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    Internet Draft                                        David M'Raihi
                                                               VeriSign
    Category:                                              Johan Rydell
      Informational                                            Portwise
    Document:                                            David Naccache
      draft-mraihi-mutual-oath-hotp-variants-01.txt                 ENS
                                                          Salah Machani
                                                             Diversinet
 
    Expires:
      June 2006                                           December 2005
 
           Mutual OATH: HOTP Extensions for mutual authentication
 
 Status of this Memo
 
    By submitting this Internet-Draft, each author represents that any
    applicable patent or other IPR claims of which he or she is aware
    have been or will be disclosed, and any of which he or she becomes
    aware will be disclosed, in accordance with Section 6 of BCP 79.
 
    Internet-Drafts are working documents of the Internet Engineering
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    http://www.ietf.org/shadow.html
 
 Abstract
 
    This document describes Mutual OATH, a mechanism for mutual
    authentication based on the HOTP algorithm [HOTP] introduced
    recently by OATH (initiative for Open AuTHentication) [OATH] and
    submitted as an individual draft to the IETF last year. The
    security and strength of Mutual OATH depends on the properties of
    the underlying building block HOTP, which is a construction based
    on HMAC [RFC2104] using SHA-1 as the hash function.
 
 
 
 
 
 
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 Table of Contents
 
    1.   Introduction...............................................2
    2.   Requirements Terminology...................................3
    3.   Algorithm Requirements.....................................3
    4.   Mutual OATH Definition.....................................4
    4.1  HOTP Algorithm.............................................4
    4.2  Algorithm Identifier.......................................5
    4.3  Mutual OATH Authentication.................................6
    4.4  Dual-key Mode..............................................6
    4.5  Chained Mode...............................................7
    4.6  PIN/Password "salted" HOTP Response........................7
    4.7  Signature - Using HOTP with challenge and fixed value......8
    5.   Security Considerations....................................8
    6.   IANA Considerations........................................9
    7.   Conclusion.................................................9
    8.   Acknowledgements...........................................9
    9.   References.................................................9
    10.1   Normative................................................9
    10.2   Informative..............................................9
    10.  Authors' Addresses........................................10
    12. Full Copyright Statement...................................11
    13. Intellectual Property......................................11
 
 
   1. Introduction
 
    The HOTP algorithm is counter based and intended for synchronous
    authentication mode systems. It has been implemented under various
    form factors such as USB tokens, software client applications,
    validation servers, applets for smart-cards, etc.
 
    OATH has identified several user cases and scenarios that require
    an asynchronous variant to accommodate users who do not want to
    maintain a synchronized authentication system. The commonly
    accepted method for this is to use a challenge-response scheme.
 
    This draft introduces the concept of randomized versions of HOTP,
    where the counter field is replaced or extended by a function of
    several parameters such as a random question and a personal
    identification code or password.
 
    Challenge response mode of authentication is widely adopted in the
    industry. Several vendors already offer software applications and
    hardware devices implementing challenge-response - but each of
    those uses vendor-specific proprietary algorithms. For the benefits
    of users we need a standardized challenge-response algorithm to
 
 
 
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    allow multi-sourcing of token purchases and validation systems to
    facilitate the democratization of strong authentication.
 
   2. Requirements 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 BCP 14.
 
   3. Algorithm Requirements
 
    This section presents the main requirements that drove this
    algorithm design. A lot of emphasis was placed on flexibility and
    usability, under the constraints and specificity of the HOTP
    algorithm and hardware token capabilities.
 
    R1 - The shared secret MUST be embedded in a tamper resistance
    device or securely implemented in a software application.
 
    R2 - The secret SHOULD be stored in a hardware device for
    additional flexibility (mobility) and security.
 
    R3 - The algorithm MUST be capable of supporting different modes of
    authentications.
 
    R4 - The challenge value MUST be randomly generated for each use of
    the authentication protocol and SHALL NOT be re-used.
 
    R5 - The algorithm MUST allow for the challenge to be presented to
    the user in numeric or alphanumeric form.
 
    R6 - If the challenge is displayed to the user, the challenge
    length SHOULD be selectable; the specification should define the
    maximum length for a challenge.
 
    R7 - There MUST be a fixed randomly generated secret (key) for each
    token/soft token that is shared between the token and the
    authentication server.
 
    R8 - The challenge MAY be generated with integrity checking (e.g.,
    parity bits). This will allow tokens with pin pads to perform
    simple error checking if the user enters the value into a token.
 
    R9 - The algorithm MAY include a counter function.
 
    R10 - The algorithm SHOULD be used in dual-key mode. The single key
    is mainly to support legacy tokens.
 
 
 
 
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    R11 - All the HOTP communications MUST take place over a secure
    channel e.g. SSL/TLS, IPsec connections.
 
   4. Mutual OATH Definition
 
    In this section, we introduce the HOTP algorithm variants for
    mutual authentication.
 
    Words used in this chapter:
 
    K - Key, a shared secret known to all parties
    C - Counter, a predefined sequence of values that can be calculated
    by all parties
    P - Hashed version of PIN (Personal Identification Number) or
    password that is known to all parties
    Q - Challenge, a predefined value that may change and is known to
    all parties during execution of the algorithm
    T - Fixed value that is used in a transaction
    AI - Algorithm identifier, described in details in section 4.2
 
    4.1  HOTP Algorithm
 
    The HOTP algorithm is based on an increasing counter value and a
    static symmetric key known only to the prover and verifier parties.
    HOTP has been introduced as an internet draft and is currently in
    the RFC-editor queue for final review.
 
    As a reminder:
 
                   HOTP(K,C) = Truncate(HMAC-SHA-1(K,C))
 
    Where Truncate represents the function that converts an HMAC-SHA-1
    value into an HOTP value of 6 to 8 digits.
 
    The Key (K), the Counter (C) and Data values are hashed high-order
    byte first. The HOTP values generated by the HOTP generator are
    treated as big endian.
 
    We refer the reader to [HOTP] for the full description and further
    details on the rationale and security analysis of HOTP.
 
    We introduced in [HOTP] the notion of a Data field that would be
    used for generating the One-Time Password values. Using a Data
    field opens for more flexibility in the algorithm implementation,
    provided that the Data field is clearly specified.
 
    A straightforward variant is to replace the counter value C by a
    random question Q to define a Response as a function of the
    challenge Q:
 
 
 
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       Response = HOTP (K,Q) = Truncate (HMAC-SHA-1(K,Q))
 
    The present draft describes the different variants based on similar
    constructions.
 
    4.2  Algorithm Identifier
 
    It is important for both parties willing to interact to know the
    operations to be performed, namely which variant is to be used.
 
    Let AI bit a binary string, and the different bits indicate which
    combination of parameters is used to compute an HOTP response.
 
    We could also specify the algorithm itself - by default, HOTP is
    based on HMAC-SHA-1 but recent attacks on hash functions and new
    standard works might result in recommendations to use another
    function for HMAC.
 
    We define the algorithm identifier AI as follows:
 
    AI = Type | Pb | Qb | Cb | Tb | Mode
 
    Where:
       Type is a 4-bit value to specify the algorithm type; at the
    moment, 4 types are defined:
          0000: HMAC-SHA-1 without truncation
          0001: HMAC-SHA-1 with Dynamic Truncation for 6-digit
          0010: HMAC-SHA-1 with Dynamic Truncation for 7-digit
          0011: HMAC-SHA-1 with Dynamic Truncation for 8-digit
       Pb - single bit that indicates whether or not a hash of a PIN or
    Password will be used in the computation;
       Qb - single bit that indicates whether or not a random Q will be
    used in the computation;
       Cb - single bit that indicates whether or not a counter C will
    be used in the computation;
       Tb - single bit that indicates whether or not a fixed value T
    will be used in the computation.
       Mode is a 4-bit value that defines the different modes of
    computation; at the moment, 4 types are defined:
          0000: plain mode, single key
          0001: plain mode, dual key
          0010: chained mode, single key
          0011: chained mode, dual key
          With the chained mode as described in section 4.5
 
    For instance, if the algorithm used is HOTP (K, C) with Dynamic
    Truncation to generate 6-digit HOTP values as described in [HOTP]
    as default mode then AI = 0001 0010 0000.
 
 
 
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    4.3  Mutual OATH Authentication
 
    A straightforward variant is to replace the counter value C by a
    random question Q to define a randomized Response as a function of
    the challenge Q:
 
       Response = HOTP (K,Q) = Truncate (HMAC-SHA-1(K,Q))
 
    The challenge-response sequence is the following for parties A and
    B sharing knowledge of secret (key) K and willing to perform Mutual
    OATH authentication:
 
       1- A sends a random question Q_A to B and AI = 0001 0100 0000
       2- B computes Response_B = HOTP (K,Q_A) and sends it to A with
    his own random question Q_B
       3- A checks Response_B and if correct, computes Response_A =
    HOTP (K,Q_B)
       4- B checks Response_A and if correct, acknowledges party A
 
    Both parties are authenticated.
 
    4.4  Dual-key Mode
 
    The key SHOULD be pre-divided into different usages. K1 is used for
    computing responses and K2 to check responses. The opposite is used
    for the other party. Depending on whether storage or computation is
    cheaper, we could either store a longer key K = K1 | K2 or set K1 =
    K and K2 = K xor Constant or K2 = SHA-1(K) for instance.
 
    The challenge-response sequence is the following for parties A and
    B sharing knowledge of secret (key) K. K is divided into K1 and K2.
 
       1- A sends a random question Q_A to B and AI = 0001 0100 0001
       2- B computes Response_B = HOTP (K2,Q_A) and sends it to A with
    his own random question Q_B
       3- A checks Response_B and if correct, computes Response_A =
    HOTP (K1,Q_B)
       4- B checks Response_A and if correct, acknowledges party A
 
    Both parties are authenticated. Separating into K1 and K2 protects
    against another party sending predefined questions "Q_A"'s to B for
    querying specific information.
 
    We recommend this mode of computation for mutual OATH.
 
 
 
 
 
 
 
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 HOTP: An HMAC-based One Time Password Algorithm          December 2005
 
 
    4.5  Chained Mode
 
    Another interesting computation mode is to chain the computations
    of different responses. It might have valuable applications such as
    to verify that a certain sequence of operations has taken place.
 
    The challenge-response sequence is the following for parties A and
    B sharing knowledge of secret (key) K and willing to perform Mutual
    OATH authentication in chained mode:
 
       1- A sends a random question Q_A to B and AI = 0001 0100 0011
       2- B computes Response_B = HOTP (K2,Q_A) and sends it to A with
    his own random question Q_B
       3- A checks Response_B and if correct, computes Response_A =
    HOTP (K1,Q_B|Response_B)
       4- B checks Response_A and if correct, acknowledges party A
 
    Both parties are authenticated.
 
    4.6  PIN/Password "salted" HOTP Response
 
    In this case, the PIN/password value used to control the usage of
    the token and/or protect access to the key embedded in the hardware
    (or software) token can be injected in the Response computation, as
    well as the random question Q:
 
       Response = HOTP (K, Q|P)
 
    Where | stands for concatenation.
 
    The challenge-response sequence is the following for parties A and
    B sharing knowledge of secret (key) K and willing to perform Mutual
    OATH authentication:
 
       1- A sends a random question Q_A to B and AI = 000110000
       2- B computes Response_B = HOTP (K2,Q_A|P) and sends it to A
    with his own random question Q_B
       3- A checks Response_B and if correct, computes Response_A =
    HOTP (K1,Q_B|P)
       4- B checks Response_A and if correct, acknowledges party A
 
    Obviously, the string P can be empty if party A or B does not have
    a PIN or Password - e.g. a validation sever computing a response to
    be authenticated by a user will probably not have the usage of a
    PIN or password.
 
    It opens for interesting combinations where the algorithm
    identifier AI could be used to specify computations both ways -
    e.g. the token could include the PIN in his computation and the
 
 
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    server, knowing the hash checks the HOTP response; in its
    calculation on the other hand, no PIN value would be included since
    the server would not have one.
 
    4.7  Signature - Using HOTP with challenge and fixed value
 
    The combination of a fixed value and random value enables to
    generate different signature values for different fixed values -
    the computation is randomized by the question Q. This is important
    when signing the same value more then once.
 
       Signature = HOTP (K, Q|T) where | stands for concatenation.
 
    The sequence for B generating a signature to be checked by A is the
    following, assuming the mode of computation is dual-key plain mode:
       1- A sends a fixed value C, a random question Q to B and AI =
    0001 0110 0001
       2- B computes Response_B = HOTP (K2,Q|T) and sends it to A
       3- A checks Response_B and if correct, acknowledges party B has
    validated the value T
 
    In an hostile environment a third party can trick party B to reveal
    the correct response for a given value. Again, all exchanges should
    take place over a secure channel - using SSL/TLS, or similar
    encryption mechanism.
 
   5. Security Considerations
 
    The keys for HOTP can be of any length equal or longer than 20
    bytes. Keys longer than 20 bytes are acceptable; they are first
    hashed using the supported hash function, e.g. SHA-1, to become
    usable. Nevertheless, the extra length would not significantly
    increase the cryptographic strength of Mutual OATH, provided the
    randomness of the original key material is sufficient.
 
    Keys need to be chosen at random or using a cryptographically
    strong pseudo-random generator properly seeded with a random value.
    We RECOMMEND to follow the recommendations in [RFC1750] for all
    pseudo-random and random generations. The pseudo-random numbers
    used for generating the keys SHOULD successfully pass the
    randomness test specified in [CN].
 
    The conclusion of the security analysis detailed in [HOTP] is that,
    for all practical purposes, the outputs of the dynamic truncation
    on distinct counter inputs are uniformly and independently
    distributed strings.
 
    The analysis demonstrates that the best possible attack against the
    HOTP function is the brute force attack.
 
 
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   6. IANA Considerations
 
    This document has no actions for IANA.
 
   7. Conclusion
 
    This draft introduced several variants of HOTP for randomized
    response and short signature-like computations.
 
    The algorithm identifier AI provides for an easy integration and
    support of different HOTP flavors within an authentication and
    validation system.
 
    Mutual OATH should enable cross-authentication both in connected
    and off-line modes, with the support of different response sizes
    and mode of operations.
 
   8. Acknowledgements
 
    We would like to thank Siddharth Bajaj, Philip Hoyer, Jon
    Martinsson and Stu Vaeth for their comments and suggestions to
    improve this draft document.
 
   9. References
 
    10.1 Normative
 
    [RFC2104]   M. Bellare, R. Canetti and H. Krawczyk, "HMAC:
                Keyed-Hashing for Message Authentication", IETF Network
                Working Group, RFC 2104, February 1997.
 
    [RFC1750]  D. Eastlake, 3rd., S. Crocker and J. Schiller,
                "Randomness Recommendantions for Security", IETF
                Network Working Group, RFC 1750, December 2004.
 
    [RFC2119]   S. Bradner, "Key words for use in RFCs to Indicate
                Requirement Levels", BCP 14, RFC 2119, March 1997.
 
    [RFC3668]  S. Bradner, "Intellectual Propery Rights in IETF
                Technology", BCP 79, RFC 3668, February 2004.
 
    10.2 Informative
 
    [HOTP]     D. M'Raihi, M. Bellare, F. Hoornaert, D. Naccache and
                O. Ranen, HOTP: HOTP: An HMAC-based One Time Password
                Algorithm", Internet Draft, Informational.
    http://www.ietf.org/internet-drafts/draft-mraihi-oath-hmac-otp-
    04.txt
 
 
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    [BCK]       M. Bellare, R. Canetti and H. Krawczyk, "Keyed Hash
                Functions and Message Authentication", Proceedings of
                Crypto'96, LNCS Vol. 1109, pp. 1-15.
 
    [OATH]     Initiative for Open AuTHentication
    http://www.openauthentication.org
 
    [CN]       J.S. Coron and D. Naccache, "An accurate evaluation of
                Maurer's universal test" by Jean-Sebastien Coron and
                David Naccache In Selected Areas in Cryptography (SAC
                '98), vol. 1556 of Lecture Notes in Computer Science,
                S. Tavares and H. Meijer, Eds., pp. 57-71, Springer-
                Verlag, 1999
 
 
   10. Authors' Addresses
 
    Primary point of contact (for sending comments and question):
 
    David M'Raihi
    VeriSign, Inc.
    685 E. Middlefield Road          Phone: 1-650-426-3832
    Mountain View, CA 94043 USA      Email: dmraihi@verisign.com
 
 
    Other Authors' contact information:
 
    Johan Rydell
    Portwise, Inc.
    624 Ellis Street, Suite 102      Phone: 1-650-472-3582
    Mountain View, CA 94043 USA      Email: johan.rydell@portwise.com
 
 
    David Naccache
    ENS, DI
    45 rue d'Ulm                     Phone: +33 6 16 59 83 49
    75005, Paris France              Email: david.naccache@ens.fr
 
 
    Salah Machani
    Diversinet Corp.
    2225 Sheppard Avenue East
    Suite 1801
    Toronto, Ontario M2J 5C2         Phone: 1-416-756-2324 Ext. 321
    Canada                           Email: smachani@diversinet.com
 
 
 
 
 
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   12.
      Full Copyright Statement
 
    Copyright (C) The Internet Society (2005).
 
    This document is subject to the rights, licenses and restrictions
    contained in BCP 78, and except as set forth therein, the authors
    retain all their rights.
 
    This document and the information contained herein are provided on
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   13.
      Intellectual Property
 
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