rfc1938.txt

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Network Working Group                                          N. Haller
Request for Comments: 1938                                      Bellcore
Category: Standards Track                                        C. Metz
                                              Kaman Sciences Corporation
                                                                May 1996


                       A One-Time Password System

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

1.0 ABSTRACT

   This document describes a one-time password authentication system
   (OTP). The system provides authentication for system access (login)
   and other applications requiring authentication that is secure
   against passive attacks based on replaying captured reusable
   passwords. OTP evolved from the S/KEY (S/KEY is a trademark of
   Bellcore) One-Time Password System that was released by Bellcore and
   is described in references [3] and [5].

2.0 OVERVIEW

   One form of attack on networked computing systems is eavesdropping on
   network connections to obtain authentication information such as the
   login IDs and passwords of legitimate users. Once this information is
   captured, it can be used at a later time to gain access to the
   system. One-time password systems are designed to counter this type
   of attack, called a "replay attack" [4].

   The authentication system described in this document uses a secret
   pass-phrase to generate a sequence of one-time (single use)
   passwords.  With this system, the user's secret pass-phrase never
   needs to cross the network at any time such as during authentication
   or during pass-phrase changes. Thus, it is not vulnerable to replay
   attacks.  Added security is provided by the property that no secret
   information need be stored on any system, including the server being
   protected.

   The OTP system protects against external passive attacks against the
   authentication subsystem. It does not prevent a network eavesdropper
   from gaining access to private information and does not provide



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   protection against either "social engineering" or active attacks [9].

3.0 INTRODUCTION

   There are two entities in the operation of the OTP one-time password
   system. The generator must produce the appropriate one-time password
   from the user's secret pass-phrase and from information provided in
   the challenge from the server. The server must send a challenge that
   includes the appropriate generation parameters to the generator, must
   verify the one-time password received, must store the last valid
   one-time password it received, and must store the corresponding one-
   time password sequence number. The server must also facilitate the
   changing of the user's secret pass-phrase in a secure manner.

   The OTP system generator passes the user's secret pass-phrase, along
   with a seed received from the server as part of the challenge,
   through multiple iterations of a secure hash function to produce a
   one-time password. After each successful authentication, the number
   of secure hash function iterations is reduced by one.  Thus, a unique
   sequence of passwords is generated.  The server verifies the one-time
   password received from the generator by computing the secure hash
   function once and comparing the result with the previously accepted
   one-time password.  This technique was first suggested by Leslie
   Lamport [1].

4.0 REQUIREMENTS TERMINOLOGY

   In this document, the words that are used to define the significance
   of each particular requirement are usually capitalized.  These words
   are:

    - MUST

      This word or the adjective "REQUIRED" means that the item is an
      absolute requirement of the specification.

    - SHOULD

      This word or the adjective "RECOMMENDED" means that there might
      exist valid reasons in particular circumstances to ignore this
      item, but the full implications should be understood and the
      case carefully weighed before taking a different course.

    - MAY

      This word or the adjective "OPTIONAL" means that this item is
      truly optional.  One vendor might choose to include the item
      because a particular marketplace requires it or because it



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      enhances the product, for example; another vendor may omit the
      same item.

5.0 SECURE HASH FUNCTION

   The security of the OTP system is based on the non-invertability of a
   secure hash function. Such a function must be tractable to compute in
   the forward direction, but computationally infeasible to invert.

   The interfaces are currently defined for three such hash algorithms,
   MD4 [2] and MD5 [6] by Ronald Rivest, and SHA [7] by NIST.  All
   conforming implementations of both server and generators MUST support
   MD5.  They SHOULD support SHA and MAY also support MD4.  Clearly, the
   generator and server must use the same algorithm in order to
   interoperate. Other hash algorithms may be specified for use with
   this system by publishing the appropriate interfaces.

   The secure hash algorithms listed above have the property that they
   accept an input that is arbitrarily long and produce a fixed size
   output. The OTP system folds this output to 64 bits using the
   algorithms in the Appendix A. 64 bits is also the length of the one-
   time passwords. This is believed to be long enough to be secure and
   short enough to be entered manually (see below, Form of Output) when
   necessary.

6.0 GENERATION OF ONE-TIME PASSWORDS

   This section describes the generation of the one-time passwords.
   This process consists of an initial step in which all inputs are
   combined, a computation step where the secure hash function is
   applied a specified number of times, and an output function where the
   64 bit one-time password is converted to a human readable form.

   Initial Step

      In principle, the user's secret pass-phrase may be of any length.
      To reduce the risk from techniques such as exhaustive search or
      dictionary attacks, character string pass-phrases MUST contain at
      least 10 characters (see Form of Inputs below).  All
      implementations MUST support a pass-phrases of at least 63
      characters.  The secret pass-phrase is frequently, but is not
      required to be, textual information provided by a user.

      In this step, the pass phrase is concatenated with a seed that is
      transmitted from the server in clear text. This non-secret seed
      allows clients to use the same secret pass-phrase on multiple
      machines (using different seeds) and to safely recycle their
      secret pass-phrases by changing the seed.



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      The result of the concatenation is passed through the secure hash
      function and then is reduced to 64 bits using one of the function
      dependent algorithms shown in Appendix A.

   Computation Step

      A sequence of one-time passwords is produced by applying the
      secure hash function multiple times to the output of the initial
      step (called S). That is, the first one-time password to be used
      is produced by passing S through the secure hash function a number
      of times (N) specified by the user. The next one-time password to
      be used is generated by passing S though the secure hash function
      N-1 times. An eavesdropper who has monitored the transmission of a
      one- time password would not be able to generate the next required
      password because doing so would mean inverting the hash function.

   Form of Inputs

      The secret pass-phrase is seen only by the OTP generator. To allow
      interchangeability of generators, all generators MUST support a
      secret pass-phrase of 10 to 63 characters. Implementations MAY
      support a longer pass-phrase, but such implementations risk the
      loss of interchangeability with implementations supporting only
      the minimum.

      The seed MUST consist of purely alphanumeric characters and MUST
      be of one to 16 characters in length. The seed is a string of
      characters that MUST not contain any blanks and SHOULD consist of
      strictly alphanumeric characters from the ISO-646 Invariant Code
      Set.  The seed MUST be case insensitive and MUST be internally
      converted to lower case before it is processed.

      The sequence number and seed together constitute a larger unit of
      data called the challenge. The challenge gives the generator the
      parameters it needs to calculate the correct one-time password
      from the secret pass-phrase. The challenge MUST be in a standard
      syntax so that automated generators can recognize the challenge in
      context and extract these parameters. The syntax of the challenge
      is:

          otp-<algorithm identifier> <sequence integer> <seed>

      The three tokens MUST be separated by a white space (defined as
      any number of spaces and/or tabs) and the entire challenge string
      MUST be terminated with either a space or a new line. The string
      "otp-" MUST be in lower case.  The algorithm identifier is case
      sensitive (the existing identifiers are all lower case), and the
      seed is case insensitive and converted before use to lower case.



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      If additional algorithms are defined, appropriate identifiers
      (short, but not limited to three or four characters) must be
      defined. The currently defined algorithm identifiers are:

      md4        MD4 Message Digest
      md5        MD5 Message Digest
      sha1       NIST Secure Hash Algorithm Revision 1

      An example of an OTP challenge is:   otp-md5 487 dog2

   Form of Output

      The one-time password generated by the above procedure is 64 bits
      in length. Entering a 64 bit number is a difficult and error prone
      process. Some generators insert this password into the input
      stream and some others make it available for system "cut and
      paste." Still other arrangements require the one-time password to
      be entered manually. The OTP system is designed to facilitate this
      manual entry without impeding automatic methods. The one-time
      password therefore MAY be converted to, and all servers MUST be
      capable of accepting it as, a sequence of six short (1 to 4
      letter) easily typed words that only use characters from ISO-646
      IVCS. Each word is chosen from a dictionary of 2048 words; at 11
      bits per word, all one-time passwords may be encoded.

      The two extra bits in this encoding are used to store a checksum.
      The 64 bits of key are broken down into pairs of bits, then these
      pairs are summed together. The two least significant bits of this
      sum are encoded in the last two bits of the six word sequence with
      the least significant bit of the sum as the last bit encoded. All
      OTP generators MUST calculate this checksum and all OTP servers
      MUST verify this checksum explicitly as part of the operation of
      decoding this representation of the one-time password.

      Generators that produce the six-word format MUST present the words
      in upper case with single spaces used as separators. All servers
      MUST accept six-word format without regard to case and white space
      used as a separator. The two lines below represent the same one-
      time password.  The first is valid as output from a generator and
      as input a server, the second is valid only as human input to a
      server.

          OUST COAT FOAL MUG BEAK TOTE
          oust coat foal  mug  beak  tote

      Interoperability requires that all OTP servers and generators use
      the same dictionary. The standard dictionary was originally
      specified in the "S/KEY One Time Password System" that is



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      described in RFC 1760 [5].  This dictionary is included in this
      document as Appendix C.

      To facilitate the implementation of smaller generators,
      hexadecimal output is an acceptable alternative for the
      presentation of the one-time password. All implementations of the
      server software MUST accept case-insensitive hexadecimal as well
      as six-word format. The hexadecimal digits may be separated by
      white space so servers are REQUIRED to ignore all white space.  If
      the representation is partitioned by white space, leading zeros
      must be retained. Examples of hexadecimal format are:

         Representation                Value

         3503785b369cda8b              0x3503785b369cda8b
         e5cc a1b8 7c13 096b           0xe5cca1b87c13096b
         C7 48 90 F4 27 7B A1 CF       0xc74890f4277ba1cf
         47 9 A68 28 4C 9D 0 1BC       0x479a68284c9d01bc

      In addition to accepting six-word and hexadecimal encodings of the
      64 bit one-time password, servers SHOULD accept the alternate
      dictionary encoding described in Appendix B.  The six words in
      this encoding MUST not overlap the set of words in the standard
      dictionary.  To avoid ambiguity with the hexadecimal
      representation, words in the alternate dictionary MUST not be
      comprised solely of the letters A-F.  Decoding words thus encoded
      does not require any knowledge of the alternative dictionary used
      so the acceptance of any alternate dictionary implies the
      acceptance of all alternate dictionaries.  Words in the
      alternative dictionaries are case sensitive.  Generators and
      servers MUST preserve the case in the processing of these words.

      In summary, all conforming servers MUST accept six-word input that
      uses the Standard Dictionary (RFC 1760 and Appendix C), MUST
      accept hexadecimal encoding, and SHOULD accept six-word input that
      uses the Alternative Dictionary technique (Appendix B).  As there
      is a remote possibility that a hexadecimal encoding of a one-time
      password will look like a valid six-word standard dictionary
      encoding, all implementations MUST use the following scheme.  If a
      six-word encoded one-time password is valid, it is accepted.
      Otherwise, if the one-time password can be interpreted as
      hexadecimal, and with that decoding it is valid, then it is
      accepted.








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