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rfc:rfc1938

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

Haller & Metz Standards Track [Page 1] RFC 1938 A One-Time Password System May 1996

 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:
  1. MUST
    This word or the adjective "REQUIRED" means that the item is an
    absolute requirement of the specification.
  1. 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.
  1. 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

Haller & Metz Standards Track [Page 2] RFC 1938 A One-Time Password System May 1996

    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.

Haller & Metz Standards Track [Page 3] RFC 1938 A One-Time Password System May 1996

    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.

Haller & Metz Standards Track [Page 4] RFC 1938 A One-Time Password System May 1996

    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

Haller & Metz Standards Track [Page 5] RFC 1938 A One-Time Password System May 1996

    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.

Haller & Metz Standards Track [Page 6] 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

Haller & Metz Standards Track [Page 7] 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.

Haller & Metz Standards Track [Page 8] 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.

Haller & Metz Standards Track [Page 9] RFC 1938 A One-Time Password System May 1996

13.0 AUTHORS' ADDRESSES

 Neil Haller
 Bellcore
 MCC 1C-265B
 445 South Street
 Morristown, NJ, 07960-6438, USA
 Phone: +1 201 829-4478
 Fax:   +1 201 829-2504
 EMail: nmh@bellcore.com
 Craig Metz
 Kaman Sciences Corporation
 For NRL Code 5544
 4555 Overlook Avenue, S.W.
 Washington, DC, 20375-5337, USA
 Phone: +1 202 404-7122
 Fax:   +1 202 404-7942
 EMail: cmetz@cs.nrl.navy.mil

Haller & Metz Standards Track [Page 10] 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.

Haller & Metz Standards Track [Page 11] 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",

Haller & Metz Standards Track [Page 12] RFC 1938 A One-Time Password System May 1996

"BAN", "BAR", "BAT", "BAY", "BE", "BED", "BEE", "BEG", "BEN", "BET", "BEY", "BIB", "BID", "BIG", "BIN", "BIT", "BOB", "BOG", "BON", "BOO", "BOP", "BOW", "BOY", "BUB", "BUD", "BUG", "BUM", "BUN", "BUS", "BUT", "BUY", "BY", "BYE", "CAB", "CAL", "CAM", "CAN", "CAP", "CAR", "CAT", "CAW", "COD", "COG", "COL", "CON", "COO", "COP", "COT", "COW", "COY", "CRY", "CUB", "CUE", "CUP", "CUR", "CUT", "DAB", "DAD", "DAM", "DAN", "DAR", "DAY", "DEE", "DEL", "DEN", "DES", "DEW", "DID", "DIE", "DIG", "DIN", "DIP", "DO", "DOE", "DOG", "DON", "DOT", "DOW", "DRY", "DUB", "DUD", "DUE", "DUG", "DUN", "EAR", "EAT", "ED", "EEL", "EGG", "EGO", "ELI", "ELK", "ELM", "ELY", "EM", "END", "EST", "ETC", "EVA", "EVE", "EWE", "EYE", "FAD", "FAN", "FAR", "FAT", "FAY", "FED", "FEE", "FEW", "FIB", "FIG", "FIN", "FIR", "FIT", "FLO", "FLY", "FOE", "FOG", "FOR", "FRY", "FUM", "FUN", "FUR", "GAB", "GAD", "GAG", "GAL", "GAM", "GAP", "GAS", "GAY", "GEE", "GEL", "GEM", "GET", "GIG", "GIL", "GIN", "GO", "GOT", "GUM", "GUN", "GUS", "GUT", "GUY", "GYM", "GYP", "HA", "HAD", "HAL", "HAM", "HAN", "HAP", "HAS", "HAT", "HAW", "HAY", "HE", "HEM", "HEN", "HER", "HEW", "HEY", "HI", "HID", "HIM", "HIP", "HIS", "HIT", "HO", "HOB", "HOC", "HOE", "HOG", "HOP", "HOT", "HOW", "HUB", "HUE", "HUG", "HUH", "HUM", "HUT", "I", "ICY", "IDA", "IF", "IKE", "ILL", "INK", "INN", "IO", "ION", "IQ", "IRA", "IRE", "IRK", "IS", "IT", "ITS", "IVY", "JAB", "JAG", "JAM", "JAN", "JAR", "JAW", "JAY", "JET", "JIG", "JIM", "JO", "JOB", "JOE", "JOG", "JOT", "JOY", "JUG", "JUT", "KAY", "KEG", "KEN", "KEY", "KID", "KIM", "KIN", "KIT", "LA", "LAB", "LAC", "LAD", "LAG", "LAM", "LAP", "LAW", "LAY", "LEA", "LED", "LEE", "LEG", "LEN", "LEO", "LET", "LEW", "LID", "LIE", "LIN", "LIP", "LIT", "LO", "LOB", "LOG", "LOP", "LOS", "LOT", "LOU", "LOW", "LOY", "LUG", "LYE", "MA", "MAC", "MAD", "MAE", "MAN", "MAO", "MAP", "MAT", "MAW", "MAY", "ME", "MEG", "MEL", "MEN", "MET", "MEW", "MID", "MIN", "MIT", "MOB", "MOD", "MOE", "MOO", "MOP", "MOS", "MOT", "MOW", "MUD", "MUG", "MUM", "MY", "NAB", "NAG", "NAN", "NAP", "NAT", "NAY", "NE", "NED", "NEE", "NET", "NEW", "NIB", "NIL", "NIP", "NIT", "NO", "NOB", "NOD", "NON", "NOR", "NOT", "NOV", "NOW", "NU", "NUN", "NUT", "O", "OAF", "OAK", "OAR", "OAT", "ODD", "ODE", "OF", "OFF", "OFT", "OH", "OIL", "OK", "OLD", "ON", "ONE", "OR", "ORB", "ORE", "ORR", "OS", "OTT", "OUR", "OUT", "OVA", "OW", "OWE", "OWL", "OWN", "OX", "PA", "PAD", "PAL", "PAM", "PAN", "PAP", "PAR", "PAT", "PAW", "PAY", "PEA", "PEG", "PEN", "PEP", "PER", "PET", "PEW", "PHI", "PI", "PIE", "PIN", "PIT", "PLY", "PO", "POD", "POE", "POP", "POT", "POW", "PRO", "PRY", "PUB", "PUG", "PUN", "PUP", "PUT",

Haller & Metz Standards Track [Page 13] RFC 1938 A One-Time Password System May 1996

"QUO", "RAG", "RAM", "RAN", "RAP", "RAT", "RAW", "RAY", "REB", "RED", "REP", "RET", "RIB", "RID", "RIG", "RIM", "RIO", "RIP", "ROB", "ROD", "ROE", "RON", "ROT", "ROW", "ROY", "RUB", "RUE", "RUG", "RUM", "RUN", "RYE", "SAC", "SAD", "SAG", "SAL", "SAM", "SAN", "SAP", "SAT", "SAW", "SAY", "SEA", "SEC", "SEE", "SEN", "SET", "SEW", "SHE", "SHY", "SIN", "SIP", "SIR", "SIS", "SIT", "SKI", "SKY", "SLY", "SO", "SOB", "SOD", "SON", "SOP", "SOW", "SOY", "SPA", "SPY", "SUB", "SUD", "SUE", "SUM", "SUN", "SUP", "TAB", "TAD", "TAG", "TAN", "TAP", "TAR", "TEA", "TED", "TEE", "TEN", "THE", "THY", "TIC", "TIE", "TIM", "TIN", "TIP", "TO", "TOE", "TOG", "TOM", "TON", "TOO", "TOP", "TOW", "TOY", "TRY", "TUB", "TUG", "TUM", "TUN", "TWO", "UN", "UP", "US", "USE", "VAN", "VAT", "VET", "VIE", "WAD", "WAG", "WAR", "WAS", "WAY", "WE", "WEB", "WED", "WEE", "WET", "WHO", "WHY", "WIN", "WIT", "WOK", "WON", "WOO", "WOW", "WRY", "WU", "YAM", "YAP", "YAW", "YE", "YEA", "YES", "YET", "YOU", "ABED", "ABEL", "ABET", "ABLE", "ABUT", "ACHE", "ACID", "ACME", "ACRE", "ACTA", "ACTS", "ADAM", "ADDS", "ADEN", "AFAR", "AFRO", "AGEE", "AHEM", "AHOY", "AIDA", "AIDE", "AIDS", "AIRY", "AJAR", "AKIN", "ALAN", "ALEC", "ALGA", "ALIA", "ALLY", "ALMA", "ALOE", "ALSO", "ALTO", "ALUM", "ALVA", "AMEN", "AMES", "AMID", "AMMO", "AMOK", "AMOS", "AMRA", "ANDY", "ANEW", "ANNA", "ANNE", "ANTE", "ANTI", "AQUA", "ARAB", "ARCH", "AREA", "ARGO", "ARID", "ARMY", "ARTS", "ARTY", "ASIA", "ASKS", "ATOM", "AUNT", "AURA", "AUTO", "AVER", "AVID", "AVIS", "AVON", "AVOW", "AWAY", "AWRY", "BABE", "BABY", "BACH", "BACK", "BADE", "BAIL", "BAIT", "BAKE", "BALD", "BALE", "BALI", "BALK", "BALL", "BALM", "BAND", "BANE", "BANG", "BANK", "BARB", "BARD", "BARE", "BARK", "BARN", "BARR", "BASE", "BASH", "BASK", "BASS", "BATE", "BATH", "BAWD", "BAWL", "BEAD", "BEAK", "BEAM", "BEAN", "BEAR", "BEAT", "BEAU", "BECK", "BEEF", "BEEN", "BEER", "BEET", "BELA", "BELL", "BELT", "BEND", "BENT", "BERG", "BERN", "BERT", "BESS", "BEST", "BETA", "BETH", "BHOY", "BIAS", "BIDE", "BIEN", "BILE", "BILK", "BILL", "BIND", "BING", "BIRD", "BITE", "BITS", "BLAB", "BLAT", "BLED", "BLEW", "BLOB", "BLOC", "BLOT", "BLOW", "BLUE", "BLUM", "BLUR", "BOAR", "BOAT", "BOCA", "BOCK", "BODE", "BODY", "BOGY", "BOHR", "BOIL", "BOLD", "BOLO", "BOLT", "BOMB", "BONA", "BOND", "BONE", "BONG", "BONN", "BONY", "BOOK", "BOOM", "BOON", "BOOT", "BORE", "BORG", "BORN", "BOSE", "BOSS", "BOTH", "BOUT", "BOWL", "BOYD", "BRAD", "BRAE", "BRAG", "BRAN", "BRAY", "BRED", "BREW", "BRIG", "BRIM", "BROW", "BUCK", "BUDD", "BUFF", "BULB", "BULK", "BULL", "BUNK", "BUNT", "BUOY", "BURG", "BURL", "BURN", "BURR", "BURT", "BURY", "BUSH", "BUSS", "BUST", "BUSY", "BYTE", "CADY", "CAFE", "CAGE", "CAIN", "CAKE", "CALF", "CALL", "CALM", "CAME", "CANE", "CANT", "CARD", "CARE", "CARL", "CARR", "CART", "CASE", "CASH", "CASK", "CAST", "CAVE", "CEIL", "CELL", "CENT", "CERN", "CHAD", "CHAR", "CHAT", "CHAW", "CHEF", "CHEN", "CHEW",

Haller & Metz Standards Track [Page 14] RFC 1938 A One-Time Password System May 1996

"CHIC", "CHIN", "CHOU", "CHOW", "CHUB", "CHUG", "CHUM", "CITE", "CITY", "CLAD", "CLAM", "CLAN", "CLAW", "CLAY", "CLOD", "CLOG", "CLOT", "CLUB", "CLUE", "COAL", "COAT", "COCA", "COCK", "COCO", "CODA", "CODE", "CODY", "COED", "COIL", "COIN", "COKE", "COLA", "COLD", "COLT", "COMA", "COMB", "COME", "COOK", "COOL", "COON", "COOT", "CORD", "CORE", "CORK", "CORN", "COST", "COVE", "COWL", "CRAB", "CRAG", "CRAM", "CRAY", "CREW", "CRIB", "CROW", "CRUD", "CUBA", "CUBE", "CUFF", "CULL", "CULT", "CUNY", "CURB", "CURD", "CURE", "CURL", "CURT", "CUTS", "DADE", "DALE", "DAME", "DANA", "DANE", "DANG", "DANK", "DARE", "DARK", "DARN", "DART", "DASH", "DATA", "DATE", "DAVE", "DAVY", "DAWN", "DAYS", "DEAD", "DEAF", "DEAL", "DEAN", "DEAR", "DEBT", "DECK", "DEED", "DEEM", "DEER", "DEFT", "DEFY", "DELL", "DENT", "DENY", "DESK", "DIAL", "DICE", "DIED", "DIET", "DIME", "DINE", "DING", "DINT", "DIRE", "DIRT", "DISC", "DISH", "DISK", "DIVE", "DOCK", "DOES", "DOLE", "DOLL", "DOLT", "DOME", "DONE", "DOOM", "DOOR", "DORA", "DOSE", "DOTE", "DOUG", "DOUR", "DOVE", "DOWN", "DRAB", "DRAG", "DRAM", "DRAW", "DREW", "DRUB", "DRUG", "DRUM", "DUAL", "DUCK", "DUCT", "DUEL", "DUET", "DUKE", "DULL", "DUMB", "DUNE", "DUNK", "DUSK", "DUST", "DUTY", "EACH", "EARL", "EARN", "EASE", "EAST", "EASY", "EBEN", "ECHO", "EDDY", "EDEN", "EDGE", "EDGY", "EDIT", "EDNA", "EGAN", "ELAN", "ELBA", "ELLA", "ELSE", "EMIL", "EMIT", "EMMA", "ENDS", "ERIC", "EROS", "EVEN", "EVER", "EVIL", "EYED", "FACE", "FACT", "FADE", "FAIL", "FAIN", "FAIR", "FAKE", "FALL", "FAME", "FANG", "FARM", "FAST", "FATE", "FAWN", "FEAR", "FEAT", "FEED", "FEEL", "FEET", "FELL", "FELT", "FEND", "FERN", "FEST", "FEUD", "FIEF", "FIGS", "FILE", "FILL", "FILM", "FIND", "FINE", "FINK", "FIRE", "FIRM", "FISH", "FISK", "FIST", "FITS", "FIVE", "FLAG", "FLAK", "FLAM", "FLAT", "FLAW", "FLEA", "FLED", "FLEW", "FLIT", "FLOC", "FLOG", "FLOW", "FLUB", "FLUE", "FOAL", "FOAM", "FOGY", "FOIL", "FOLD", "FOLK", "FOND", "FONT", "FOOD", "FOOL", "FOOT", "FORD", "FORE", "FORK", "FORM", "FORT", "FOSS", "FOUL", "FOUR", "FOWL", "FRAU", "FRAY", "FRED", "FREE", "FRET", "FREY", "FROG", "FROM", "FUEL", "FULL", "FUME", "FUND", "FUNK", "FURY", "FUSE", "FUSS", "GAFF", "GAGE", "GAIL", "GAIN", "GAIT", "GALA", "GALE", "GALL", "GALT", "GAME", "GANG", "GARB", "GARY", "GASH", "GATE", "GAUL", "GAUR", "GAVE", "GAWK", "GEAR", "GELD", "GENE", "GENT", "GERM", "GETS", "GIBE", "GIFT", "GILD", "GILL", "GILT", "GINA", "GIRD", "GIRL", "GIST", "GIVE", "GLAD", "GLEE", "GLEN", "GLIB", "GLOB", "GLOM", "GLOW", "GLUE", "GLUM", "GLUT", "GOAD", "GOAL", "GOAT", "GOER", "GOES", "GOLD", "GOLF", "GONE", "GONG", "GOOD", "GOOF", "GORE", "GORY", "GOSH", "GOUT", "GOWN", "GRAB", "GRAD", "GRAY", "GREG", "GREW", "GREY", "GRID", "GRIM", "GRIN", "GRIT", "GROW", "GRUB", "GULF", "GULL", "GUNK", "GURU", "GUSH", "GUST", "GWEN", "GWYN", "HAAG", "HAAS", "HACK", "HAIL", "HAIR", "HALE", "HALF", "HALL", "HALO", "HALT", "HAND", "HANG", "HANK", "HANS", "HARD", "HARK", "HARM", "HART", "HASH", "HAST", "HATE", "HATH", "HAUL", "HAVE", "HAWK", "HAYS", "HEAD", "HEAL", "HEAR", "HEAT", "HEBE",

Haller & Metz Standards Track [Page 15] RFC 1938 A One-Time Password System May 1996

"HECK", "HEED", "HEEL", "HEFT", "HELD", "HELL", "HELM", "HERB", "HERD", "HERE", "HERO", "HERS", "HESS", "HEWN", "HICK", "HIDE", "HIGH", "HIKE", "HILL", "HILT", "HIND", "HINT", "HIRE", "HISS", "HIVE", "HOBO", "HOCK", "HOFF", "HOLD", "HOLE", "HOLM", "HOLT", "HOME", "HONE", "HONK", "HOOD", "HOOF", "HOOK", "HOOT", "HORN", "HOSE", "HOST", "HOUR", "HOVE", "HOWE", "HOWL", "HOYT", "HUCK", "HUED", "HUFF", "HUGE", "HUGH", "HUGO", "HULK", "HULL", "HUNK", "HUNT", "HURD", "HURL", "HURT", "HUSH", "HYDE", "HYMN", "IBIS", "ICON", "IDEA", "IDLE", "IFFY", "INCA", "INCH", "INTO", "IONS", "IOTA", "IOWA", "IRIS", "IRMA", "IRON", "ISLE", "ITCH", "ITEM", "IVAN", "JACK", "JADE", "JAIL", "JAKE", "JANE", "JAVA", "JEAN", "JEFF", "JERK", "JESS", "JEST", "JIBE", "JILL", "JILT", "JIVE", "JOAN", "JOBS", "JOCK", "JOEL", "JOEY", "JOHN", "JOIN", "JOKE", "JOLT", "JOVE", "JUDD", "JUDE", "JUDO", "JUDY", "JUJU", "JUKE", "JULY", "JUNE", "JUNK", "JUNO", "JURY", "JUST", "JUTE", "KAHN", "KALE", "KANE", "KANT", "KARL", "KATE", "KEEL", "KEEN", "KENO", "KENT", "KERN", "KERR", "KEYS", "KICK", "KILL", "KIND", "KING", "KIRK", "KISS", "KITE", "KLAN", "KNEE", "KNEW", "KNIT", "KNOB", "KNOT", "KNOW", "KOCH", "KONG", "KUDO", "KURD", "KURT", "KYLE", "LACE", "LACK", "LACY", "LADY", "LAID", "LAIN", "LAIR", "LAKE", "LAMB", "LAME", "LAND", "LANE", "LANG", "LARD", "LARK", "LASS", "LAST", "LATE", "LAUD", "LAVA", "LAWN", "LAWS", "LAYS", "LEAD", "LEAF", "LEAK", "LEAN", "LEAR", "LEEK", "LEER", "LEFT", "LEND", "LENS", "LENT", "LEON", "LESK", "LESS", "LEST", "LETS", "LIAR", "LICE", "LICK", "LIED", "LIEN", "LIES", "LIEU", "LIFE", "LIFT", "LIKE", "LILA", "LILT", "LILY", "LIMA", "LIMB", "LIME", "LIND", "LINE", "LINK", "LINT", "LION", "LISA", "LIST", "LIVE", "LOAD", "LOAF", "LOAM", "LOAN", "LOCK", "LOFT", "LOGE", "LOIS", "LOLA", "LONE", "LONG", "LOOK", "LOON", "LOOT", "LORD", "LORE", "LOSE", "LOSS", "LOST", "LOUD", "LOVE", "LOWE", "LUCK", "LUCY", "LUGE", "LUKE", "LULU", "LUND", "LUNG", "LURA", "LURE", "LURK", "LUSH", "LUST", "LYLE", "LYNN", "LYON", "LYRA", "MACE", "MADE", "MAGI", "MAID", "MAIL", "MAIN", "MAKE", "MALE", "MALI", "MALL", "MALT", "MANA", "MANN", "MANY", "MARC", "MARE", "MARK", "MARS", "MART", "MARY", "MASH", "MASK", "MASS", "MAST", "MATE", "MATH", "MAUL", "MAYO", "MEAD", "MEAL", "MEAN", "MEAT", "MEEK", "MEET", "MELD", "MELT", "MEMO", "MEND", "MENU", "MERT", "MESH", "MESS", "MICE", "MIKE", "MILD", "MILE", "MILK", "MILL", "MILT", "MIMI", "MIND", "MINE", "MINI", "MINK", "MINT", "MIRE", "MISS", "MIST", "MITE", "MITT", "MOAN", "MOAT", "MOCK", "MODE", "MOLD", "MOLE", "MOLL", "MOLT", "MONA", "MONK", "MONT", "MOOD", "MOON", "MOOR", "MOOT", "MORE", "MORN", "MORT", "MOSS", "MOST", "MOTH", "MOVE", "MUCH", "MUCK", "MUDD", "MUFF", "MULE", "MULL", "MURK", "MUSH", "MUST", "MUTE", "MUTT", "MYRA", "MYTH", "NAGY", "NAIL", "NAIR", "NAME", "NARY", "NASH", "NAVE", "NAVY", "NEAL", "NEAR", "NEAT", "NECK", "NEED", "NEIL", "NELL", "NEON", "NERO", "NESS", "NEST", "NEWS", "NEWT", "NIBS", "NICE", "NICK", "NILE", "NINA", "NINE", "NOAH", "NODE", "NOEL", "NOLL", "NONE", "NOOK", "NOON", "NORM", "NOSE",

Haller & Metz Standards Track [Page 16] RFC 1938 A One-Time Password System May 1996

"NOTE", "NOUN", "NOVA", "NUDE", "NULL", "NUMB", "OATH", "OBEY", "OBOE", "ODIN", "OHIO", "OILY", "OINT", "OKAY", "OLAF", "OLDY", "OLGA", "OLIN", "OMAN", "OMEN", "OMIT", "ONCE", "ONES", "ONLY", "ONTO", "ONUS", "ORAL", "ORGY", "OSLO", "OTIS", "OTTO", "OUCH", "OUST", "OUTS", "OVAL", "OVEN", "OVER", "OWLY", "OWNS", "QUAD", "QUIT", "QUOD", "RACE", "RACK", "RACY", "RAFT", "RAGE", "RAID", "RAIL", "RAIN", "RAKE", "RANK", "RANT", "RARE", "RASH", "RATE", "RAVE", "RAYS", "READ", "REAL", "REAM", "REAR", "RECK", "REED", "REEF", "REEK", "REEL", "REID", "REIN", "RENA", "REND", "RENT", "REST", "RICE", "RICH", "RICK", "RIDE", "RIFT", "RILL", "RIME", "RING", "RINK", "RISE", "RISK", "RITE", "ROAD", "ROAM", "ROAR", "ROBE", "ROCK", "RODE", "ROIL", "ROLL", "ROME", "ROOD", "ROOF", "ROOK", "ROOM", "ROOT", "ROSA", "ROSE", "ROSS", "ROSY", "ROTH", "ROUT", "ROVE", "ROWE", "ROWS", "RUBE", "RUBY", "RUDE", "RUDY", "RUIN", "RULE", "RUNG", "RUNS", "RUNT", "RUSE", "RUSH", "RUSK", "RUSS", "RUST", "RUTH", "SACK", "SAFE", "SAGE", "SAID", "SAIL", "SALE", "SALK", "SALT", "SAME", "SAND", "SANE", "SANG", "SANK", "SARA", "SAUL", "SAVE", "SAYS", "SCAN", "SCAR", "SCAT", "SCOT", "SEAL", "SEAM", "SEAR", "SEAT", "SEED", "SEEK", "SEEM", "SEEN", "SEES", "SELF", "SELL", "SEND", "SENT", "SETS", "SEWN", "SHAG", "SHAM", "SHAW", "SHAY", "SHED", "SHIM", "SHIN", "SHOD", "SHOE", "SHOT", "SHOW", "SHUN", "SHUT", "SICK", "SIDE", "SIFT", "SIGH", "SIGN", "SILK", "SILL", "SILO", "SILT", "SINE", "SING", "SINK", "SIRE", "SITE", "SITS", "SITU", "SKAT", "SKEW", "SKID", "SKIM", "SKIN", "SKIT", "SLAB", "SLAM", "SLAT", "SLAY", "SLED", "SLEW", "SLID", "SLIM", "SLIT", "SLOB", "SLOG", "SLOT", "SLOW", "SLUG", "SLUM", "SLUR", "SMOG", "SMUG", "SNAG", "SNOB", "SNOW", "SNUB", "SNUG", "SOAK", "SOAR", "SOCK", "SODA", "SOFA", "SOFT", "SOIL", "SOLD", "SOME", "SONG", "SOON", "SOOT", "SORE", "SORT", "SOUL", "SOUR", "SOWN", "STAB", "STAG", "STAN", "STAR", "STAY", "STEM", "STEW", "STIR", "STOW", "STUB", "STUN", "SUCH", "SUDS", "SUIT", "SULK", "SUMS", "SUNG", "SUNK", "SURE", "SURF", "SWAB", "SWAG", "SWAM", "SWAN", "SWAT", "SWAY", "SWIM", "SWUM", "TACK", "TACT", "TAIL", "TAKE", "TALE", "TALK", "TALL", "TANK", "TASK", "TATE", "TAUT", "TEAL", "TEAM", "TEAR", "TECH", "TEEM", "TEEN", "TEET", "TELL", "TEND", "TENT", "TERM", "TERN", "TESS", "TEST", "THAN", "THAT", "THEE", "THEM", "THEN", "THEY", "THIN", "THIS", "THUD", "THUG", "TICK", "TIDE", "TIDY", "TIED", "TIER", "TILE", "TILL", "TILT", "TIME", "TINA", "TINE", "TINT", "TINY", "TIRE", "TOAD", "TOGO", "TOIL", "TOLD", "TOLL", "TONE", "TONG", "TONY", "TOOK", "TOOL", "TOOT", "TORE", "TORN", "TOTE", "TOUR", "TOUT", "TOWN", "TRAG", "TRAM", "TRAY", "TREE", "TREK", "TRIG", "TRIM", "TRIO", "TROD", "TROT", "TROY", "TRUE", "TUBA", "TUBE", "TUCK", "TUFT", "TUNA", "TUNE", "TUNG", "TURF", "TURN", "TUSK", "TWIG", "TWIN", "TWIT", "ULAN", "UNIT", "URGE", "USED", "USER", "USES", "UTAH", "VAIL", "VAIN", "VALE", "VARY", "VASE", "VAST", "VEAL", "VEDA", "VEIL", "VEIN", "VEND", "VENT", "VERB", "VERY", "VETO", "VICE", "VIEW", "VINE", "VISE", "VOID", "VOLT", "VOTE", "WACK", "WADE",

Haller & Metz Standards Track [Page 17] RFC 1938 A One-Time Password System May 1996

"WAGE", "WAIL", "WAIT", "WAKE", "WALE", "WALK", "WALL", "WALT", "WAND", "WANE", "WANG", "WANT", "WARD", "WARM", "WARN", "WART", "WASH", "WAST", "WATS", "WATT", "WAVE", "WAVY", "WAYS", "WEAK", "WEAL", "WEAN", "WEAR", "WEED", "WEEK", "WEIR", "WELD", "WELL", "WELT", "WENT", "WERE", "WERT", "WEST", "WHAM", "WHAT", "WHEE", "WHEN", "WHET", "WHOA", "WHOM", "WICK", "WIFE", "WILD", "WILL", "WIND", "WINE", "WING", "WINK", "WINO", "WIRE", "WISE", "WISH", "WITH", "WOLF", "WONT", "WOOD", "WOOL", "WORD", "WORE", "WORK", "WORM", "WORN", "WOVE", "WRIT", "WYNN", "YALE", "YANG", "YANK", "YARD", "YARN", "YAWL", "YAWN", "YEAH", "YEAR", "YELL", "YOGA", "YOKE" };

Haller & Metz Standards Track [Page 18]

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