rfc1938.txt

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RFC 1938               A One-Time Password System               May 1996


7.0 VERIFICATION OF ONE-TIME PASSWORDS

   An application on the server system that requires OTP authentication
   is expected to issue an OTP challenge as described above. Given the
   parameters from this challenge and the secret pass-phrase, the
   generator can compute (or lookup) the one-time password that is
   passed to the server to be verified.

   The server system has a database containing, for each user, the one-
   time password from the last successful authentication or the first
   OTP of a newly initialized sequence. To authenticate the user, the
   server decodes the one-time password received from the generator into
   a 64-bit key and then runs this key through the secure hash function
   once. If the result of this operation matches the stored previous
   OTP, the authentication is successful and the accepted one-time
   password is stored for future use.

8.0 PASS-PHRASE CHANGES

   Because the number of hash function applications executed by the
   generator decreases by one each time, at some point the user must
   reinitialize the system or be unable to authenticate.

   Although some installations may not permit users to initialize
   remotely, implementations MUST provide a means to do so that does not
   reveal the user's secret pass-phrase.  One way is to provide a means
   to reinitialize the  sequence through explicit specification of the
   first one-time password.

   When the sequence of one-time passwords is reinitialized,
   implementations MUST verify that the seed or the pass-phrase is
   changed.  Installations SHOULD discourage any operation that sends
   the secret pass-phrase over a network in clear-text as such practice
   defeats the concept of a one-time password.

   Implementations MAY use the following technique for
   [re]initialization:

    o  The user picks a new seed and hash count (default values may
       be offered).  The user provides these, along with the
       corresponding generated one-time password, to the host system.

    o  The user MAY also provide the corresponding generated one
       time password for count-1 as an error check.

    o  The user SHOULD provide the generated one-time password for
       the old seed and old hash count to protect an idle terminal
       or workstation (this implies that when the count is 1, the



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RFC 1938               A One-Time Password System               May 1996


       user can login but cannot then change the seed or count).

   In the future a specific protocol may be defined for reinitialization
   that will permit smooth and possibly automated interoperation of all
   hosts and generators.

9.0 PROTECTION AGAINST RACE ATTACK

   All conforming server implementations MUST protect against the race
   condition described in this section.  A defense against this attack
   is outlined; implementations MAY use this approach or MAY select an
   alternative defense.

   It is possible for an attacker to listen to most of a one-time
   password, guess the remainder, and then race the legitimate user to
   complete the authentication.  Multiple guesses against the last word
   of the six-word format are likely to succeed.

   One possible defense is to prevent a user from starting multiple
   simultaneous authentication sessions. This means that once the
   legitimate user has initiated authentication, an attacker would be
   blocked until the first authentication process has completed.  In
   this approach, a timeout is necessary to thwart a denial of service
   attack.

10.0 SECURITY CONSIDERATIONS

   This entire document discusses an authentication system that improves
   security by limiting the danger of eavesdropping/replay attacks that
   have been used against simple password systems [4].

   The use of the OTP system only provides protections against passive
   eavesdropping/replay attacks.  It does not provide for the privacy of
   transmitted data, and it does not provide protection against active
   attacks.  Active attacks against TCP connections are known to be
   present in the current Internet [9].

   The success of the OTP system to protect host systems is dependent on
   the non-invertability of the secure hash functions used.  To our
   knowledge, none of the hash algorithms have been broken, but it is
   generally believed [6] that MD4 is not as strong as MD5.  If a server
   supports multiple hash algorithms, it is only as secure as the
   weakest algorithm.








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RFC 1938               A One-Time Password System               May 1996


11.0 ACKNOWLEDGMENTS

   The idea behind OTP authentication was first proposed by Leslie
   Lamport [1]. Bellcore's S/KEY system, from which OTP is derived, was
   proposed by Phil Karn, who also wrote most of the Bellcore reference
   implementation.

12.0 REFERENCES

  [1]  Leslie Lamport, "Password Authentication with Insecure
       Communication", Communications of the ACM 24.11 (November
       1981), 770-772

  [2]  Rivest, R., "The MD4 Message-Digest Algorithm, RFC 1320",
       MIT and RSA Data Security, Inc., April 1992.

  [3]  Neil Haller, "The S/KEY One-Time Password System", Proceedings
       of the ISOC Symposium on Network and Distributed System
       Security, February 1994, San Diego, CA

  [4]  Haller, N., and R. Atkinson, "On Internet Authentication",
       RFC 1704, Bellcore and Naval Research Laboratory, October 1994.

  [5]  Haller, N., "The S/KEY One-Time Password System", RFC 1760,
       Bellcore, February 1995.

  [6]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
       MIT and RSA Data Security, Inc., April 1992.

  [7]  National Institute of Standards and Technology (NIST),
       "Announcing the Secure Hash Standard", FIPS 180-1, U.S.
       Department of Commerce, April 1995.

  [8]  International Standard - Information Processing -- ISO 7-bit
       coded character set for information interchange (Invariant Code
       Set), ISO-646, International Standards Organization, Geneva,
       Switzerland, 1983

  [9]  Computer Emergency Response Team (CERT), "IP Spoofing and
       Hijacked Terminal Connections", CA-95:01, January 1995.
       Available via anonymous ftp from info.cert.org in
       /pub/cert_advisories.









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13.0 AUTHORS' ADDRESSES

   Neil Haller
   Bellcore
   MCC 1C-265B
   445 South Street
   Morristown, NJ, 07960-6438, USA

   Phone: [phone]
   Fax:   [phone]
   EMail: [e-mail]


   Craig Metz
   Kaman Sciences Corporation
   For NRL Code 5544
   4555 Overlook Avenue, S.W.
   Washington, DC, 20375-5337, USA

   Phone: [phone]
   Fax:   [phone]
   EMail: [e-mail]





























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RFC 1938               A One-Time Password System               May 1996


Appendix A   -   Interfaces to Secure Hash Algorithms

MD4 Message Digest (see reference [2])

          strcpy(buf,seed);
          strcat(buf,passwd);
          MDbegin(&md)
          MDupdate(&md,(unsigned char *)buf,8*buflen);

          /* Fold result to 64 bits */
          md.buffer[0] ^= md.buffer[2];
          md.buffer[1] ^= md.buffer[3];


MD5 Message Digest (see reference [6])

          MD5_CTX mdCxt;

          strcpy(buf,seed);
          strcat(buf,passwd);

          /* Crunch the key through MD5  */
          MD5Init(&mdCxt);
          MD5Update(&mdCxt,(unsigned char *)bits,strlen(bits));
          MD5Update(&mdCxt,(unsigned char *)buf,buflen);
          MD5Final(&mdCxt);

          /* Fold result to 64 bits */
          for( i = 0; i < 8; i++ )
              result[i] = mdCxt.digest[i] ^ mdCxt.digest[i+8];


SHA Secure Hash Algorithm (see reference [7])


          /*  Fold 160 bit result to 64 bits */
          md.buffer[0] ^= md.buffer[2];
          md.buffer[1] ^= md.buffer[3];
          md.buffer[0] ^= md.buffer[4];

Appendix B   -   Alternative Dictionary Algorithm

   The purpose of alternative dictionary encoding of the OTP one-time
   password is to allow the use of language specific or friendly words.
   As case translation is not always well defined, the alternative
   dictionary encoding is case insensitive.  Servers SHOULD accept this
   encoding in addition to the standard 6-word and hexadecimal
   encodings.



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RFC 1938               A One-Time Password System               May 1996


GENERATOR ENCODING USING AN ALTERNATE DICTIONARY

  The standard 6-word encoding uses the placement of a word in the
  dictionary to represent an 11-bit number. The 64-bit one-time
  password can then be represented by six words.

  An alternative dictionary of 2048 words may be created such that
  each word W and position of the word in the dictionary N obey the
  relationship:

          alg( W ) % 2048 == N
  where
          alg is the hash algorithm used (e.g. MD4, MD5, SHA1).

  In addition, no words in the standard dictionary may be chosen.

  The generator expands the 64-bit one-time password to 66 bits by
  computing parity as with the standard 6-word encoding.  The six 11-
  bit numbers are then converted to words using the dictionary that
  was created such that the above relationship holds.


SERVER DECODING OF ALTERNATE DICTIONARY ONE-TIME PASSWORDS

  The server accepting alternative dictionary encoding converts each
  word to an 11-bit number using the above encoding. These numbers are
  then used in the same way as the decoded standard dictionary words
  to form the 66-bit one-time password.

  The server does not need to have access to the alternate dictionary
  that was used to create the one-time password it is authenticating.
  This is because the decoding from word to 11-bit number does not
  make any use of the dictionary.  As a result of the independence of
  the dictionary, a server accepting one alternate dictionary accept
  all alternate dictionaries.

Appendix C - Dictionary for Converting Between 6-Word and Binary
Formats

   This dictionary is from the module put.c in the original Bellcore
   reference distribution.

{        "A",    "ABE",   "ACE",   "ACT",   "AD",    "ADA",   "ADD",
"AGO",   "AID",  "AIM",   "AIR",   "ALL",   "ALP",   "AM",    "AMY",
"AN",    "ANA",  "AND",   "ANN",   "ANT",   "ANY",   "APE",   "APS",
"APT",   "ARC",  "ARE",   "ARK",   "ARM",   "ART",   "AS",    "ASH",
"ASK",   "AT",   "ATE",   "AUG",   "AUK",   "AVE",   "AWE",   "AWK",
"AWL",   "AWN",  "AX",    "AYE",   "BAD",   "BAG",   "BAH",   "BAM",



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RFC 1938               A One-Time Password System               May 1996