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 Internet Draft                                           David M'Raihi
                                                               VeriSign
    Category:                                              Johan Rydell
      Informational                                            PortWise
    Document:                                            David Naccache
      draft-mraihi-mutual-oath-hotp-variants-06.txt                 ENS
                                                          Salah Machani
                                                             Diversinet
                                                        Siddharth Bajaj
                                                               VeriSign
    Expires:
      June 2008                                      December 2007
 
                  OCRA: OATH Challenge-Response Algorithms
 
 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
    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".
 
    The list of current Internet-Drafts can be accessed at
    http://www.ietf.org/1id-abstracts.html
    The list of Internet-Draft Shadow Directories can be accessed at
    http://www.ietf.org/shadow.html
 
 Abstract
 
    This document describes the OATH algorithm for challenge-response
    authentication and signatures. This algorithm is based on the HOTP
    algorithm [RFC4226] that was introduced by OATH (initiative for
    Open AuTHentication) [OATH] and submitted as an individual draft to
    the IETF last year.
 
 
 
 
 
 
 
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                             Table of Contents
 
 
 
 
 
 
    1.   Introduction...............................................3
    2.   Requirements Terminology...................................3
    3.   Algorithm Requirements.....................................3
    4.   OCRA Background............................................4
    4.1  HOTP Algorithm.............................................4
    5.   Definition of OCRA.........................................5
    5.1 DataInput Parameters........................................5
    5.2 CryptoFunction..............................................6
    6.   The OCRASuite..............................................7
    7.   Algorithm Modes for Authentication.........................8
    7.1 One way Challenge-Response..................................8
    7.2 Mutual Challenge-Response...................................9
    8.   Algorithm Modes for Signature.............................10
    8.1  Plain Signature...........................................10
    8.2  Signature with Server Authentication......................11
    9.   Security Considerations...................................13
    9.1 Security Analysis of the OCRA algorithm....................13
    9.2 Implementation Considerations..............................13
    10.  IANA Considerations.......................................15
    11.  Conclusion................................................15
    12.  Acknowledgements..........................................15
    13.  References................................................15
    13.1 Normative.................................................15
    13.2 Informative...............................................16
    Appendix A: Source Code........................................16
    Appendix B: Test Vectors.......................................19
    14.  Authors' Addresses........................................20
    15.  Full Copyright Statement..................................21
    16.  Intellectual Property.....................................21
 
 
 
 
 
 
 
 
 
 
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   1. Introduction
 
    OATH has identified several use 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.
 
    Such 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
    allow multi-sourcing of token purchases and validation systems to
    facilitate the democratization of strong authentication.
    Additionally, this specification can also be used to create
    symmetric key based digital signatures. Such systems are variants
    of challenge-response mode where the data to be signed becomes the
    challenge.
 
 
   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 RFC 2119
    [RFC2119].
 
 
   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 algorithm MUST support asynchronous challenge-response
    based authentication.
 
    R2 - The algorithm MUST be capable of supporting symmetric key
    based digital signatures. Essentially this is a variation of
    challenge-response where the challenge is derived from the data
    that needs to be signed.
 
    R3 - The algorithm MUST be capable of supporting server-
    authentication, whereby the user can verify that he/she is talking
    to a valid server.
 
    R4 - The algorithm SHOULD use HOTP [RFC4226] as a key building
    block.
 
 
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    R5 - The length and format for the input challenge SHOULD be
    configurable.
 
    R6 - The output length and format for the response SHOULD be
    configurable.
 
    R7 - 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.
 
    R8 - There MUST be a unique secret (key) for each token/soft token
    that is shared between the token and the authentication server. The
    keys MUST be randomly generated or derived using some key
    derivation algorithm.
 
    R9 - The algorithm MUST enable additional data attributes such as a
    counter, a time function or session information to be included in
    the computation. These data inputs MAY be used individually or all
    together.
 
 
   4. OCRA Background
 
    OATH introduced the HOTP algorithm as a first open, freely
    available building block toward hardening authentication for end-
    users in a variety of applications. One-time passwords are very
    efficient at solving specific security issues thanks to the dynamic
    nature of OTP computations.
 
    After carefully analyzing different use cases, OATH came to the
    conclusion that providing for extensions to the HOTP algorithms was
    important. A very natural extension is to introduce a challenge
    mode for computing HOTP values based on random questions. Equally
    beneficial, being able to perform mutual authentication between two
    parties, or short-signature computation for authenticating
    transaction was also identified as critical for improving the
    security of e-commerce applications.
 
    4.1  HOTP Algorithm
 
    The HOTP algorithm, as defined in [RFC4226] is based on an
    increasing counter value and a static symmetric key known only to
    the prover and verifier parties.
 
    As a reminder:
 
                    HOTP(K,C) = Truncate(HMAC-SHA1(K,C))
 
 
 
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    Where Truncate represents the function that converts an HMAC-SHA-1
    value into an HOTP value.
 
    We refer the reader to [RFC4226] for the full description and
    further details on the rationale and security analysis of HOTP.
 
    The present draft describes the different variants based on similar
    constructions as HOTP.
 
   5. Definition of OCRA
 
    OCRA is a generalization of HOTP with variable data inputs not
    solely based on an incremented counter and secret key values.
 
    The definition of OCRA requires a cryptographic function, a key K
    and a set of DataInput parameters. This section first formally
    introduces the OCRA algorithm and then introduces the definitions
    and default values recommended for all the parameters.
 
    In a nutshell,
                    OCRA = CryptoFunction(K, DataInput)
 
    Where:
 
    - K: a shared secret key known to both parties;
    - DataInput: a structure that contains the concatenation of the
    various input data values. Defined in details in section 5.1;
    - CryptoFunction: this is the function performing the OCRA
    computation from the secret key K and DataInput material;
    CryptoFunction is described in details in section 5.2.
 
    5.1 DataInput Parameters
 
    This structure is the concatenation of all the parameters used in
    the computation of the OCRA values, save for the secret key K.
 
    DataInput = {C | Q | P | S | T} where:
       . C is a 8-byte counter value processed high-order bit first,
         and MUST be synchronized between all parties;
       . Q is the list of (concatenated) challenge question(s)
         generated by the verifier(s);the questions SHOULD be L-byte
         values and MUST be at least t-byte values;
       . P is a SHA1-hash of PIN/password that is known to all parties
         during the execution of the algorithm;
       . S is a string that contains information about the current
         session;
       . T is a timestamp value in number of minutes since midnight UTC
         of January 1, 1970.
 
 
 
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    When computing a response, the concatenation order is always the
    following:
 
                                     C,
                 OTHER-PARTY-GENERATED-CHALLENGE-QUESTION,
                     YOUR-GENERATED-CHALLENGE-QUESTION,
                          P, S and then T values.
 
    If a value is empty (i.e. a certain input is not used in the
    computation) then the value is simply not represented in the
    string.
 
    We always start with C to be compliant and follow the HOPT RFC when
    all the other values are empty. The counter on the token or client
    is incremented every time a new computation is requested by the
    user. The server's counter value is only incremented after a
    successful OCRA authentication
 
    5.2 CryptoFunction
 
    The default CryptoFunction is HOTP-SHA1-6, i.e. the default mode of
    computation for OCRA is HOTP with the default 6-digit dynamic
    truncation and a combination of DataInput values as the message to
    compute the HMAC-SHA1 digest.
 
    We denote t as the digit-length of the truncation output. For
    instance, if t = 6, then the output of the truncation is a 6-digit
    value.
 
    We define the HOTP family of functions as an extension to HOTP:
    - HOTP-H-t: these are the different possible truncated versions of
      HOTP, using the dynamic truncation method for extracting an HOTP
      value from the HMAC output;
    - We will denote HOTP-H-t as the realization of an HOTP function
      that uses an HMAC function with the hash function H, and the
      dynamic truncation as described in [RFC 4226] to extract a t-
      digit value;
    - t=0 means that no truncation is performed and the full HMAC value
      is used for authentication purpose.
 
    We list the following preferred modes of computation, where *
    denotes the default CryptoFunction:
       . HOTP-SHA1-4: HOTP with SHA-1 as the hash function for HMAC
          and a dynamic truncation to a 4-digit value; this mode is not
          recommended in the general case but can be useful when a very
          short authentication code is needed by an application;
       . *HOTP-SHA1-6: HOTP with SHA-1 as the hash function for HMAC
          and a dynamic truncation to a 6-digit value;
 
 
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       . HOTP-SHA256-6: HOTP with SHA-256 as the hash function for
          HMAC and a dynamic truncation to a 6-digit value;
       . HOTP-SHA512-6: HOTP with SHA-512 as the hash function for
          HMAC and a dynamic truncation to a 6-digit value;
 
    This table summarizes all possible values for the CryptoFunction:
 
    Name           HMAC Function Used      Size of Truncation (t)
    --------------------------------------------------------------
    HOTP-SHA1-t       HMAC-SHA1            0 (no truncation), 4-10
    HOTP-SHA256-t     HMAC-SHA256          0 (no truncation), 4-10
    HOTP-SHA512-t     HMAC-SHA512          0 (no truncation), 4-10
 
 
   6. The OCRASuite
 
    The following values define the OCRASuite codes used in the
    description of modes of operation for the OCRA algorithm.
 
    An OCRASuite value defines an OCRA suite of operations as supported
    in the present draft and is represented as follows:
 
 
                     Algorithm:CryptoFunction:DataInput
 
    The client and server need to agree on one or two values of
    OCRASuite. These values may be agreed at time of token provisioning
    or for more sophisticated client-server interactions these values
    may be negotiated for every transaction. Note that for Mutual
    Challenge-Response or Signature with Server Authentication modes,
    the client and server will need to agree on two values of OCRASuite
    - one for server computation and another for client computation.
 
    Algorithm
    ---------
 
    Description: Indicates the version of OCRA algorithm.
    Values: OCRA-v where v represents the version number (e.g. 1, 2
    etc.). This document describes version 1 of the OCRA algorithm.
 
    CryptoFunction
    --------------
 
    Description: Indicates the function used to compute OCRA values
    Values: Permitted values are described in section 5.2
 
    DataInput
    ---------
 
 
 
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    Description: List of valid inputs for the computation; [] indicates
    a value is optional.
    Values:
    [C] | Q | [P | S | T]: Challenge-Response computation
    [C] | Q | [P | T]: Plain Signature computation
 
    Example of possible values: OCRA-1:HOTP-SHA512-8:C-Q-P means
    version 1 of the OCRA algorithm with HMAC-SHA512 function,
    truncated to an 8-digit value, using the counter, a random
    challenge and a hash of the PIN/Password as parameters.
 
   7. Algorithm Modes for Authentication
 
    In this section we describe the typical modes in which the above
    defined computation can be used for authentication.
 
    7.1 One way Challenge-Response
 
    A challenge/response is a security mechanism in which the verifier
    presents a question (challenge) to the prover who must provide a
    valid answer (response) to be authenticated.
 
    To use this algorithm for a one-way challenge-response, the
    verifier will communicate a challenge value (typically randomly
    generated) to the prover. The prover will use the challenge in the
    computation as described above. The prover then communicates the
    response to the verifier to authenticate.
 
    Therefore in this mode, the typical data inputs will be:
 
    C - Counter, optional.
    Q - Challenge question, mandatory, supplied by the verifier.
    P - Hashed version of PIN/password, optional.
    S - Session information, optional
    T - Timestamp, optional.
 
    The picture below shows the messages that are exchanged between the
    client (prover) and the server (verifier) to complete a one-way
    challenge-response authentication.
 
    We assume that the client and server have a pre-shared key K that
    is used for the computation.
 
 
 
 
 
 
 
 
 
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     CLIENT                                     SERVER
    (PROVER)                                  (VERIFIER)
      |                                           |
      |    Verifier sends challenge to prover     |
      |    Challenge = Q                          |
      |<------------------------------------------|
      |                                           |
      |    Prover Computes Response               |
      |    R = OCRA(K, {[C] | Q | [P | S | T]})   |
      |    Response = R                           |
      |------------------------------------------>|
      |                                           |
      |    Verifier Validates Response            |
      |    Response = OK                          |
      |<------------------------------------------|
      |                                           |
 
    7.2 Mutual Challenge-Response
 
    Mutual challenge-response is a variation of one-way challenge-
    response where both the client and server and mutually authenticate
    each other.
 
    To use this algorithm, the client will first send a random client-
    challenge to the server. The server computes the server-response
    and sends it to the client along with a server-challenge.
 
    The client will first verify the server-response to authenticate
    that it is talking to a valid server. It will then compute the
    client-response and send it to the server to authenticate. The
    server verifies the client-response to complete the two-way
    authentication process.
 
    In this mode there are two computations: client-response and
    server-response. There are two separate challenge questions,
    generated by both parties. We denote these challenge questions Q1
    and Q2.
 
    Typical data inputs for server-response computation will be:
    C  - Counter, optional.
    QC - Challenge question, mandatory, supplied by the client.
    QS - Challenge question, mandatory, supplied by the server.
    S  - Session information, optional.
    T  - Timestamp, optional.
 
    Typical data inputs for client-response computation will be:
    C  - Counter, optional.
    QS - Challenge question, mandatory, supplied by the server.
    QC - Challenge question, mandatory, supplied by the client.
 
 
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    P  - Hashed version of PIN/password, optional.
    S  - Session information, optional.
    T  - Timestamp, optional.
 
    The following picture shows the messages that are exchanged between
    the client and the server to complete a two-way mutual challenge-
    response authentication.
 
    We assume that the client and server have a pre-shared key K that
    is used for the computation.
 
 
    CLIENT                                              SERVER
      |                                                   |
      |    1. Client sends client-challenge               |
      |    QC = Client-challenge                          |
      |-------------------------------------------------->|
      |                                                   |
      |    2. Server computes server-response             |
      |       and sends server-challenge                  |
      |    RS = OCRA(K, [C] | QC | QS | [S | T])          |
      |    QS = Server-challenge                          |
      |    Response = RS, QS                              |
      |<--------------------------------------------------|
      |                                                   |
      |    3. Client verifies server-response             |
      |       and computes client-response                |
      |    OCRA(K, [C] | QC | QS | [S | T]) != RS -> STOP |
      |    RC = OCRA(K, [C] | QS | QC | [P | S | T])      |
      |    Response = RC                                  |
      |-------------------------------------------------->|
      |                                                   |
      |    4. Server verifies client-response             |
      |    OCRA(K, [C] | QS | QC | [P|S|T]) != RC -> STOP |
      |    Response = OK                                  |
      |<--------------------------------------------------|
      |                                                   |
 
 
   8. Algorithm Modes for Signature
 
    In this section we describe the typical modes in which the above
    defined computation can be used for digital signatures.
 
    8.1  Plain Signature
 
    To use this algorithm in plain signature mode, the server will
    communicate a signature-challenge value to the client (signer). The
 
 
 
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    signature-challenge is either the data to be signed or derived from
    the data to be signed using a hash function, for example.
 
    The client will use the signature-challenge in the computation as
    described above. The client then communicates the signature value
    (response) to the server to authenticate.
 
    Therefore in this mode, the data inputs will be:
 
    C - Counter, optional.
    QS - Signature-challenge, mandatory, supplied by the server.
    P - Hashed version of PIN/password, optional.
    T - Timestamp, optional.
 
    The picture below shows the messages that are exchanged between the
    client (prover) and the server (verifier) to complete a plain
    signature operation.
 
    We assume that the client and server have a pre-shared key K that
    is used for the computation.
 
     CLIENT                                     SERVER
    (PROVER)                                  (VERIFIER)
      |                                           |
      |    Verifier sends signature-challenge     |
      |    Challenge = QS                         |
      |<------------------------------------------|
      |                                           |
      |    Client Computes Response               |
      |    SIGN = OCRA(K, [C] | QS | [P | T])     |
      |    Response = SIGN                        |
      |------------------------------------------>|
      |                                           |
      |    Verifier Validates Response            |
      |    Response = OK                          |
      |<------------------------------------------|
      |                                           |
 
 
    8.2  Signature with Server Authentication
 
    This mode is a variation of the plain signature mode where the
    client can first authenticates the server before generating a
    digital signature.
 
    To use this algorithm, the client will first send a random client-
    challenge to the server. The server computes the server-response
    and sends it to the client along with a signature-challenge. The
    client will first verify the server-response to authenticate that
 
 
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    it is talking to a valid server. It will then compute the signature
    and send it to the server.
 
    In this mode there are two computations: client-signature and
    server-response.
 
    Typical data inputs for server-response computation will be:
    C - Counter, optional.
    QC - Challenge question, mandatory, supplied by the client.
    T - Timestamp, optional.
 
    Typical data inputs for client-signature computation will be:
    C - Counter, optional.
    QS - Signature-challenge, mandatory, supplied by the server.
    P - Hashed version of PIN/password, optional.
    T - Timestamp, optional.
 
    The picture below shows the messages that are exchanged between the
    client and the server to complete a signature with server
    authentication transaction.
 
    We assume that the client and server have a pre-shared key K that
    is used for the computation.
 
    CLIENT                                              SERVER
      |                                                   |
      |    1. Client sends client-challenge               |
      |    QC = Client-challenge                          |
      |-------------------------------------------------->|
      |                                                   |
      |    2. Server computes server-response             |
      |       and sends signature-challenge               |
      |    RS = OCRA(K, [C] | QC | QS | [T])              |
      |    QS = signature-challenge                       |
      |    Response = RS, QS                              |
      |<--------------------------------------------------|
      |                                                   |
      |    3. Client verifies server-response             |
      |       and computes signature                      |
      |    OCRA(K, [C] | QC | QS | [T]) != R1 -> STOP     |
      |    SIGN = OCRA( K, [C] | QS | QC | [P | T])       |
      |    Signature = SIGN                               |
      |-------------------------------------------------->|
      |                                                   |
      |    4. Server verifies Signature                   |
      |    OCRA(K, [C] | QS | QC | [P|T]) != SIGN -> STOP |
      |    Response = OK                                  |
      |<--------------------------------------------------|
      |                                                   |
 
 
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   9. Security Considerations
 
    Any algorithm is only as secure as the application and the
    authentication protocols that implement it. Therefore, this section
    discusses the critical security requirements that our choice of
    algorithm imposes on the authentication protocol and validation
    software.
 
    9.1 Security Analysis of the OCRA algorithm
 
    The security and strength of this algorithm 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.
 
    The conclusion of the security analysis detailed in [RFC4226] 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.
 
    9.2 Implementation Considerations
 
    S1 - In the authentication mode, the client MUST support two-factor
    authentication, i.e., the communication and verification of
    something you know (secret code such as a Password, Pass phrase,
    PIN code, etc.) and something you have (token).  The secret code is
    known only to the user and usually entered with the Response value
    for authentication purpose (two-factor authentication).
    Alternatively, instead of sending something you know to the server,
    the client may use a hash of the Password or PIN code in the
    computation itself, thus implicitly enabling two-factor
    authentication.
 
    S2 - The keys for HOTP can be of any length equal or longer than L
    bytes, where L is the byte-length of the CryptoFunction output.
    Keys longer than L 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 OCRA, provided the randomness of the
    original key material is sufficient.
 
    S3 - Keys need to be chosen at random or using a cryptographically
    strong pseudo-random generator properly seeded with a random value.
    We RECOMMEND following the recommendations in [RFC1750] for all
    pseudo-random and random generations. The pseudo-random numbers
 
 
 
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    used for generating the keys SHOULD successfully pass the
    randomness test specified in [CN].
 
    S4 - On the client side, the keys MUST be embedded in a tamper
    resistant device or securely implemented in a software application.
    Additionally, by embedding the keys in a hardware device, you also
    have the advantage of improving the flexibility (mobility).
 
    S5 - For authentication computations, the challenge value MUST be
    randomly generated and SHALL NOT be re-used. We RECOMMEND following
    the recommendations in [RFC1750] for all pseudo-random and random
    generations.
 
    S6 - All the communications SHOULD take place over a secure channel
    e.g. SSL/TLS, IPsec connections.
 
    S7 - The OCRA algorithm when used in mutual authentication mode or
    in signature with server authentication mode SHOULD use dual key
    mode - i.e. there are two keys that are shared between the client
    and the server. One shared key is used to generate the server
    response on the server side and to verify it on the client side.
    The other key is used to create the response or signature on the
    client side and to verify the same on the server side.
 
    S8 - We recommend that implementations MAY use the session
    information, S as an additional input in the computation. For
    example, S could be the session identifier from the TLS session.
    This will enable you to counter certain types of man-in-the-middle
    attacks. However, this will introduce the additional dependency
    that first of all the prover needs to have access to the session
    identifier to compute the response and the verifier will need
    access to the session identifier to verify the response.
 
    S9 - In the signature mode, whenever the counter or time (defined
    as optional elements) are not used in the computation, there might
    be a risk of replay attack and the implementers should carefully
    consider this issue in the light of their specific application
    requirements and security guidelines.
 
    S10 - We also RECOMMEND storing the shared secrets securely in the
    validation system, and more specifically encrypting the shared
    secrets using tamper-resistant hardware encryption and exposing
    them only when required: for example, the shared secret is
    decrypted when needed to verify an HOTP value, and re-encrypted
    immediately to limit exposure in the RAM for a short period of
    time.  The data store holding the shared secrets MUST be in a
    secure area, to avoid as much as possible direct attack on the
    validation system and secrets database.
 
 
 
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    Particularly, access to the shared secrets should be limited to
    programs and processes required by the validation system only.  We
    will not elaborate on the different security mechanisms to put in
    place, but obviously, the protection of shared secrets is of the
    uttermost importance.
 
   10. IANA Considerations
 
    This document has no actions for IANA.
 
 
   11. Conclusion
 
    This draft introduced several variants of HOTP for challenge-
    response based authentication and short signature-like
    computations.
 
    The OCRASuite provides for an easy integration and support of
    different flavors within an authentication and validation system.
 
    Finally, OCRA should enable cross-authentication both in connected
    and off-line modes, with the support of different response sizes
    and mode of operations.
 
 
   12. Acknowledgements
 
    We would like to thank Philip Hoyer, Jonathan Tuliani, Shuh Chang,
    Stu Vaeth, Jon Martinsson, Jeff Burstein, Frederik Mennes, Oanh
    Hoang, Mingliang Pei and Enrique Rodriguez for their comments and
    suggestions to improve this draft document.
 
 
   13. References
 
    13.1 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 Recommendations 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.
 
 
 
 
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 OCRA: OATH Challenge Response Algorithms                 December 2007
 
 
    [RFC3668]  S. Bradner, "Intellectual Property Rights in IETF
                Technology", BCP 79, RFC 3668, February 2004.
 
    [RFC4226]   D. M'Raihi, M. Bellare, F. Hoornaert, D. Naccache and
                O. Ranen, "HOTP: An HMAC-based One Time Password
                Algorithm", IETF Network Working Group, RFC 4226,
                December 2005.
 
 
    13.2 Informative
 
    [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
 
 
 
 
 Appendix A: Source Code
 
    import java.lang.reflect.UndeclaredThrowableException;
    import java.security.GeneralSecurityException;
    import javax.crypto.Mac;
    import javax.crypto.spec.SecretKeySpec;
 
    /**
     * This an example implementation of the OATH OCRA algorithm.
     * Visit www.openauthentication.org for more information.
     *
     * @author Johan Rydell, PortWise
     */
    public class OCRA {
       private OCRA() {}
 
       /**
        * This method uses the JCE to provide the crypto
        * algorithm.
        * HMAC computes a Hashed Message Authentication Code with the
        * crypto hash algorithm as a parameter.
 
 
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 OCRA: OATH Challenge Response Algorithms                 December 2007
 
 
        *
        * @param crypto     the crypto algorithm
        *                   (HmacSHA1, HmacSHA256, HmacSHA512)
        * @param keyBytes   the bytes to use for the HMAC key
        * @param text       the message or text to be authenticated.
        */
       public static byte[] hmac_sha1(String crypto,
                                       byte[] keyBytes,
                                       byte[] text)
       {
          try {
             Mac hmac;
             hmac = Mac.getInstance(crypto);
             SecretKeySpec macKey =
                new SecretKeySpec(keyBytes, "RAW");
             hmac.init(macKey);
             return hmac.doFinal(text);
          } catch (GeneralSecurityException gse) {
             throw new UndeclaredThrowableException(gse);
          }
       }
 
       private static final int[] DIGITS_POWER
       // 0 1  2   3    4     5      6       7        8
       = {1,10,100,1000,10000,100000,1000000,10000000,100000000};
 
       /**
        * This method generates an OCRA HOTP value for the given
        * set of parameters.
        *
        * @param crypto              the crypto algorithm
        * @param key                 the shared secret
        * @param movingFactor        the counter that changes
        *                             on a per use basis
        * @param question            the challenge question
        * @param password            a password that can be used
        * @param sessionInformation    Static information that
        *                            identifies the current session
        * @param timeStamp           a value that reflects a time
        * @param codeDigits          number of digits in the OTP
        *
        * @return A numeric String in base 10 that includes
        * {@link truncationDigits} digits
        */
       static public String generateOTP(String crypto,
             String key,
             String movingFactor,
             String question,
             String password,
 
 
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 OCRA: OATH Challenge Response Algorithms                 December 2007
 
 
             String sessionInformation,
             String timeStamp,
             int codeDigits)
       {
          String result = null;
          String messageStr =
             question + password +
             sessionInformation + timeStamp ;
          byte[] msg;
 
          // Using the counter
          if (0 < movingFactor.length()){
             // First 8 bytes are for the movingFactor
             // Complient with RFC 4226
             messageStr = "00000000" + messageStr;
             msg = messageStr.getBytes();
             long mFactor = Long.decode(movingFactor);
             for (int i = 7; i >= 0; i--) {
                msg[i] = (byte) (mFactor & 0xff);
                mFactor >>= 8;
             }
          }else
             msg = messageStr.getBytes();
 
          // compute hmac hash
          byte[] hash = hmac_sha1(crypto, key.getBytes(), msg);
 
          // put selected bytes into result int
          int offset = hash[hash.length - 1] & 0xf;
 
          int binary =
             ((hash[offset] & 0x7f) << 24) |
               ((hash[offset + 1] & 0xff) << 16) |
             ((hash[offset + 2] & 0xff) << 8) |
               (hash[offset + 3] & 0xff);
 
          int otp = binary % DIGITS_POWER[codeDigits];
 
          result = Integer.toString(otp);
          while (result.length() < codeDigits) {
             result = "0" + result;
          }
          return result;
       }
    }
 
 
 
 
 
 
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 OCRA: OATH Challenge Response Algorithms                 December 2007
 
 
 Appendix B: Test Vectors
 
    Plain challenge response
    ========================
 
    OCRA-HOTP-SHA1-8-Q
    ------------------
    K = 12345678901234567890      Q = 10000000      OCRA = 57953866
    K = 12345678901234567890      Q = 10000001      OCRA = 15772773
    K = 12345678901234567890      Q = 10000002      OCRA = 68105940
 
    OCRA-HOTP-SHA256-8-Q
    --------------------
    K = 12345678901234567890      Q = 10000000      OCRA = 79730854
    K = 12345678901234567890      Q = 10000001      OCRA = 22925447
    K = 12345678901234567890      Q = 10000002      OCRA = 15947867
 
    OCRA-HOTP-SHA512-8-Q
    --------------------
    K = 12345678901234567890      Q = 10000000      OCRA = 68325835
    K = 12345678901234567890      Q = 10000001      OCRA = 53995836
    K = 12345678901234567890      Q = 10000002      OCRA = 89008345
 
 
    Mutual challenge response
    =========================
 
    OCRA-HOTP-SHA512-8-Q
    --------------------
    (From server) K = 12345678901234567890
    Q1 = 11111110     Q2 = 22222220     OCRA = 70933163
    (From client) K = 12345678901234567890
    Q1 = 11111110     Q2 = 22222220     OCRA = 63875222
 
    (From server) K = 12345678901234567890
    Q1 = 11111111     Q2 = 22222221     OCRA = 08364053
    (From client) K = 12345678901234567890
    Q1 = 11111111     Q2 = 22222221     OCRA = 91844292
 
    (From server) K = 12345678901234567890
    Q1 = 11111112     Q2 = 22222222     OCRA = 70960179
    (From client) K = 12345678901234567890
    Q1 = 11111112     Q2 = 22222222     OCRA = 75789938
 
 
 
 
 
 
 
 
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 OCRA: OATH Challenge Response Algorithms                 December 2007
 
 
    Plain signature
    ===============
 
    OCRA-HOTP-SHA512-8-Q
    --------------------
    K = 12345678901234567890      Q (value) = 00010000
    OCRA (signature) = 13175449
    K = 12345678901234567890      Q (value) = 00011000
    OCRA (signature) = 41866883
    K = 12345678901234567890      Q (value) = 00012000
    OCRA (signature) = 82912137
 
 
   14. 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.
    275 Hawthorne Ave, Suite 119     Phone: 1-650-515-3569
    Palo Alto, CA 94301 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
 
    Siddharth Bajaj
    VeriSign, Inc.
    487 E. Middlefield Road          Phone: 1-650-426-3458
    Mountain View, CA 94043 USA      Email: sbajaj@verisign.com
 
 
 
 
 
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 OCRA: OATH Challenge Response Algorithms                 December 2007
 
 
   15. Full Copyright Statement
 
    Copyright (C) The IETF Trust (2007).
 
    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
    an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
    REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE
    IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL
    WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY
    WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE
    ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS
    FOR A PARTICULAR PURPOSE.
 
 
   16. Intellectual Property
 
    The IETF takes no position regarding the validity or scope of any
    Intellectual Property Rights or other rights that might be claimed
    to pertain to the implementation or use of the technology described
    in this document or the extent to which any license under such
    rights might or might not be available; nor does it represent that
    it has made any independent effort to identify any such rights.
    Information on the procedures with respect to rights in RFC
    documents can be found in BCP 78 and BCP 79.
 
    Copies of IPR disclosures made to the IETF Secretariat and any
    assurances of licenses to be made available, or the result of an
    attempt made to obtain a general license or permission for the use
    of such proprietary rights by implementers or users of this
    specification can be obtained from the IETF on-line IPR repository
    at http://www.ietf.org/ipr.
 
    The IETF invites any interested party to bring to its attention any
    copyrights, patents or patent applications, or other proprietary
    rights that may cover technology that may be required to implement
    this standard. Please address the information to the IETF at ietf-
    ipr@ietf.org.
 
 
 
 
 
 
 
 
 
 
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