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Independent Submission                                        V. CakulevRequest for Comments: 6539                                   G. SundaramCategory: Informational                                      I. BroustisISSN: 2070-1721                                           Alcatel Lucent                                                              March 2012IBAKE: Identity-Based Authenticated Key ExchangeAbstract   Cryptographic protocols based on public-key methods have been   traditionally based on certificates and Public Key Infrastructure   (PKI) to support certificate management.  The emerging field of   Identity-Based Encryption (IBE) protocols allows simplification of   infrastructure requirements via a Private-Key Generator (PKG) while   providing the same flexibility.  However, one significant limitation   of IBE methods is that the PKG can end up being a de facto key escrow   server, with undesirable consequences.  Another observed deficiency   is a lack of mutual authentication of communicating parties.  This   document specifies the Identity-Based Authenticated Key Exchange   (IBAKE) protocol.  IBAKE does not suffer from the key escrow problem   and in addition provides mutual authentication as well as perfect   forward and backward secrecy.Status of This Memo   This document is not an Internet Standards Track specification; it is   published for informational purposes.   This is a contribution to the RFC Series, independently of any other   RFC stream.  The RFC Editor has chosen to publish this document at   its discretion and makes no statement about its value for   implementation or deployment.  Documents approved for publication by   the RFC Editor are not a candidate for any level of Internet   Standard; seeSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc6539.Independent Submissions Editor Note   This document specifies the Identity-Based Authenticated Key Exchange   (IBAKE) protocol.  Due to its specialized nature, this document   experienced limited review within the Internet Community.  Readers of   this RFC should carefully evaluate its value for implementation and   deployment.Cakulev, et al.               Informational                     [Page 1]

RFC 6539                          IBAKE                       March 2012Copyright Notice   Copyright (c) 2012 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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.Table of Contents1. Introduction ....................................................22. Requirements Notation ...........................................32.1. IBE: Definition ............................................32.2. Abbreviations ..............................................32.3. Conventions ................................................43. Identity-Based Authenticated Key Exchange .......................53.1. Overview ...................................................53.2. IBAKE Message Exchange .....................................63.3. Discussion .................................................74. Security Considerations .........................................94.1. General ....................................................94.2. IBAKE Protocol ............................................105. References .....................................................125.1. Normative References ......................................125.2. Informative References ....................................121.  Introduction   Authenticated key agreements are cryptographic protocols where two or   more participants authenticate each other and agree on key material   used for securing future communication.  These protocols could be   symmetric key or asymmetric public-key protocols.  Symmetric-key   protocols require an out-of-band security mechanism to bootstrap a   secret key.  On the other hand, public-key protocols traditionally   require certificates and a large-scale Public Key Infrastructure   (PKI).  Clearly, public-key methods are more flexible; however, the   requirement for certificates and a large-scale PKI have proved to be   challenging.  In particular, efficient methods to support large-scale   certificate revocation and management have proved to be elusive.   Recently, Identity-Based Encryption (IBE) protocols have been   proposed as a viable alternative to public-key methods by replacing   the PKI with a Private-Key Generator (PKG).  However, one significant   limitation of IBE methods is that the PKG can end up being a de factoCakulev, et al.               Informational                     [Page 2]

RFC 6539                          IBAKE                       March 2012   key escrow entity (i.e., an entity that has sufficient information to   decrypt communicated data), with undesirable consequences.  Another   limitation is a lack of mutual authentication between communicating   parties.  This document specifies an Identity-Based Authenticated Key   Encryption (IBAKE) protocol that does not suffer from the key escrow   problem and that provides mutual authentication.  In addition, the   scheme described in this document allows the use of time-bound public   identities and corresponding public and private keys, resulting in   automatic expiration of private keys at the end of a time span   indicated in the identity itself.  With the self-expiration of the   public identities, the traditional real-time validity verification   and revocation procedures used with certificates are not required.   For example, if the public identity is bound to one day, then, at the   end of the day, the public/private key pair issued to this peer will   simply not be valid anymore.  Nevertheless, just as with public-key-   based certificate systems, if there is a need to revoke keys before   the designated expiry time, communication with a third party will be   needed.  Finally, the protocol also provides forward and backward   secrecy of session keys; i.e., a session key produced using IBAKE is   always fresh and unrelated to any past or future sessions between the   protocol participants.2.  Requirements Notation   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].2.1.  IBE: Definition   Identity-Based Encryption (IBE) is a public-key encryption technology   that allows a public key to be calculated from an identity and a set   of public parameters, and the corresponding private key to be   calculated from the public key.  The public key can then be used by   an Initiator to encrypt messages that the recipient can decrypt using   the corresponding private key.  The IBE framework is defined in   [RFC5091], [RFC5408], and [RFC5409].2.2.  Abbreviations   EC          Elliptic Curve   IBE         Identity-Based Encryption   IBAKE       Identity-Based Authenticated Key Exchange   IDi         Initiator's IdentityCakulev, et al.               Informational                     [Page 3]

RFC 6539                          IBAKE                       March 2012   IDr         Responder's Identity   K_PUB       Public Key   PKG         Private-Key Generator   PKI         Public Key Infrastructure2.3.  Conventions   o  E is an elliptic curve over a finite field F.   o  P is a point on E of large prime order.   o  s is a non-zero positive integer.  s is a secret stored in a PKG.      This is a system-wide secret and not revealed outside the PKG.   o  sP is the public key of the system that is known to all      participants.  sP denotes a point on E, and denotes the point P      added to itself s times where addition refers to the group      operation on E.   o  H1 is a known hash function that takes a string and assigns it to      a point on the elliptic curve, i.e., H1(A) = QA on E, where A is      usually based on the identity.   o  E(k, A) denotes that A is IBE-encrypted with the key k.   o  s||t denotes concatenation of the strings s and t.   o  K_PUBx denotes a public key of x.Cakulev, et al.               Informational                     [Page 4]

RFC 6539                          IBAKE                       March 20123.  Identity-Based Authenticated Key Exchange3.1.  Overview   IBAKE consists of a three-way exchange between an Initiator and a   Responder.  In the figure below, a conceptual signaling diagram of   IBAKE is depicted.                 +---+                             +---+                 | I |                             | R |                 +---+                             +---+                                MESSAGE_1                   ---------------------------------->                                MESSAGE_2                   <----------------------------------                                MESSAGE_3                   ---------------------------------->                 Figure 1: Example IBAKE Message Exchange   The Initiator (I) and Responder (R) are attempting to mutually   authenticate each other and agree on a key using IBAKE.  This   specification assumes that the Initiator and the Responder trust a   third party -- the PKG.  Rather than a single PKG, different PKGs may   be involved, e.g., one for the Initiator and one for the Responder.   The Initiator and the Responder do not share any credentials;   however, they know or can obtain each other's public identity (key)   as well as the public parameters of each other's PKG.  This   specification does not make any assumption on when and how the   private keys are obtained.  However, to complete the protocol   described (i.e., to decrypt encrypted messages in the IBAKE protocol   exchange), the Initiator and the Responder need to have their   respective private keys.  The procedures needed to obtain the private   keys and public parameters are outside the scope of this   specification.  The details of these procedures can be found in   [RFC5091] and [RFC5408].  Finally, the protocol described in this   document relies on the use of elliptic curves.Section 3.3 discusses   the choice of elliptic curves.  However, how the Initiator and the   Responder agree on a specific elliptic curve is left to the   application that is leveraging the IBAKE protocol (see [EAP-IBAKE],   for example).   The Initiator chooses a random x.  In the first step, the Initiator   computes xP (i.e., P, as a point on E, added to itself x times using   the addition law on E); encrypts xP, the IDi, and the IDr using the   Responder's public key (e.g., K_PUBr=H1(IDr||date)); and includesCakulev, et al.               Informational                     [Page 5]

RFC 6539                          IBAKE                       March 2012   this encrypted information in MESSAGE_1 sent to the Responder.  In   this step, encryption refers to IBE as described in [RFC5091] and   [RFC5408].   The Responder, upon receiving the message, IBE-decrypts it using its   private key (e.g., a private key for that date), and obtains xP.  The   Responder further chooses a random y and computes yP.  The Responder   then IBE-encrypts the Initiator's identity (IDi), its own identity   (IDr), xP, and yP using the Initiator's public key (e.g.,   K_PUBi=H1(IDi||date)).  The Responder includes this encrypted   information in MESSAGE_2 sent to the Initiator.   The Initiator, upon receiving and IBE-decrypting MESSAGE_2, obtains   yP.  Subsequently, the Initiator sends MESSAGE_3, which includes the   IBE-encrypted IDi, IDr, and yP, to the Responder.  At this point,   both the Initiator and the Responder are able to compute the same   session key as xyP.3.2.  IBAKE Message Exchange   Initially, the Initiator selects a random x and computes xP; the   Initiator MUST use a fresh, random value for x on each run of the   protocol.  The Initiator then encrypts xP, the IDi, and the IDr using   the Responder's public key (e.g., K_PUBr=H1(IDr||date)).  The   Initiator includes this encrypted information in MESSAGE_1 and sends   it to the Responder, as shown below.   Initiator   ---->   Responder      MESSAGE_1 = E(K_PUBr, IDi || IDr || xP)   Upon receiving MESSAGE_1, the Responder SHALL perform the following:   o  Decrypt the message as specified in [RFC5091] and [RFC5408].   o  Obtain xP.   o  Select a random y and compute yP.  The Responder MUST use a fresh,      random value for x on each run of the protocol.   o  Encrypt the Initiator's identity (IDi), its own identity (IDr),      xP, and yP using the Initiator's public key (K_PUBi).   Responder   ---->   Initiator      MESSAGE_2 = E(K_PUBi, IDi || IDr || xP || yP)Cakulev, et al.               Informational                     [Page 6]

RFC 6539                          IBAKE                       March 2012   Upon receiving MESSAGE_2, the Initiator SHALL perform the following:   o  Decrypt the message as specified in [RFC5091] and [RFC5408].   o  Verify that the received xP is the same as that sent in MESSAGE_1.   o  Obtain yP.   o  Encrypt its own identity (IDi), the Responder's identity (IDr),      and yP using the Responder's public key (K_PUBi).   Initiator   ---->   Responder      MESSAGE_3 = E(K_PUBr, IDi || IDr || yP)   Upon receiving MESSAGE_3, the Responder SHALL perform the following:   o  Decrypt the message as specified in [RFC5091] and [RFC5408].   o  Verify that the received yP is the same as that sent in MESSAGE_2.   If any of the above verifications fail, the protocol halts;   otherwise, following this exchange, both the Initiator and the   Responder have authenticated each other and are able to compute xyP   as the session key.  At this point, both protocol participants MUST   discard all intermediate cryptographic values, including x and y.   Similarly, both parties MUST immediately discard these values   whenever the protocol terminates as a result of a verification   failure or timeout.3.3.  Discussion   Properties of the protocol are as follows:   o  Immunity from key escrow: Observe that all of the steps in the      protocol exchange are encrypted using IBE.  So, clearly, the PKG      can decrypt all of the exchanges.  However, given the assumption      that PKGs are trusted and well behaved (e.g., PKGs will not mount      an active man-in-the-middle (MitM) attack), they cannot compute      the session key.  This is because of the hardness of the Elliptic      Curve Diffie-Hellman problem.  In other words, given xP and yP, it      is computationally hard to compute xyP.   o  Mutually authenticated key agreement: Observe that all of the      steps in the protocol exchange are encrypted using IBE.  In      particular, only the Responder and its corresponding PKG can      decrypt the contents of MESSAGE_1 and MESSAGE_3 sent by the      Initiator, and similarly only the Initiator and its correspondingCakulev, et al.               Informational                     [Page 7]

RFC 6539                          IBAKE                       March 2012      PKG can decrypt the contents of MESSAGE_2 sent by the Responder.      Again, given the assumption made above -- that PKGs are trusted      and well behaved (e.g., a PKG will not impersonate a user to which      it issued a private key) -- upon receiving MESSAGE_2, the      Initiator can verify the Responder's authenticity, since xP could      have been sent in MESSAGE_2 only after decryption of the contents      of MESSAGE_1 by the Responder.  Similarly, upon receiving      MESSAGE_3, the Responder can verify the Initiator's authenticity,      since yP could have been sent back in MESSAGE_3 only after correct      decryption of the contents of MESSAGE_2 by the Initiator.      Finally, both the Initiator and the Responder can agree on the      same session key.  In other words, IBAKE is a mutually      authenticated key agreement protocol based on IBE.  The hardness      of the key agreement protocol relies on the hardness of the      Elliptic Curve Diffie-Hellman problem.  Thus, in any practical      implementation, care should be devoted to the choice of elliptic      curve.   o  Perfect forward and backward secrecy: Since x and y are random,      xyP is always fresh and unrelated to any past or future sessions      between the Initiator and the Responder.   o  No passwords: Clearly, the IBAKE protocol does not require any      offline exchange of passwords or secret keys between the Initiator      and the Responder.  In fact, the method is applicable to any two      parties communicating for the first time through any communication      network.  The only requirement is to ensure that both the      Initiator and the Responder are aware of each other's public keys      and the public parameters of the PKG that generated the      corresponding private keys.   o  PKG availability: Observe that PKGs need not be contacted during      an IBAKE protocol exchange, which dramatically reduces the      availability requirements on PKGs.   o  Choice of elliptic curves: This specification relies on the use of      elliptic curves for both IBE and Elliptic Curve Diffie-Hellman      exchange.  When making a decision on the choice of elliptic      curves, it is beneficial to choose two different elliptic curves      -- a non-supersingular curve for the internal calculations of      Elliptic Curve Diffie-Hellman values xP and yP, and a      supersingular curve for the IBE encryption/decryption.  For the      calculations of Elliptic Curve Diffie-Hellman values, it is      beneficial to use the curves recommended by NIST [FIPS-186].      These curves make the calculations simpler while keeping the      security high.  On the other hand, IBE systems are based on      bilinear pairings.  Therefore, the choice of an elliptic curve forCakulev, et al.               Informational                     [Page 8]

RFC 6539                          IBAKE                       March 2012      IBE is restricted to a family of supersingular elliptic curves      over finite fields of large prime characteristic.  The appropriate      elliptic curves for IBE are described in [RFC5091].   o  Implementation considerations: An implementation of IBAKE would      consist of two primary modules, i.e., point addition operations      over a NIST curve, and IBE operations over a supersingular curve.      The implementation of both modules only needs to be aware of the      following parameters: (a) the full description of the curves that      are in use (fixed or negotiated), (b) the public parameters of the      PKG used for the derivation of IBE private keys, and (c) the exact      public identity of each IBAKE participant.  The knowledge of these      parameters is sufficient to perform Elliptic Curve Cryptography      (ECC) operations in different terminals and produce the same      results, independently of the implementation.4.  Security Considerations   This document is based on the basic IBE protocol, as specified in   [BF], [RFC5091]), [RFC5408], and [RFC5409], and as such inherits some   properties of that protocol.  For instance, by concatenating the   "date" with the identity (to derive the public key), the need for any   key revocation mechanisms is virtually eliminated.  Moreover, by   allowing the participants to acquire multiple private keys (e.g., for   duration of contract) the availability requirements on the PKG are   also reduced without any reduction in security.  The granularity   associated with the date is a matter of security policy and as such   is a decision made by the PKG administrator.  However, the   granularity applicable to any given participant should be publicly   available and known to other participants.  For example, this   information can be made available in the same venue that provides   "public information" on a PKG server (i.e., P, sP) needed to   execute IBE.4.1.  General   Attacks on the cryptographic algorithms used in IBE are outside the   scope of this document.  It is assumed that any administrator will   pay attention to the desired strengths of the relevant cryptographic   algorithms based on an up-to-date understanding of the strength of   these algorithms from published literature, as well as to known   attacks.   It is assumed that the PKGs are secure, not compromised, trusted, and   will not engage in launching active attacks independently or in a   collaborative environment.  Nevertheless, if an active adversary can   fool the parties into believing that it is a legitimate PKG, then it   can mount a successful MitM attack.  Therefore, care should be takenCakulev, et al.               Informational                     [Page 9]

RFC 6539                          IBAKE                       March 2012   when choosing a PKG.  In addition, any malicious insider could   potentially launch passive attacks (by decryption of one or more   message exchanges offline).  While it is in the best interest of   administrators to prevent such an issue, it is hard to eliminate this   problem.  Hence, it is assumed that such problems will persist, and   hence the session key agreement protocols are designed to protect   participants from passive adversaries.   It is also assumed that the communication between participants and   their respective PKGs is secure.  Therefore, in any implementation of   the protocols described in this document, administrators of any PKG   have to ensure that communication with participants is secure and not   compromised.   Finally, concatenating the date to the identity ensures that the   corresponding private key is applicable only to that date.  This   serves to limit the damage related to a leakage or compromise of   private keys to just that date.  This, in particular, eliminates the   revocation mechanisms that are typical to various certificate-based   public key protocols.4.2.  IBAKE Protocol   For the basic IBAKE protocol, from a cryptographic perspective, the   following security considerations apply.   In every step, IBE is used, with the recipient's public key.  This   guarantees that only the intended recipient of the message and its   corresponding PKG can decrypt the message [BF].   Next, the use of identities within the encrypted payload is intended   to eliminate some basic reflection attacks.  For instance, suppose we   did not use identities as part of the encrypted payload, in the first   step of the IBAKE protocol exchange (i.e., MESSAGE_1 of Figure 1 inSection 3.1).  Furthermore, assume that an adversary has access to   the conversation between the Initiator and the Responder and can   actively snoop packets and drop/modify them before routing them to   the destination.  For instance, assume that the IP source address and   destination address can be modified by the adversary.  After the   first message is sent by the Initiator (to the Responder), the   adversary can take over and trap the packet.  Next, the adversary can   modify the IP source address to include the adversary's IP address,   before routing it on to the Responder.  The Responder will assume   that the request for an IBAKE session came from the adversary, and   will execute step 2 of the IBAKE protocol exchange (i.e., MESSAGE_2   of Figure 1 inSection 3.1) but encrypt it using the adversary's   public key.  The above message can be decrypted by the adversary (and   only by the adversary).  In particular, since the second messageCakulev, et al.               Informational                    [Page 10]

RFC 6539                          IBAKE                       March 2012   includes the challenge sent by the Initiator to the Responder, the   adversary will now learn the challenge sent by the Initiator.   Following this, the adversary can carry on a conversation with the   Initiator, "pretending" to be the Responder.  This attack will be   eliminated if identities are used as part of the encrypted payload.   In summary, at the end of the exchange, both the Initiator and the   Responder can mutually authenticate each other and agree on a   session key.   Recall that IBE guarantees that only the recipient of the message can   decrypt the message using the private key, with the caveat that the   PKG that generated the private key of the recipient of the message   can decrypt the message as well.  However, the PKG cannot learn the   public key xyP given xP and yP, based on the hardness of the Elliptic   Curve Diffie-Hellman problem.  This property of resistance to passive   key escrow from the PKG is not applicable to the basic IBE protocols   proposed in [RFC5091]), [RFC5408], and [RFC5409].   Observe that the protocol works even if the Initiator and Responder   belong to two different PKGs.  In particular, the parameters used for   encryption to the Responder and parameters used for encryption to the   Initiator can be completely different and independent of each other.   Moreover, the elliptic curve used to generate the session key xyP can   be completely different and can be chosen during the key exchange.   If such flexibility is desired, then it would be required to add   optional extra data to the protocol to exchange the algebraic   primitives used in deriving the session key.   In addition to mutual authentication and resistance to passive   escrow, the Diffie-Hellman property of the session key exchange   guarantees perfect secrecy of keys.  In other words, accidental   leakage of one session key does not compromise past or future session   keys between the same Initiator and Responder.Cakulev, et al.               Informational                    [Page 11]

RFC 6539                          IBAKE                       March 20125.  References5.1.  Normative References   [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate               Requirement Levels",BCP 14,RFC 2119, March 1997.5.2.  Informative References   [BF]        Boneh, D. and M. Franklin, "Identity-Based Encryption               from the Weil Pairing", in SIAM Journal on Computing,               Vol. 32, No. 3, pp. 586-615, 2003.   [EAP-IBAKE] Cakulev, V. and I. Broustis, "An EAP Authentication               Method Based on Identity-Based Authenticated Key               Exchange", Work in Progress, February 2012.   [FIPS-186]  National Institute of Standards and Technology, "Digital               Signature Standard (DSS)", FIPS Pub 186-3, June 2009.   [RFC5091]   Boyen, X. and L. Martin, "Identity-Based Cryptography               Standard (IBCS) #1: Supersingular Curve Implementations               of the BF and BB1 Cryptosystems",RFC 5091,               December 2007.   [RFC5408]   Appenzeller, G., Martin, L., and M. Schertler, "Identity-               Based Encryption Architecture and Supporting Data               Structures",RFC 5408, January 2009.   [RFC5409]   Martin, L. and M. Schertler, "Using the Boneh-Franklin               and Boneh-Boyen Identity-Based Encryption Algorithms with               the Cryptographic Message Syntax (CMS)",RFC 5409,               January 2009.Cakulev, et al.               Informational                    [Page 12]

RFC 6539                          IBAKE                       March 2012Authors' Addresses   Violeta Cakulev   Alcatel Lucent   600 Mountain Ave.   3D-517   Murray Hill, NJ  07974   US   Phone: +1 908 582 3207   EMail: violeta.cakulev@alcatel-lucent.com   Ganapathy S. Sundaram   Alcatel Lucent   600 Mountain Ave.   3D-517   Murray Hill, NJ  07974   US   Phone: +1 908 582 3209   EMail: ganesh.sundaram@alcatel-lucent.com   Ioannis Broustis   Alcatel Lucent   600 Mountain Ave.   3D-526   Murray Hill, NJ  07974   US   Phone: +1 908 582 3744   EMail: ioannis.broustis@alcatel-lucent.comCakulev, et al.               Informational                    [Page 13]