This is a purely informative rendering of an RFC that includes verified errata. This rendering may not be used as a reference.

The following 'Verified' errata have been incorporated in this document: EID 163
Network Working Group                                         D. M'Raihi
Request for Comments: 4226                                      VeriSign
Category: Informational                                       M. Bellare
                                                            F. Hoornaert
                                                             D. Naccache
                                                                O. Ranen
                                                           December 2005

           HOTP: An HMAC-Based One-Time Password Algorithm

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2005).


   This document describes an algorithm to generate one-time password
   values, based on Hashed Message Authentication Code (HMAC).  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 Virtual Private Network
   (VPN) access, Wi-Fi network logon to transaction-oriented Web

   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 ................................................6
      5.3. Generating an HOTP Value ...................................6
      5.4. Example of HOTP Computation for Digit = 6 ..................7
   6. Security Considerations .........................................8
   7. Security Requirements ...........................................9
      7.1. Authentication Protocol Requirements .......................9
      7.2. Validation of HOTP Values .................................10
      7.3. Throttling at the Server ..................................10
      7.4. Resynchronization of the Counter ..........................11
      7.5. Management of Shared Secrets ..............................11
   8. Composite Shared Secrets .......................................14
   9. Bi-Directional Authentication ..................................14
   10. Conclusion ....................................................15
   11. Acknowledgements ..............................................15
   12. Contributors ..................................................15
   13. References ....................................................15
      13.1. Normative References .....................................15
      13.2. Informative References ...................................16
   Appendix A - HOTP Algorithm Security: Detailed Analysis ...........17
      A.1. Definitions and Notations .................................17
      A.2. The Idealized Algorithm: HOTP-IDEAL .......................17
      A.3. Model of Security .........................................18
      A.4. Security of the Ideal Authentication Algorithm ............19
           A.4.1. From Bits to Digits ................................19
           A.4.2. Brute Force Attacks ................................21
           A.4.3. Brute force attacks are the best possible attacks ..22
      A.5. Security Analysis of HOTP .................................23
   Appendix B - SHA-1 Attacks ........................................25
      B.1. SHA-1 Status ..............................................25
      B.2. HMAC-SHA-1 Status .........................................26
      B.3. HOTP Status ...............................................26
   Appendix C - HOTP Algorithm: Reference Implementation .............27
   Appendix D - HOTP Algorithm: Test Values ..........................32
   Appendix E - Extensions ...........................................33
      E.1. Number of Digits ..........................................33
      E.2. Alphanumeric Values .......................................33
      E.3. Sequence of HOTP values ...................................34
      E.4. A Counter-Based Resynchronization Method ..................34
      E.5. Data Field ................................................35

1.  Overview

   The document introduces first the context around an algorithm that
   generates one-time password values based on HMAC [BCK1] and, thus, is
   named the HMAC-Based One-Time Password (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 10 concludes this document.  In Appendix A, the interested
   reader will find a detailed, full-fledged 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 requires 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 as 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 Public-Key Infrastructure (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 document 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 [RFC3979].

   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",
   document are to be interpreted as described in [RFC2119].

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 prerequisite.

   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 support 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.  Section 7.4 and Appendix E.4 details the
   resynchronization mechanism proposed in this document

   R6 - The algorithm MUST use a strong shared secret.  The length of
   the shared secret MUST be at least 128 bits.  This document
   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 that 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-SHA-1 calculation is 160 bits, we must
   truncate this value to something that can be easily entered by a

                   HOTP(K,C) = Truncate(HMAC-SHA-1(K,C))


     - 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

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 below,
                        //  returns a 31-bit string

   Step 3: Compute an HOTP value
   Let Snum  = StToNum(Sbits)   // Convert S to a number in
   Return D = Snum mod 10^Digit //  D is a number in the range

   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-SHA-1 result.

    DT(String) // String = String[0]...String[19]
     Let OffsetBits be the low-order 4 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-
   SHA-1 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                                               |
   | Byte Value                                                |

   * The last byte (byte 19) has the hex value 0x5a.
   * The value of the lower 4 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

   The conclusion of the security analysis detailed in the Appendix 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 the 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 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

     - Sec is the probability of success of the adversary;
     - s is the look-ahead synchronization window size;
     - v is the number of verification attempts;
     - Digit is 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

7.  Security Requirements

   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

   The parameters T and s discussed in this section have a significant
   impact on the security -- further details in Section 6 elaborate on
   the relations between these parameters and their impact on the system

   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 attacks
   aimed at breaking confidentiality and privacy of transactions.

7.1.  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

   RP1 - P 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 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 over a secure channel in order to
   protect users' privacy and avoid replay attacks.

7.2.  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.

7.3.  Throttling at the Server

   Truncating the HMAC-SHA-1 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 that 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.

7.4.  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 denial-of-service 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 that usability is not impacted.

7.5.  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 techniques to perform these
   different operations with respect to the state of the art in data

   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 Hardware Security Module (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 following
   the recommendations in [RFC4086] and selecting 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 that
   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 selecting 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:
   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.

   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.

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.  Bi-Directional Authentication

EID 163 (Verified) is as follows:

Section: 9

Original Text:

   Oracle AuthO()
      A = ALG(K,C)
      C = C + 1
      Return O to B

Corrected Text:

   Oracle AuthO()
      A = ALG(K,C)
      C = C + 1
      Return A to B

Section A.4.1, Paragraph 3, Lemma 1 definition, top of page 19

The description of Lemma 1 defines P_ {N,m} (z) using the term Z_ {n}
and it should actually be Z_ {N}.
P_{N,m}(z) = Pr [x mod m = z : x randomly pick in Z_{n}]
Should be:
P_{N,m}(z) = Pr [x mod m = z : x randomly pick in Z_{N}]

Section E.2, Paragraph 4, bottom of page 32
32^8 > 10^12 so the security of an 8-alphanumeric HOTP code is
significantly better than a 9-digit HOTP value.
Should be:
32^8 > 10^12 so the security of an 8-alphanumeric HOTP code is
significantly better than a 12-digit HOTP value.

In Author's Addresses, Page 35, David Naccache's contact information should be:

David Naccache
45 rue d'Ulm
75005 Paris, France
Information Security Group,
Royal Holloway,
University of London, Egham,
Surrey TW20 0EX, UK

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 a secure channel, e.g., SSL/TLS, IPsec connections. 10. Conclusion This document 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 document 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 document 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 document would not be the same. 13. References 13.1. Normative References [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] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- Hashing for Message Authentication", RFC 2104, February 1997. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3979] Bradner, S., "Intellectual Property Rights in IETF Technology", BCP 79, RFC 3979, March 2005. [RFC4086] Eastlake, D., 3rd, Schiller, J., and S. Crocker, "Randomness Requirements for Security", BCP 106, RFC 4086, June 2005. 13.2. Informative References [OATH] Initiative for Open AuTHentication [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 articleID=60402150 [Sha1] Bruce Schneier. SHA-1 broken. February 15, 2005. sha1_broken.html [Res] Researchers: Digital encryption standard flawed 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. 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 and the impact of the truncation and make 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 document. 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 {0,1}^c to {0,1}^n, 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 one to assess 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, to 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 appears with probability slightly greater than 10^-6, the rest with probability slightly less, meaning that the distribution is slightly non-uniform. However, as the table above 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 that 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 that 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 equal 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 in Section 13. 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 in [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 that the impact on the security of signatures is significant. Indeed, one can read in the press that SHA-1 is "broken" [Sha1] and that encryption and SSL are "broken" [Res]. 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 better 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 /* * * 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 EXPRESSaLY 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; import; import; import ; import java.lang.reflect.UndeclaredThrowableException; import; import; import; 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-SHA-1. * * @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-SHA-1 * 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-SHA-1 key * @param text the message or text to be authenticated. * * @throws NoSuchAlgorithmException if no provider makes * either HmacSHA1 or HMAC-SHA-1 * digest algorithms available. * @throws InvalidKeyException * The secret provided was not a valid HMAC-SHA-1 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-SHA-1"); } 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-SHA-1 * digest algorithms available. * @throws InvalidKeyException * The secret provided was not * a valid HMAC-SHA-1 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 "12345678901234567890" for the secret: Secret = 0x3132333435363738393031323334353637383930 Table 1 details for each count, the intermediate HMAC value. Count Hexadecimal HMAC-SHA-1(secret, count) 0 cc93cf18508d94934c64b65d8ba7667fb7cde4b0 1 75a48a19d4cbe100644e8ac1397eea747a2d33ab 2 0bacb7fa082fef30782211938bc1c5e70416ff44 3 66c28227d03a2d5529262ff016a1e6ef76557ece 4 a904c900a64b35909874b33e61c5938a8e15ed1c 5 a37e783d7b7233c083d4f62926c7a25f238d0316 6 bc9cd28561042c83f219324d3c607256c03272ae 7 a4fb960c0bc06e1eabb804e5b397cdc4b45596fa 8 1b3c89f65e6c9e883012052823443f048b4332db 9 1637409809a679dc698207310c8c7fc07290d9e5 Table 2 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-SHA-1 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. Alphanumeric 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, twice a week, etc.) and to achieve this purpose, ask for a sequence of L HOTP values. E.4. A Counter-Based Resynchronization 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 used 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, which 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. For example, 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. Authors' Addresses David M'Raihi (primary contact for sending comments and questions) VeriSign, Inc. 685 E. Middlefield Road Mountain View, CA 94043 USA Phone: 1-650-426-3832 EMail: 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: Frank Hoornaert VASCO Data Security, Inc. Koningin Astridlaan 164 1780 Wemmel, Belgium EMail: David Naccache Gemplus Innovation 34 rue Guynemer, 92447, Issy les Moulineaux, France and Information Security Group, Royal Holloway, University of London, Egham, Surrey TW20 0EX, UK EMail:, Ohad Ranen Aladdin Knowledge Systems Ltd. 15 Beit Oved Street Tel Aviv, Israel 61110 EMail: 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. 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