuegler & Sheffer Expires January 2, 2011 [Page 8] Internet-Draft IKEv2 with PACE July 2010 5.3. The ENONCE Payload This protocol defines a new ENONCE (encrypted nonce) payload, with payload type TBD by IANA. Its format is as follows: 0 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Initialization Vector | | (optional, length is block size for encryption algorithm) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Encrypted Nonce ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1: ENONCE Payload Structure The length of the nonce MUST be an integer multiple of the selected encryption algorithm's block size (if any). Note that the payload format does not support any padding of the encrypted data. 5.4. The PKE (PKEi/PKEr) Payloads These payloads have an identical format to the IKEv2 KE payload. However, this protocol defines a new payload type named PKE (Public Key - Ephemeral), whose value is TBD by IANA. 5.5. PACE and Session Resumption A session resumption [RFC5723] ticket may be requested during the IKE_PACE/IKE_PACE_AUTH exchanges. The request MUST be sent in the IKE_PACE request, and any response MUST be sent in the IKE_PACE_AUTH response. PACE should be considered an "authentication method", in the sense of Sec. 5 of [RFC5723], which means that its use MUST be noted in the protected ticket. Note that even if the initial authentication used PACE and its new exchange types, session resumption will still include the normal IKE_AUTH exchange. Kuegler & Sheffer Expires January 2, 2011 [Page 9] Internet-Draft IKEv2 with PACE July 2010 6. Security Considerations 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. What follows is an outline of the security considerations for this protocol. 6.1. Credential Security Assumptions 6.2. Vulnerability to Passive and Active Attacks 6.3. Perfect Forward Secrecy 6.4. Identity Protection 6.5. Denial of Service 6.6. Choice of Encryption Algorithms 6.7. Security Model and Security Proof 6.8. Long-Term Credential Storage 7. IANA Considerations IANA is requested to allocate (has allocated) the following values: o The PACE_SUPPORTED notification type (TBD) from the "IKEv2 Notify Message Types - Status Types" registry. o A payload type (TBD) for the Encrypted Nonce (ENONCE) payload from the "IKEv2 Payload Types" registry. o A payload type (TBD) for the Ephemeral Public Key (PKE) payload from the same registry. o An authentication method (TBD) for Password Authenticated Connection Establishment (PACE) from the "IKEv2 Authentication Method" registry. o An exchange type (TBD) for the IKE_PACE exchange, from the "IKEv2 Exchange Types" registry. o An exchange type (TBD) for the IKE_PACE_AUTH exchange, from the same registry. This document does not define any new registries. 8. References Kuegler & Sheffer Expires January 2, 2011 [Page 10] Internet-Draft IKEv2 with PACE July 2010 8.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2631] Rescorla, E., "Diffie-Hellman Key Agreement Method", RFC 2631, June 1999. [RFC3454] Hoffman, P. and M. Blanchet, "Preparation of Internationalized Strings ("stringprep")", RFC 3454, December 2002. [RFC4013] Zeilenga, K., "SASLprep: Stringprep Profile for User Names and Passwords", RFC 4013, February 2005. [RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", RFC 4306, December 2005. 8.2. Informative References [I-D.harkins-ipsecme-pake-criteria] Harkins, D., "Password-Based Authentication in IKEv2: Selection Criteria and Considerations", draft-harkins-ipsecme-pake-criteria-00 (work in progress), April 2010. [I-D.ietf-ipsecme-ikev2bis] Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen, "Internet Key Exchange Protocol: IKEv2", draft-ietf-ipsecme-ikev2bis-11 (work in progress), May 2010. [PACEsec] "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. [RFC5723] Sheffer, Y. and H. Tschofenig, "Internet Key Exchange Protocol Version 2 (IKEv2) Session Resumption", RFC 5723, January 2010. [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. Kuegler & Sheffer Expires January 2, 2011 [Page 11] Internet-Draft IKEv2 with PACE July 2010 Appendix A. Protocol Selection Criteria 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. A.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. A.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. A.3. Miscellaneous Criteria MISC1: One additional exchange is required. MISC2: The protocol requires the following operations per entity: * one key derivation from the password, * one symmetric encryption or decryption, * one multi-exponentiation for the mapping, * one exponentiation for the key pair generation, * one exponentiation for the shared secret calculation, and * two symmetric authentications (generation and 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. Kuegler & Sheffer Expires January 2, 2011 [Page 12] Internet-Draft IKEv2 with PACE July 2010 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. Appendix B. Change Log Note to RFC Editor: please remove this appendix before publication. B.1. draft-kuegler-ipsecme-pace-ikev2-01 Formalized the protocol: added payload formats, error behavior etc. Authors' Addresses Dennis Kuegler Bundesamt fuer Sicherheit in der Informationstechnik (BSI) Postfach 200363 Bonn 53133 Germany Email: dennis.kuegler@bsi.bund.de Yaron Sheffer Independent Email: yaronf.ietf@gmail.com Kuegler & Sheffer Expires January 2, 2011 [Page 13]