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Versions: 00 01 02 03 04 RFC 4226

    Internet Draft                                           D. M'Raihi
    Category: Informational                                    VeriSign
    Document: draft-mraihi-oath-hmac-otp-04.txt              M. Bellare
    Expires: April 2005                                            UCSD
                                                           F. Hoornaert
                                                                  Vasco
                                                            D. Naccache
                                                                Gemplus
                                                               O. Ranen
                                                                Aladdin
                                                           October 2004
 
              HOTP: An HMAC-based One Time Password Algorithm
 
 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 an algorithm to generate one-time password
    values, based on HMAC [BCK1]. A security analysis of the algorithm
    is presented, and important parameters related to the secure
    deployment of the algorithm are discussed. The proposed algorithm
    can be used across a wide range of network applications ranging
    from remote VPN access, Wi-Fi network logon to transaction-oriented
    Web applications.
 
    This work is a joint effort by the OATH (Open AuTHentication)
    membership to specify an algorithm that can be freely distributed
    to the technical community. The authors believe that a common and
    shared algorithm will facilitate adoption of two-factor
    authentication on the Internet by enabling interoperability across
    commercial and open-source implementations. 
 
 
 
 Table of Contents
 
    1.   Overview...................................................3
    2.   Introduction...............................................3
    3.   Requirements Terminology...................................4
    4.   Algorithm Requirements.....................................4
    5.   HOTP Algorithm.............................................5
    5.1  Notation and Symbols.......................................5
    5.2  Description................................................5
    5.3  Generating an HOTP value...................................6
    5.4  Example of HOTP computation for Digit = 6..................7
    6.   Security Considerations....................................7
    6.1  Authentication Protocol Requirements.......................8
    6.2  Validation of HOTP values..................................8
    6.3  Bi-directional Authentication..............................9
    6.4  Throttling at the server...................................9
    6.5  Resynchronization of the counter...........................9
    6.6  Management of Shared Secrets..............................10
    7.   HOTP Algorithm Security: Overview.........................12
    8.   Composite Shared Secrets..................................13
    9.   IANA Considerations.......................................13
    10.  Conclusion................................................13
    11.  Acknowledgements..........................................13
    12.  Contributors..............................................13
    13.  References................................................14
    12.1   Normative...............................................14
    12.2   Informative.............................................14
    14.  Authors' Addresses........................................15
    15. Full Copyright Statement...................................15
    16. Intellectual Property......................................16
    Appendix A - HOTP Algorithm Security: Detailed Analysis........16
    A.1 Definitions and Notations..................................16
    A.2 The idealized algorithm: HOTP-IDEAL........................17
    A.3 Model of Security..........................................17
    A.4 Security of the ideal authentication algorithm.............19
    A.4.1 From bits to digits......................................19
    A.4.2 Brute force attacks......................................20
    A.4.3 Brute force attacks are the best possible attacks........21
    A.5 Security Analysis of HOTP..................................22
    Appendix B - SHA-1 Attacks.....................................23
    B.1 SHA-1 status...............................................23
    B.2 HMAC-SHA-1 status..........................................24
    B.3 HOTP status................................................25
    Appendix C - HOTP Algorithm: Reference Implementation..........25
    Appendix D - HOTP Algorithm: Test Values.......................29
    Appendix E - Extensions........................................29
    E.1  Number of Digits..........................................30
    E.2  Alpha-numeric Values......................................30
    E.3  Sequence of HOTP values...................................30
    E.4  A Counter-based Re-Synchronization Method.................31
    E.5  Data Field................................................31
 
     
 
 
 
   1. Overview
 
    The document introduces first the context around the HOTP
    algorithm. In section 4, the algorithm requirements are listed and
    in section 5, the HOTP algorithm is described. Sections 6 and 7
    focus on the algorithm security. Section 8 proposes some extensions
    and improvements, and Section 9 concludes this document. The
    interested reader will find in the Appendix a detailed, full-fledge
    analysis of the algorithm security: an idealized version of the
    algorithm is evaluated, and then the HOTP algorithm security is
    analyzed.
 
   2. Introduction
 
    Today, deployment of two-factor authentication remains extremely
    limited in scope and scale. Despite increasingly higher levels of
    threats and attacks, most Internet applications still rely on weak
    authentication schemes for policing user access. The lack of
    interoperability among hardware and software technology vendors has
    been a limiting factor in the adoption of two-factor authentication
    technology. In particular, the absence of open specifications has
    led to solutions where hardware and software components are tightly
    coupled through proprietary technology, resulting in high cost
    solutions, poor adoption and limited innovation.
 
    In the last two years, the rapid rise of network threats has
    exposed the inadequacies of static passwords as the primary mean of
    authentication on the Internet. At the same time, the current
    approach that requires an end-user to carry an expensive,
    single-function device that is only used to authenticate to the
    network is clearly not the right answer.  For two factor
    authentication to propagate on the Internet, it will have to be
    embedded in more flexible devices that can work across a wide range
    of applications.
 
    The ability to embed this base technology while ensuring broad
    interoperability require that it be made freely available to the
    broad technical community of hardware and software developers. Only
    an open system approach will ensure that basic two-factor
    authentication primitives can be built into the next-generation of
    consumer devices such USB mass storage devices, IP phones, and
    personal digital assistants).
 
    One Time Password is certainly one of the simplest and most popular
    forms of two-factor authentication for securing network access. For
    example, in large enterprises, Virtual Private Network access often
    requires the use of One Time Password tokens for remote user
    authentication. One Time Passwords are often preferred to stronger
    forms of authentication such as PKI or biometrics because an
    air-gap device does not require the installation of any client
    desktop software on the user machine, therefore allowing them to
    roam across multiple machines including home computers, kiosks and
    personal digital assistants. 
 
 
 
 
    This draft proposes a simple One Time Password algorithm that can
    be implemented by any hardware manufacturer or software developer
    to create interoperable authentication devices and software agents.
    The algorithm is event-based so that it can be embedded in high
    volume devices such as Java smart cards, USB dongles and GSM SIM
    cards. The presented algorithm is made freely available to the
    developer community under the terms and conditions of the IETF
    Intellectual Property Rights [RFC3668].
 
    The authors of this document are members of the Open AuTHentication
    initiative [OATH]. The initiative was created in 2004 to facilitate
    collaboration among strong authentication technology providers.
 
   3. 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.
 
   4. Algorithm Requirements
 
    This section presents the main requirements that drove this
    algorithm design. A lot of emphasis was placed on end-consumer
    usability as well as the ability for the algorithm to be
    implemented by low cost hardware that may provide minimal user
    interface capabilities. In particular, the ability to embed the
    algorithm into high volume SIM and Java cards was a fundamental
    pre-requisite.
 
    R1 - The algorithm MUST be sequence or counter-based: One of the
    goals is to have the HOTP algorithm embedded in high volume devices
    such as Java smart cards, USB dongles and GSM SIM cards.
 
    R2 - The algorithm SHOULD be economical to implement in hardware by
    minimizing requirements on battery, number of buttons,
    computational horsepower, and size of LCD display.
 
    R3 - The algorithm MUST work with tokens that do not supports any
    numeric input, but MAY also be used with more sophisticated devices
    such as secure PIN-pads.
 
    R4 - The value displayed on the token MUST be easily read and
    entered by the user: This requires the HOTP value to be of
    reasonable length. The HOTP value must be at least a 6-digit value.
    It is also desirable that the HOTP value be 'numeric only' so that
    it can be easily entered on restricted devices such as phones.
 
    R5 - There MUST be user-friendly mechanisms available to
    resynchronize the counter. The sections 6.4 and 8.4 detail the
    resynchronization mechanism proposed in this draft.
     
 
 
 
    R6 - The algorithm MUST use a strong shared secret. The length of
    the shared secret MUST be at least 128 bits. This draft RECOMMENDs
    a shared secret length of 160 bits.
 
   5. HOTP Algorithm
 
    In this section, we introduce the notation and describe the HOTP
    algorithm basic blocks - the base function to compute an HMAC-SHA-1
    value and the truncation method to extract an HOTP value.
 
    5.1  Notation and Symbols
 
    A string always means a binary string, meaning a sequence of zeros
    and ones.
 
    If s is a string then |s| denotes its length.
 
    If n is a number then |n| denotes its absolute value.
 
    If s is a string then s[i] denotes its i-th bit. We start numbering
    the bits at 0, so s = s[0]s[1]..s[n-1] where n = |s| is the length
    of s.
 
    Let StToNum (String to Number) denote the function which as input a
    string s returns the number whose binary representation is s.
    (For example StToNum(110) = 6).
 
    Here is a list of symbols used in this document.
 
    Symbol   Represents
    -------------------------------------------------------------------
    C       8-byte counter value, the moving factor. This counter
            MUST be synchronized between the HOTP generator (client)
            and the HOTP validator (server);
 
    K       shared secret between client and server; each HOTP
             generator has a different and unique secret K;
 
    T        throttling parameter: the server will refuse connections
             from a user after T unsuccessful authentication attempts;
 
    s        resynchronization parameter: the server will attempt to
             verify a received authenticator across s consecutive
             counter values;
 
    Digit    number of digits in an HOTP value; system parameter.
 
    5.2  Description
 
    The HOTP algorithm is based on an increasing counter value and a
    static symmetric key known only to the token and the validation
    service. In order to create the HOTP value, we will use the  
 
 
 
    HMAC-SHA-1 algorithm, as defined in RFC 2104 [BCK2].
 
    As the output of the HMAC-SHA1 calculation is 160 bits, we must
    truncate this value to something that can be easily entered by a
    user.
 
                   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 as defined in Section 5.3.
 
    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.
 
    5.3  Generating an HOTP value
 
    We can describe the operations in 3 distinct steps:
 
    Step 1: Generate an HMAC-SHA-1 value
    Let HS = HMAC-SHA-1(K,C)  // HS is a 20 byte string
 
    Step 2: Generate a 4-byte string (Dynamic Truncation)
    Let Sbits = DT(HS)   //  DT, defined in Section 6.3.1
                         //  returns a 31 bit string
 
    Step 3: Compute an HOTP value
    Let Snum  = StToNum(S)        // Convert S to a number in
                                     0...2^{31}-1
    Return D = Snum mod 10^Digit //  D is a number in the range
                                     0...10^{Digit}-1
 
    The Truncate function performs Step 2 and Step 3, i.e. the dynamic
    truncation and then the reduction modulo 10^Digit. The purpose of
    the dynamic offset truncation technique is to extract a 4-byte
    dynamic binary code from a 160-bit (20-byte) HMAC-SHA1 result.
 
    DT(String) // String = String[0]...String[19]
     Let OffsetBits be the low order four bits of String[19]
     Offset = StToNum(OffSetBits) // 0 <= OffSet <= 15
     Let P = String[OffSet]...String[OffSet+3]
     Return the Last 31 bits of P
 
    The reason for masking the most significant bit of P is to avoid
    confusion about signed vs. unsigned modulo computations. Different
    processors perform these operations differently, and masking out
    the signed bit removes all ambiguity.
     
 
 
 
    Implementations MUST extract a 6-digit code at a minimum and
    possibly 7 and 8-digit code. Depending on security requirements,
    Digit = 7 or more SHOULD be considered in order to extract a longer
    HOTP value.
 
    The following paragraph is an example of using this technique for
    Digit = 6, i.e. that a 6-digit HOTP value is calculated from the
    HMAC value.
 
    5.4  Example of HOTP computation for Digit = 6
 
    The following code example describes the extraction of a dynamic
    binary code given that hmac_result is a byte array with the
    HMAC-SHA1 result:
 
        int offset   =  hmac_result[19] & 0xf ;
        int bin_code = (hmac_result[offset]  & 0x7f) << 24
           | (hmac_result[offset+1] & 0xff) << 16
           | (hmac_result[offset+2] & 0xff) <<  8
           | (hmac_result[offset+3] & 0xff) ;
 
 
    SHA-1 HMAC Bytes (Example)
 
    -------------------------------------------------------------
    | Byte Number                                               |
    -------------------------------------------------------------
    |00|01|02|03|04|05|06|07|08|09|10|11|12|13|14|15|16|17|18|19|
    -------------------------------------------------------------
    | Byte Value                                                |
    -------------------------------------------------------------
    |1f|86|98|69|0e|02|ca|16|61|85|50|ef|7f|19|da|8e|94|5b|55|5a|
    -------------------------------***********----------------++|
 
    * The last byte (byte 19) has the hex value 0x5a.
    * The value of the lower four bits is 0xa (the offset value).
    * The offset value is byte 10 (0xa).
    * The value of the 4 bytes starting at byte 10 is 0x50ef7f19,
      which is the dynamic binary code DBC1
    * The MSB of DBC1 is 0x50 so DBC2 = DBC1 = 0x50ef7f19
    * HOTP = DBC2 modulo 10^6 = 872921.
 
    We treat the dynamic binary code as a 31-bit, unsigned, big-endian
    integer; the first byte is masked with a 0x7f.
 
    We then take this number modulo 1,000,000 (10^6) to generate the
    6-digit HOTP value 872921 decimal.
 
   6. Security Considerations
 
    Any One-Time Password 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.
 
    The parameters T and s discussed in this section have a significant
    impact on the security - further details in Section 7 elaborate on
    the relations between these parameters and their impact on the
    system security.
 
    It is also important to remark that the HOTP algorithm is not a
    substitute for encryption and does not provide for the privacy of
    data transmission. Other mechanisms should be used to defeat
 
    6.1  Authentication Protocol Requirements
 
    We introduce in this section some requirements for a protocol P
    implementing HOTP as the authentication method between a prover and
    a verifier.
 
    RP1 - P MUST be two-factor, i.e. 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 one-time password value for authentication purpose
    (two-factor authentication).
 
    RP2 - P SHOULD NOT be vulnerable to brute force attacks. This
    implies that a throttling/lockout scheme is RECOMMENDED on the
    validation server side.
 
    RP3 - P SHOULD be implemented with respect to the state of the art
    in terms of security, in order to avoid the usual attacks and risks
    associated with the transmission of sensitive data over a public
    network (privacy, replay attacks, etc.)
 
    6.2  Validation of HOTP values
 
    The HOTP client (hardware or software token) increments its counter
    and then calculates the next HOTP value HOTP-client. If the value
    received by the authentication server matches the value calculated
    by the client, then the HOTP value is validated. In this case, the
    server increments the counter value by one.
 
    If the value received by the server does not match the value
    calculated by the client, the server initiate the resynch protocol
    (look-ahead window) before it requests another pass.
 
    If the resynch fails, the server asks then for another
    authentication pass of the protocol to take place, until the
    maximum number of authorized attempts is reached.
     
 
 
 
    If and when the maximum number of authorized attempts is reached,
    the server SHOULD lock out the account and initiate a procedure to
    inform the user.
 
    6.3  Bi-directional Authentication
 
    Interestingly enough, the HOTP client could also be used to
    authenticate the validation server, claiming that it is a genuine
    entity knowing the shared secret.
 
    Since the HOTP client and the server are synchronized and share the
    same secret (or a method to recompute it) a simple 3-pass protocol
    could be put in place:
    1- The end user enter the TokenID and a first OTP value OTP1;
    2- The server checks OTP1 and if correct, sends back OTP2;
    3- The end user checks OTP2 using his HOTP device and if correct,
    uses the web site.
 
    Obviously, as indicated previously, all the OTP communications have
    to take place over secure https (SSL) connections.
 
    6.4  Throttling at the server
 
    Truncating the HMAC-SHA1 value to a shorter value makes a brute
    force attack possible. Therefore, the authentication server needs
    to detect and stop brute force attacks.
 
    We RECOMMEND setting a throttling parameter T, which defines the
    maximum number of possible attempts for One-Time-Password
    validation. The validation server manages individual counters per
    HOTP device in order to take note of any failed attempt. We
    RECOMMEND T not to be too large, particularly if the
    resynchronization method used on the server is window-based, and
    the window size is large. T SHOULD be set as low as possible, while
    still ensuring usability is not significantly impacted.
 
    Another option would be to implement a delay scheme to avoid a
    brute force attack. After each failed attempt A, the authentication
    server would wait for an increased T*A number of seconds, e.g. say
    T = 5, then after 1 attempt, the server waits for 5 seconds, at the
    second failed attempt, it waits for 5*2 = 10 seconds, etc.
 
    The delay or lockout schemes MUST be across login sessions to
    prevent attacks based on multiple parallel guessing techniques.
 
    6.5  Resynchronization of the counter
 
    Although the server's counter value is only incremented after a
    successful HOTP authentication, the counter on the token is
    incremented every time a new HOTP is requested by the user. Because
    of this, the counter values on the server and on the token might be
    out of synchronization.
     
 
 
 
    We RECOMMEND setting a look-ahead parameter s on the server, which
    defines the size of the look-ahead window. In a nutshell, the
    server can recalculate the next s HOTP-server values, and check
    them against the received HOTP-client.
 
    Synchronization of counters in this scenario simply requires the
    server to calculate the next HOTP values and determine if there is
    a match. Optionally, the system MAY require the user to send a
    sequence of (say 2, 3) HOTP values for resynchronization purpose,
    since forging a sequence of consecutive HOTP values is even more
    difficult than guessing a single HOTP value.
 
    The upper bound set by the parameter s ensures the server does not
    go on checking HOTP values forever (causing a DoS attack) and also
    restricts the space of possible solutions for an attacker trying to
    manufacture HOTP values. s SHOULD be set as low as possible, while
    still ensuring usability is not impacted.
 
    6.6  Management of Shared Secrets
 
    The operations dealing with the shared secrets used to generate and
    verify OTP values must be performed securely, in order to mitigate
    risks of any leakage of sensitive information. We describe in this
    section different modes of operations and techniquest to perform
    these different operations with respect of the state of the art in
    terms of data security.
 
    We can consider two different avenues for generating and storing
    (securely) shared secrets in the Validation system:
      * Deterministic Generation: secrets are derived from a master
    seed, both at provisioning and verification stages and generated
    on-the-fly whenever it is required;
      * Random Generation: secrets are generated randomly at
    provisioning stage, and must be stored immediately and kept secure
    during their life cycle.
 
    Deterministic Generation
    ------------------------
 
    A possible strategy is to derive the shared secrets from a master
    secret. The master secret will be stored at the server only. A
    tamper resistant device MUST be used to store the master key and
    derive the shared secrets from the master key and some public
    information. The main benefit would be to avoid the exposure of the
    shared secrets at any time and also avoid specific requirements on
    storage, since the shared secrets could be generated on-demand when
    needed at provisioning and validation time.
 
    We distinguish two different cases:
     - A single master key MK is used to derive the shared secrets;
        each HOTP device has a different secret, K_i = SHA-1 (MK,i)
        where i stands for a public piece of information that
        identifies uniquely the HOTP device such as a serial number, a 
 
 
 
        token ID, etc.; obviously, this is in the context of an
        application or service - different application or service
        providers will have different secrets and settings;
     - Several master keys MK_i are used and each HOTP device stores a
        set of different derived secrets, {K_i,j = SHA-1(MK_i,j)} where
        j stands for a public piece of information identifying the
        device. The idea would be to store ONLY the active master key
        at the validation server, in the HSM, and keep in a safe place,
        using secret sharing methods such as [Shamir] for instance. In
        this case, if a master secret MK_i is compromised, then it is
        possible to switch to another secret without replacing all the
        devices.
 
    The drawback in the deterministic case is that the exposure of the
    master secret would obviously enable an attacker to rebuild any
    shared secret based on correct public information. The revocation
    of all secrets would be required, or switching to a new set of
    secrets in the case of multiple master keys.
 
    On the other hand, the device used to store the master key(s) and
    generate the shared secrets MUST be tamper resistant. Furthermore,
    the HSM will not be exposed outside the security perimeter of the
    validation system, therefore reducing the risk of leakage.
 
    Random Generation
    -----------------
 
    The shared secrets are randomly generated. We RECOMMEND to follow
    the recommendations in [RFC1750] and to select a good and secure
    random source for generating these secrets. A (true) random
    generator requires a naturally occurring source of randomness.
    Practically, there are two possible avenues to consider for the
    generation of the shared secrets:
 
      * Hardware-based generators: they exploit the randomness which
    occurs in physical phenomena. A nice implementation can be based on
    oscillators, and built in such ways that active attacks are more
    difficult to perform.
 
      * Software-based generators: designing a good software random
    generator is not an easy task. A simple, but efficient,
    implementation should be based on various sources, and apply to the
    sampled sequence a one-way function such as SHA-1.
 
    We RECOMMEND to select proven products, being hardware or software
    generators for the computation of shared secrets.
 
    We also RECOMMEND storing the shared secrets securely, and more
    specifically encrypting the shared secrets when stored using
    tamper-resistant hardware encryption, and exposing them only when
    required: e.g. 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.
 
    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.
 
   7. HOTP Algorithm Security: Overview
 
    The conclusion of the security analysis detailed in the Appendix
    section is that, for all practical purposes, the outputs of the
    dynamic truncation (DT) on distinct counter inputs are uniformly
    and independently distributed 31-bit strings.
 
    The security analysis then details the impact of the conversion
    from a string to an integer and the final reduction modulo
    10^Digit, where Digit is the number of digits in an HOTP value.
 
    The analysis demonstrates that these final steps introduce a
    negligible bias, which does not impact the security of the HOTP
    algorithm, in the sense that the best possible attack against the
    HOTP function is the brute force attack.
 
    Assuming an adversary is able to observe numerous protocol
    exchanges and collect sequences of successful authentication
    values. This adversary, trying to build a function F to generate
    HOTP values based on his observations, will not have a significant
    advantage over a random guess.
 
    The logical conclusion is simply that is best strategy will once
    again be to perform a brute force attack to enumerate and try all
    the possible values.
 
    Considering the security analysis in the Appendix section of this
    document, without loss of generality, we can approximate closely
    the security of the HOTP algorithm by the following formula:
 
                             Sec = sv/10^Digit
 
    Where:
     - Sec is the probability of success of the adversary
     - s stands for the look-ahead synchronization window size;
     - v stands for the number of verification attempts;
     - Digit stands for the number of digits in HOTP values.
 
    Obviously, we can play with s, T (the Throttling parameter that
    would limit the number of attempts by an attacker) and Digit until
    achieving a certain level of security, still preserving the system
    usability.
     
 
 
 
   8. Composite Shared Secrets
 
    It may be desirable to include additional authentication factors in
    the shared secret K. These additional factors can consist of any
    data known at the token but not easily obtained by others. Examples
    of such data include:
       * PIN or Password obtained as user input at the token
       * Phone number
       * Any unique identifier programmatically available at the token
 
    In this scenario the composite shared secret K is constructed
    during the provisioning process from a random seed value combined
    with one or more additional authentication factors. The server
    could either build on-demand or store composite secrets - in any
    case, depending on implementation choice, the token only stores the
    seed value. When the token performs the HOTP calculation it
    computes K from the seed value and the locally derived or input
    values of the other authentication factors.
 
    The use of composite shared secrets can strengthen HOTP based
    authentication systems through the inclusion of additional
    authentication factors at the token. To the extent that the token
    is a trusted device this approach has the further benefit of not
    requiring exposure of the authentication factors (such as the user
    input PIN) to other devices.
 
   9. IANA Considerations
 
    This document has no actions for IANA.
 
   10. Conclusion
 
    This draft describes HOTP, a HMAC-based One-Time Password
    algorithm. It also recommends the preferred implementation and
    related modes of operations for deploying the algorithm.
 
    The draft also exhibits elements of security and demonstrates that
    the HOTP algorithm is practical and sound, the best possible attack
    being a brute force attack that can be prevented by careful
    implementation of countermeasures in the validation server.
 
    Eventually, several enhancements have been proposed, in order to
    improve security if needed for specific applications.
 
   11. Acknowledgements
 
    The authors would like to thank Siddharth Bajaj, Alex Deacon, Loren
    Hart and Nico Popp for their help during the conception and
    redaction of this document.
 
   12. Contributors
     
 
 
 
    The authors of this draft would like to emphasize the role of three
    persons who have made a key contribution to this document:
 
    - Laszlo Elteto is system architect with SafeNet, Inc.
 
    - Ernesto Frutos is director of Engineering with Authenex, Inc.
 
    - Fred McClain is Founder and CTO with Boojum Mobile, Inc.
 
    Without their advice and valuable inputs, this draft would not be
    the same.
 
   13. References
 
    12.1 Normative
 
    [BCK1]      M. Bellare, R. Canetti and H. Krawczyk, "Keyed Hash
                Functions and Message Authentication", Proceedings of
                Crypto'96, LNCS Vol. 1109, pp. 1-15.
 
    [BCK2]      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.
 
    12.2 Informative
 
    [OATH]     Initiative for Open AuTHentication
    http://www.openauthentication.org
 
    [PrOo]     B. Preneel and P. van Oorschot, "MD-x MAC and building
                fast MACs from hash functions", Advances in Cryptology
                CRYPTO '95, Lecture Notes in Computer Science Vol. 963,
                D. Coppersmith ed., Springer-Verlag, 1995.
 
    [Crack]     Crack in SHA-1 code 'stuns' security gurus
    http://www.eetimes.com/showArticle.jhtml?articleID=60402150
 
    [Sha1]      Bruce Schneier. SHA-1 broken. February 15, 2005.
    http://www.schneier.com/blog/archives/2005/02/sha1_broken.html
 
 
 
     
 
 
 
    [Res]       Researchers: Digital encryption standard flawed
    http://news.com.com/Researchers+Digital+encryption+standard+flawed/
    2100-1002-5579881.html?part=dht&tag=ntop&tag=nl.e703
 
    [Shamir]    How to Share a Secret, by Adi Shamir. In Communications
    of the ACM, Vol. 22, No. 11, pp. 612-613, November, 1979.
 
   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:
 
    Mihir Bellare
    Dept of Computer Science and Engineering, Mail Code 0114
    University of California at San Diego
    9500 Gilman Drive
    La Jolla, CA 92093, USA          Email: mihir@cs.ucsd.edu
 
    Frank Hoornaert
    VASCO Data Security, Inc.
    Koningin Astridlaan 164
    1780 Wemmel, Belgium             Email: frh@vasco.com
 
    David Naccache
    Gemplus Innovation
    34 rue Guynemer, 92447,
    Issy les Moulineaux, France      Email: david.naccache@gemplus.com
    and
    Information Security Group,
    Royal Holloway,
    University of London, Egham,
    Surrey TW20 0EX, UK              Email: david.naccache@rhul.ac.uk
 
    Ohad Ranen
    Aladdin Knowledge Systems Ltd.
    15 Beit Oved Street
    Tel Aviv, Israel 61110           Email: Ohad.Ranen@ealaddin.com
 
   15. 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
    an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
    REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY 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.
 
 Appendix A - HOTP Algorithm Security: Detailed Analysis
 
    The security analysis of the HOTP algorithm is summarized in this
    section. We first detail the best attack strategies, and then
    elaborate on the security under various assumptions, the impact of
    the truncation and some recommendations regarding the number of
    digits.
 
    We focus this analysis on the case where Digit = 6, i.e. an HOTP
    function that produces 6-digit values, which is the bare minimum
    recommended in this draft.
 
   A.1 Definitions and Notations
 
    We denote by {0,1}^l the set of all strings of length l.
 
    Let Z_{n} = {0,.., n - 1}.
 
    Let IntDiv(a,b) denote the integer division algorithm that takes
    input integers a, b where a >= b >= 1 and returns integers (q,r) 
 
 
 
    the quotient and remainder, respectively, of the division of a by
    b. (Thus a = bq + r and 0 <= r < b.)
 
    Let H: {0,1}^k x {0,1}^c --> {0,1}^n be the base function that
    takes a k-bit key K and c-bit counter C and returns an n-bit output
    H(K,C). (In the case of HOTP, H is HMAC-SHA-1; we use this formal
    definition for generalizing our proof of security)
 
   A.2 The idealized algorithm: HOTP-IDEAL
 
    We now define an idealized counterpart of the HOTP algorithm. In
    this algorithm, the role of H is played by a random function that
    forms the key.
 
    To be more precise, let Maps(c,n) denote the set of all functions
    mapping from {0,1}^c to {0,1}^n. The idealized algorithm has key
    space Maps(c,n), so that a "key" for such an algorithm is a
    function h from {0,1}^c to {0,1}^n. We imagine this key (function)
    to be drawn at random. It is not feasible to implement this
    idealized algorithm, since the key, being a function from is way
    too large to even store. So why consider it?
 
    Our security analysis will show that as long as H satisfies a
    certain well-accepted assumption, the security of the actual and
    idealized algorithms is for all practical purposes the same. The
    task that really faces us, then, is to assess the security of the
    idealized algorithm.
 
    In analyzing the idealized algorithm, we are concentrating on
    assessing the quality of the design of the algorithm itself,
    independently of HMAC-SHA-1. This is in fact the important issue.
 
   A.3 Model of Security
 
    The model exhibits the type of threats or attacks that are being
    considered and enables to asses the security of HOTP and
    HOTP-IDEAL. We denote ALG as either HOTP or HOTP-IDEAL for the
    purpose of this security analysis.
 
    The scenario we are considering is that a user and server share a
    key K for ALG. Both maintain a counter C, initially zero, and the
    user authenticates itself by sending ALG(K,C) to the server. The
    latter accepts if this value is correct.
 
    In order to protect against accidental increment of the user
    counter, the server, upon receiving a value z, will accept as long
    as z equals ALG(K,i) for some i in the range C,...,C + s-1, where s
    is the resynchronization parameter and C is the server counter. If
    it accepts with some value of i, it then increments its counter to
    i+ 1. If it does not accept, it does not change its counter value.
 
    The model we specify captures what an adversary can do and what it
    needs to achieve in order to "win." First, the adversary is assumed
 
 
 
    to be able to eavesdrop, meaning see the authenticator transmitted
    by the user. Second, the adversary wins if it can get the server to
    accept an authenticator relative to a counter value for which the
    user has never transmitted an authenticator.
 
    The formal adversary, which we denote by B, starts out knowing
    which algorithm ALG is being used, knowing the system design and
    knowing all system parameters. The one and only thing it is not
    given a priori is the key K shared between the user and the server.
 
    The model gives B full control of the scheduling of events. It has
    access to an authenticator oracle representing the user. By calling
    this oracle, the adversary can ask the user to authenticate itself
    and get back the authenticator in return. It can call this oracle
    as often as it wants and when it wants, using the authenticators it
    accumulates to perhaps "learn" how to make authenticators itself.
    At any time, it may also call a verification oracle, supplying the
    latter with a candidate authenticator of its choice. It wins if the
    server accepts this accumulator.
 
    Consider the following game involving an adversary B that is
    attempting to compromise the security of an authentication
    algorithm ALG: K x {0,1}^c --> R.
 
    Initializations - A key K is selected at random from K, a counter C
    is initialized to 0, and the Boolean value win is set to false.
 
    Game execution - Adversary B is provided with the two following
    oracles:
 
    Oracle AuthO()
    --------------
       A = ALG(K,C)
       C = C + 1
       Return O to B
 
    Oracle VerO(A)
    --------------
       i = C
       While (i <= C + s - 1 and Win == FALSE) do
          If A == ALG(K,i) then Win = TRUE; C = i + 1
          Else i = i + 1
       Return Win to B
 
    AuthO() is the authenticator oracle and VerO(A) is the verification
    oracle.
 
    Upon execution, B queries the two oracles at will. Let Adv(B) be
    the probability that win gets set to true in the above game. This
    is the probability that the adversary successfully impersonates the
    user.
     
 
 
 
    Our goal is to assess how large this value can be as a function of
    the number v of verification queries made by B, the number a of
    authenticator oracle queries made by B, and the running time t of
    B. This will tell us how to set the throttle, which effectively
    upper bounds v.
 
   A.4 Security of the ideal authentication algorithm
 
    This section summarizes the security analysis of HOTP-IDEAL,
    starting with the impact of the conversion modulo 10^Digit and
    then, focusing on the different possible attacks.
 
    A.4.1 From bits to digits
 
    The dynamic offset truncation of a random n-bit string yields a
    random 31-bit string. What happens to the distribution when it is
    taken modulo m = 10^Digit, as done in HOTP?
 
    The following lemma estimates the biases in the outputs in this
    case.
 
    Lemma 1
    -------
    Let N >= m >= 1 be integers, and let (q,r) = IntDiv(N,m). For z in
    Z_{m} let:
 
          P_{N,m}(z) = Pr [x mod m = z : x randomly pick in Z_{n}]
 
    Then for any z in Z_{m}
 
    P_{N,m}(z) =   (q + 1) / N    if 0 <= z < r
                   q / N          if r <= z < m
 
    Proof of Lemma 1
    ----------------
    Let the random variable X be uniformly distributed over Z_{N}.
    Then:
 
    P_{N,m}(z)  = Pr [X mod m = z]
 
                = Pr [X < mq] * Pr [X mod m = z| X < mq]
                + Pr [mq <= X < N] * Pr [X mod m = z| mq <= X < N]
 
                = mq/N * 1/m +
                   (N - mq)/N * 1 / (N - mq)     if 0 <= z < N - mq
                   0                             if N - mq <= z <= m
 
                = q/N +
                   r/N * 1 / r                   if 0 <= z < N - mq
                   0                             if r <= z <= m
 
    Simplifying yields the claimed equation. 
 
 
 
 
    Let N = 2^31, d = 6 and m = 10^d. If x is chosen at random from
    Z_{N} (meaning, is a random 31-bit string), then reducing it to a
    6-digit number by taking x mod m does not yield a random 6-digit
    number.
 
    Rather, x mod m is distributed as shown in the following table:
 
    Values               Probability that each appears as output
    ----------------------------------------------------------------
    0,1,...,483647       2148/2^31 roughly equals to 1.00024045/10^6
    483648,...,999999    2147/2^31 roughly equals to 0.99977478/10^6
 
    If X is uniformly distributed over Z_{2^31} (meaning is a random
    31-bit string) then the above shows the probabilities for different
    outputs of X mod 10^6. The first set of values appear with
    probability slightly greater than 10^-6, the rest with probability
    slightly less, meaning the distribution is slightly non-uniform.
 
    However, as the Figure indicates, the bias is small and as we will
    see later, negligible: the probabilities are very close to 10^-6.
 
    A.4.2 Brute force attacks
 
    If the authenticator consisted of d random digits, then a brute
    force attack using v verification attempts would succeed with
    probability sv/10^Digit.
 
    However, an adversary can exploit the bias in the outputs of HOTP-
    IDEAL, predicted by Lemma 1, to mount a slightly better attack.
 
    Namely, it makes authentication attempts with authenticators which
    are the most likely values, meaning the ones in the range 0,...,r -
    1, where (q,r) = IntDiv(2^31,10^Digit).
 
    The following specifies an adversary in our model of security that
    mounts the attack. It estimates the success probability as a
    function of the number of verification queries.
 
    For simplicity, we assume the number of verification queries is at
    most r. With N = 2^31 and m = 10^6 we have r = 483,648, and the
    throttle value is certainly less than this, so this assumption is
    not much of a restriction.
 
    Proposition 1
    -------------
    Suppose m = 10^Digit < 2^31, and let (q,r) = IntDiv(2^31,m). Assume
    s <= m. The brute-force attack adversary B-bf attacks HOTP using v
    <= r verification oracle queries. This adversary makes no
    authenticator oracle queries, and succeeds with probability
 
                    Adv(B-bf) = 1 - (1 - v(q+1)/2^31)^s
     
 
 
 
    which is roughly equals to
 
                              sv * (q+1)/2^31
 
    With m = 10^6 we get q = 2,147. In that case, the brute force
    attack using v verification attempts succeeds with probability
 
         Adv(B-bf) roughly = sv * 2148/2^31 = sv * 1.00024045/10^6
 
    As this equation shows, the resynchronization parameter s has a
    significant impact in that the adversary's success probability is
    proportional to s. This means that s cannot be made too large
    without compromising security.
 
    A.4.3 Brute force attacks are the best possible attacks
 
    A central question is whether there are attacks any better than the
    brute force one. In particular, the brute force attack did not
    attempt to collect authenticators sent by the user and try to
    cryptanalyze them in an attempt to learn how to better construct
    authenticators. Would doing this help? Is there some way to "learn"
    how to build authenticators that result in a higher success rate
    than given by the brute-force attack?
 
    The following says the answer to these questions is no. No matter
    what strategy the adversary uses, and even if it sees, and tries to
    exploit, the authenticators from authentication attempts of the
    user, its success probability will not be above that of the brute
    force attack - this is true as long as the number of
    authentications it observes is not incredibly large. This is
    valuable information regarding the security of the scheme.
 
    Proposition 2
    -------------
    Suppose m = 10^Digit < 2^31, and let (q,r) = IntDiv(2^31,m). Let B
    be any adversary attacking HOTP-IDEAL using v verification oracle
    queries and a <= 2^c - s authenticator oracle queries. Then
 
                        Adv(B) < = sv * (q+1)/ 2^31
 
    Note: This result is conditional on the adversary not seeing more
    than 2^c - s authentications performed by the user, which is hardly
    restrictive as long as c is large enough.
 
    With m = 10^6 we get q = 2,147. In that case, Proposition 2 says
    that any adversary B attacking HOTP-IDEAL and making v verification
    attempts succeeds with probability at most
 
    Equation 1
    ----------
               sv * 2148/2^31 roughly = sv * 1.00024045/10^6
     
 
 
 
    Meaning, B's success rate is not more than that achieved by the
    brute force attack.
 
   A.5 Security Analysis of HOTP
 
    We have analyzed in the previous sections, the security of the
    idealized counterparts HOTP-IDEAL of the actual authentication
    algorithm HOTP. We now show that, under appropriate and
    well-believed assumption on H, the security of the actual
    algorithms is essentially the same as that of its idealized
    counterpart.
 
    The assumption in question is that H is a secure pseudorandom
    function, or PRF, meaning that its input-output values are
    indistinguishable from those of a random function in practice.
 
    Consider an adversary A that is given an oracle for a function f:
    {0,1}^c --> {0, 1}^n and eventually outputs a bit. We denote Adv(A)
    as the prf-advantage of A, which represents how well the adversary
    does at distinguishing the case where its oracle is H(K,.) from the
    case where its oracle is a random function of {0,1}^c to {0,1}^n.
 
    One possible attack is based on exhaustive search for the key K. If
    A runs for t steps and T denotes the time to perform one
    computation of H, its prf-advantage from this attack turns out to
    be (t/T)2^-k . Another possible attack is a birthday one [PrOo],
    whereby A can attain advantage p^2/2^n in p oracle queries and
    running time about pT.
 
    Our assumption is that these are the best possible attacks. This
    translates into the following.
 
    Assumption 1
    ------------
 
    Let T denotes the time to perform one computation of H. Then if A
    is any adversary with running time at most t and making at most p
    oracle queries,
 
                       Adv(A) <= (t/T)/2^k + p^2/2^n
 
    In practice this assumption means that H is very secure as PRF. For
    example, given that k = n = 160, an attacker with running time 2^60
    and making 2^40 oracle queries has advantage at most (about) 2^-80.
 
    Theorem 1
    ---------
    Suppose m = 10^Digit < 2^31, and let (q,r) = IntDiv(2^31,m). Let B
    be any adversary attacking HOTP using v verification oracle
    queries, a <= 2^c - s authenticator oracle queries, and running
    time t. Let T denote the time to perform one computation of H. If
    Assumption 1 is true then
     
 
 
 
         Adv(B) <= sv * (q + 1)/2^31 + (t/T)/2^k + ((sv + a)^2)/2^n
 
    In practice, the (t/T)2^-k + ((sv + a)^2)2^-n term is much smaller
    than the sv(q + 1)/2^n term, so that the above says that for all
    practical purposes the success rate of an adversary attacking HOTP
    is sv(q + 1)/2^n, just as for HOTP-IDEAL, meaning the HOTP
    algorithm is in practice essentially as good as its idealized
    counterpart.
 
    In the case m = 10^6 of a 6-digit output this means that an
    adversary making v authentication attempts will have a success rate
    that is at most that of Equation 1.
 
    For example, consider an adversary with running time at most 2^60
    that sees at most 2^40 authentication attempts of the user. Both
    these choices are very generous to the adversary, who will
    typically not have these resources, but we are saying that even
    such a powerful adversary will not have more success than indicated
    by Equation 1.
 
    We can safely assume sv <= 2^40 due to the throttling and bounds on
    s. So:
        (t/T)/2^k + ((sv + a)^2)/2^n  <= 2^60/2^160 + (2^41)^2/2^160
                                     roughly <= 2^-78
 
    which is much smaller than the success probability of Equation 1
    and negligible compared to it.
 
 Appendix B - SHA-1 Attacks
 
    This sections addresses the impact of the recent attacks on SHA-1
    on the security of the HMAC-SHA-1 based HOTP. We begin with some
    discussion of the situation of SHA-1 and then discuss the relevance
    to HMAC-SHA-1 and HOTP. Cited references are at the bottom of the
    document.
 
   B.1 SHA-1 status
 
    A collision for a hash function h means a pair x,y of different
    inputs such that h(x)=h(y). Since SHA-1 outputs 160 bits, a
    birthday attack finds a collision in 2^{80} trials. (A trial means
    one computation of the function.) This was thought to be the best
    possible until Wang, Yin and Yu announced on February 15, 2005 that
    they had an attack finding collisions in 2^{69} trials.
 
    Is SHA-1 broken? For most practical purposes we would say probably
    not, since the resources needed to mount the attack are huge.  Here
    is one way to get a sense of it: we can estimate it is about the
    same as the time we would need to factor a 760-bit RSA modulus, and
    this is currently considered out of reach.
     
 
 
 
    Burr of NIST is quoted [Crack] as saying ``Large national
    intelligence agencies could do this in a reasonable amount of time
    with a few million dollars in computer time.'' However, the
    computation may be out of reach of all but such well-funded
    agencies.
 
    One should also ask what impact finding SHA-1 collisions actually
    has on security of real applications such as signatures. To exploit
    a collision x,y to forge signatures, you need to somehow obtain a
    signature of x and then you can forge a signature of y. How
    damaging this is depends on the content of y: the y created by the
    attack may not be meaningful in the application context. Also, one
    needs a chosen-message attack to get the signature of x. This seems
    possible in some contexts, but not others. Overall, it is not clear
    the impact on the security of signatures is significant.
 
    Indeed, one can read that SHA-1 is ``broken,'' [Sha1], that
    encryption and SSL are ``broken'' [Res], in the press. The media
    have a tendency to magnify events: it would hardly be interesting
    to announce in the news that a team of cryptanalysts did very
    interesting theoretical work in attacking SHA-1.
 
    Cryptographers are excited too. But mainly because this is an
    important theoretical breakthrough. Attacks can only get beter with
    time: it is therefore important to monitor any progress in hash
    functions cryptanalysis and be prepared for any really practical
    break with a sound migration plan for the future.
 
   B.2 HMAC-SHA-1 status
 
    The new attacks on SHA-1 have no impact on the security of HMAC-
    SHA-1. The best attack on the latter remains one needing a sender
    to authenticate 2^{80} messages before an adversary can create a
    forgery. Why?
 
    HMAC is not a hash function. It is a message authentication code
    (MAC) that uses a hash function internally. A MAC depends on a
    secret key, while hash functions don't. What one needs to worry
    about with a MAC is forgery, not collisions. HMAC was designed so
    that collisions in the hash function (here SHA-1) do not yield
    forgeries for HMAC.
 
    Recall that HMAC-SHA-1(K,x) = SHA-1(K_o,SHA-1(K_i,x)) where the
    keys K_o,K_i are derived from K. Suppose the attacker finds a pair
    x,y such that SHA-1(K_i,x)=SHA-1(K_i,y). (Call this a hidden-key
    collision.) Then if it can obtain the MAC of x (itself a tall
    order), it can forge the MAC of y. (These values are the same.) But
    finding hidden-key collisions is harder than finding collisions,
    because the attacker does not know the hidden key K_i. All it may
    have is some outputs of HMAC-SHA-1 with key K. To date there are no
    claims or evidence that the recent attacks on SHA-1 extend to find
    hidden-key collisions.
     
 
 
 
    Historically, the HMAC design has already proven itself in this
    regard.  MD5 is considered broken in that collisions in this hash
    function can be found relatively easily. But there is still no
    attack on HMAC-MD5 better than the trivial 2^{64} time birthday
    one. (MD5 outputs 128 bits, not 160.) We are seeing this strength
    of HMAC coming into play again in the SHA-1 context.
 
   B.3 HOTP status
 
    Since no new weakness has surfaced in HMAC-SHA-1, there is no
    impact on HOTP. The best attacks on HOTP remain those described in
    the document, namely to try to guess output values.
 
    The security proof of HOTP requires that HMAC-SHA-1 behave like a
    pseudorandom function. The quality of HMAC-SHA-1 as a pseudorandom
    function is not impacted by the new attacks on SHA-1, and so
    neither is this proven guarantee.
 
 Appendix C - HOTP Algorithm: Reference Implementation
 
    /*
     * OneTimePasswordAlgorithm.java
     * OATH Initiative,
     * HOTP one-time password algorithm
     *
     */
 
    /* Copyright (C) 2004, OATH. All rights reserved.
     *
     * License to copy and use this software is granted provided that it
     * is identified as the "OATH HOTP Algorithm" in all material
     * mentioning or referencing this software or this function.
     *
     * License is also granted to make and use derivative works provided
     * that such works are identified as
     *  "derived from OATH HOTP algorithm"
     * in all material mentioning or referencing the derived work.
     *
     * OATH (Open AuTHentication) and its members make no
     * representations concerning either the merchantability of this
     * software or the suitability of this software for any particular
     * purpose.
     *
     * It is provided "as is" without express or implied warranty
     * of any kind and OATH AND ITS MEMBERS EXPRESSELY DISCLAIMS
     * ANY WARRANTY OR LIABILITY OF ANY KIND relating to this software.
     *
     * These notices must be retained in any copies of any part of this
     * documentation and/or software.
     */
 
    package org.openauthentication.otp; 
 
 
 
 
    import java.io.IOException;
    import java.io.File;
    import java.io.DataInputStream;
    import java.io.FileInputStream ;
    import java.lang.reflect.UndeclaredThrowableException;
 
    import java.security.GeneralSecurityException;
    import java.security.NoSuchAlgorithmException;
    import java.security.InvalidKeyException;
 
    import javax.crypto.Mac;
    import javax.crypto.spec.SecretKeySpec;
 
    /**
     * This class contains static methods that are used to calculate the
     * One-Time Password (OTP) using
     * JCE to provide the HMAC-SHA1.
     *
     * @author Loren Hart
     * @version 1.0
     */
    public class OneTimePasswordAlgorithm {
        private OneTimePasswordAlgorithm() {}
 
        // These are used to calculate the check-sum digits.
        //                                0  1  2  3  4  5  6  7  8  9
        private static final int[] doubleDigits =
                                  { 0, 2, 4, 6, 8, 1, 3, 5, 7, 9 };
 
        /**
         * Calculates the checksum using the credit card algorithm.
         * This algorithm has the advantage that it detects any single
         * mistyped digit and any single transposition of
         * adjacent digits.
         *
         * @param num the number to calculate the checksum for
         * @param digits number of significant places in the number
         *
         * @return the checksum of num
         */
        public static int calcChecksum(long num, int digits) {
            boolean doubleDigit = true;
            int     total = 0;
            while (0 < digits--) {
                int digit = (int) (num % 10);
                num /= 10;
                if (doubleDigit) {
                    digit = doubleDigits[digit];
                }
                total += digit;
                doubleDigit = !doubleDigit;
            } 
 
 
 
            int result = total % 10;
            if (result > 0) {
                result = 10 - result;
            }
            return result;
        }
 
        /**
         * This method uses the JCE to provide the HMAC-SHA1
         * algorithm.
         * HMAC computes a Hashed Message Authentication Code and
         * in this case SHA1 is the hash algorithm used.
         *
         * @param keyBytes   the bytes to use for the HMAC-SHA1 key
         * @param text       the message or text to be authenticated.
         *
         * @throws NoSuchAlgorithmException if no provider makes
         *       either HmacSHA1 or HMAC-SHA1
         *       digest algorithms available.
         * @throws InvalidKeyException
         *       The secret provided was not a valid HMAC-SHA1 key.
         *
         */
 
        public static byte[] hmac_sha1(byte[] keyBytes, byte[] text)
            throws NoSuchAlgorithmException, InvalidKeyException
        {
    //        try {
                Mac hmacSha1;
                try {
                    hmacSha1 = Mac.getInstance("HmacSHA1");
                } catch (NoSuchAlgorithmException nsae) {
                    hmacSha1 = Mac.getInstance("HMAC-SHA1");
                }
                SecretKeySpec macKey =
                new SecretKeySpec(keyBytes, "RAW");
                hmacSha1.init(macKey);
                return hmacSha1.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 OTP value for the given
         * set of parameters.
         *
         * @param secret       the shared secret
         * @param movingFactor the counter, time, or other value that  
 
 
 
         *                     changes on a per use basis.
         * @param codeDigits   the number of digits in the OTP, not
         *                     including the checksum, if any.
         * @param addChecksum  a flag that indicates if a checksum digit
         *                     should be appended to the OTP.
         * @param truncationOffset the offset into the MAC result to
         *                     begin truncation. If this value is out of
         *                     the range of 0 ... 15, then dynamic
         *                     truncation  will be used.
         *                     Dynamic truncation is when the last 4
         *                     bits of the last byte of the MAC are
         *                     used to determine the start offset.
         * @throws NoSuchAlgorithmException if no provider makes
         *                     either HmacSHA1 or HMAC-SHA1
         *                     digest algorithms available.
         * @throws InvalidKeyException
         *                     The secret provided was not
         *                     a valid HMAC-SHA1 key.
         *
         * @return A numeric String in base 10 that includes
         * {@link codeDigits} digits plus the optional checksum
         * digit if requested.
         */
        static public String generateOTP(byte[] secret,
                   long movingFactor,
                 int codeDigits,
                   boolean addChecksum,
                 int truncationOffset)
            throws NoSuchAlgorithmException, InvalidKeyException
        {
            // put movingFactor value into text byte array
       String result = null;
       int digits = addChecksum ? (codeDigits + 1) : codeDigits;
            byte[] text = new byte[8];
            for (int i = text.length - 1; i >= 0; i--) {
                text[i] = (byte) (movingFactor & 0xff);
                movingFactor >>= 8;
            }
 
            // compute hmac hash
            byte[] hash = hmac_sha1(secret, text);
 
            // put selected bytes into result int
            int offset = hash[hash.length - 1] & 0xf;
       if ( (0<=truncationOffset) &&
             (truncationOffset<(hash.length-4)) ) {
           offset = truncationOffset;
       }
            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];
       if (addChecksum) {
           otp =  (otp * 10) + calcChecksum(otp, codeDigits);
       }
       result = Integer.toString(otp);
       while (result.length() < digits) {
           result = "0" + result;
       }
       return result;
        }
    }
 
 Appendix D - HOTP Algorithm: Test Values
 
    The following test data uses the ASCII string
    "123456787901234567890" for the secret:
 
    Secret = 0x3132333435363738393031323334353637383930
 
    Table 1 details for each count, the intermediate hmac value.
 
    Count    Hexadecimal HMAC-SHA1(secret, count)
    0        cc93cf18508d94934c64b65d8ba7667fb7cde4b0
    1        75a48a19d4cbe100644e8ac1397eea747a2d33ab
    2        0bacb7fa082fef30782211938bc1c5e70416ff44
    3        66c28227d03a2d5529262ff016a1e6ef76557ece
    4        a904c900a64b35909874b33e61c5938a8e15ed1c
    5        a37e783d7b7233c083d4f62926c7a25f238d0316
    6        bc9cd28561042c83f219324d3c607256c03272ae
    7        a4fb960c0bc06e1eabb804e5b397cdc4b45596fa
    8        1b3c89f65e6c9e883012052823443f048b4332db
    9        1637409809a679dc698207310c8c7fc07290d9e5
 
    Table details for each count the truncated values (both in
    hexadecimal and decimal) and then the HOTP value.
 
                      Truncated
    Count    Hexadecimal    Decimal        HOTP
    0        4c93cf18       1284755224     755224
    1        41397eea       1094287082     287082
    2         82fef30        137359152     359152
    3        66ef7655       1726969429     969429
    4        61c5938a       1640338314     338314
    5        33c083d4        868254676     254676
    6        7256c032       1918287922     287922
    7         4e5b397         82162583     162583
    8        2823443f        673399871     399871
    9        2679dc69        645520489     520489
 
   Appendix E - Extensions
     
 
 
 
    We introduce in this section several enhancements to the HOTP
    algorithm. These are not recommended extensions or part of the
    standard algorithm, but merely variations that could be used for
    customized implementations.
 
   E.1 Number of Digits
 
    A simple enhancement in terms of security would be to extract more
    digits from the HMAC-SHA1 value.
 
    For instance, calculating the HOTP value modulo 10^8 to build an
    8-digit HOTP value would reduce the probability of success of the
    adversary from sv/10^6 to sv/10^8.
 
    This could give the opportunity to improve usability, e.g. by
    increasing T and/or s, while still achieving a better security
    overall. For instance, s = 10 and 10v/10^8 = v/10^7 < v/10^6 which
    is the theoretical optimum for 6-digit code when s = 1.
 
   E.2 Alpha-numeric Values
 
    Another option is to use A-Z and 0-9 values; or rather a subset of
    32 symbols taken from the alphanumerical alphabet in order to avoid
    any confusion between characters: 0, O and Q as well as l, 1 and I
    are very similar, and can look the same on a small display.
 
    The immediate consequence is that the security is now in the order
    of sv/32^6 for a 6-digit HOTP value and sv/32^8 for an 8-digit HOTP
    value.
 
    32^6 > 10^9 so the security of a 6-alphanumeric HOTP code is
    slightly better than a 9-digit HOTP value, which is the maximum
    length of an HOTP code supported by the proposed algorithm.
 
    32^8 > 10^12 so the security of an 8-alphanumeric HOTP code is
    significantly better than a 9-digit HOTP value.
 
    Depending on the application and token/interface used for
    displaying and entering the HOTP value, the choice of alphanumeric
    values could be a simple and efficient way to improve security at a
    reduced cost and impact on users.
 
   E.3 Sequence of HOTP values
 
    As we suggested for the resynchronization to enter a short sequence
    (say 2 or 3) of HOTP values, we could generalize the concept to the
    protocol, and add a parameter L that would define the length of the
    HOTP sequence to enter.
 
    Per default, the value L SHOULD be set to 1, but if security needs
    to be increased, users might be asked (possibly for a short period
    of time, or a specific operation) to enter L HOTP values.
     
 
 
 
    This is another way, without increasing the HOTP length or using
    alphanumeric values to tighten security.
 
    Note: The system MAY also be programmed to request synchronization
    on a regular basis (e.g. every night, or twice a week, etc.) and to
    achieve this purpose, ask for a sequence of L HOTP values.
 
   E.4 A Counter-based Re-Synchronization Method
 
    In this case, we assume that the client can access and send not
    only the HOTP value but also other information, more specifically
    the counter value.
 
    A more efficient and secure method for resynchronization is
    possible in this case. The client application will not send the
    HOTP-client value only, but the HOTP-client and the related
    C-client counter value, the HOTP value acting as a message
    authentication code of the counter.
 
    Resynchronization Counter-based Protocol (RCP)
    ----------------------------------------------
 
    The server accepts if the following are all true, where C-server is
    its own current counter value:
 
    1) C-client >= C-server
    2) C-client - C-server <= s
    3) Check that HOTP-client is valid HOTP(K,C-Client)
    4) If true, the server sets C to C-client + 1 and client is
    authenticated
 
    In this case, there is no need for managing a look-ahead window
    anymore. The probability of success of the adversary is only v/10^6
    or roughly v in one million. A side benefit is obviously to be able
    to increase s "infinitely" and therefore improve the system
    usability without impacting the security.
 
    This resynchronization protocol SHOULD be use whenever the related
    impact on the client and server applications is deemed acceptable.
 
   E.5 Data Field
 
    Another interesting option is the introduction of a Data field,
    that would be used for generating the One-Time password values:
    HOTP (K, C, [Data]) where Data is an optional field that can be the
    concatenation of various pieces of identity-related information -
    e.g. Data = Address | PIN.
 
    We could also use a Timer, either as the only moving factor or in
    combination with the Counter - in this case, e.g. Data = Timer,
    where Timer could be the UNIX-time (GMT seconds since 1/1/1970)
    divided by some factor (8, 16, 32, etc.) in order to give a
    specific time step. The time window for the One-Time Password is 
 
 
 
    then equal to the time step multiplied by the resynchronization
    parameter as defined before - e.g. if we take 64 seconds as the
    time step and 7 for the resynchronization parameter, we obtain an
    acceptance window of +/- 3 minutes.
 
    Using a Data field opens for more flexibility in the algorithm
    implementation, provided that the Data field is clearly specified.
 

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