The S/KEY One-Time Password System
RFC 1760

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Network Working Group                                          N. Haller
Request for Comments: 1760                                      Bellcore
Category: Informational                                    February 1995

                   The S/KEY One-Time Password System

Status of this Memo

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

Abstract

   This document describes the S/KEY* One-Time Password system as
   released for public use by Bellcore and as described in reference
   [3].  A reference implementation and documentation are available by
   anonymous ftp from ftp.bellcore.com in the directories pub/nmh/...

Overview

   One form of attack on computing system connected to the Internet is
   eavesdropping on network connections to obtain login id's and
   passwords of legitimate users.  The captured login id and password
   are, at a later time, used gain access to the system.  The S/KEY
   One-Time Password system is designed to counter this type of attack,
   called a replay attack.

   With the S/KEY system, only a single use password ever crosses the
   network.  The user's secret pass-phrase never crosses the network at
   any time, including during login or when executing other commands
   requiring authentication such as the UNIX commands passwd or su.
   Thus, it is not vulnerable to eavesdropping/replay attacks.  Added
   security is provided by the property that no secret information need
   be stored on any system, including the host being protected.

   The S/KEY 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 protection against "inside jobs" or against active attacks
   where the potential intruder as able to intercept and modify the
   packet stream.

Haller                                                          [Page 1]
RFC 1760           The S/KEY One-Time Password System      February 1995

Introduction

   There are two sides to the operation of the S/KEY one-time password
   system.  On the client side, the appropriate one-time password must
   be generated.  On the host side, the server must verify the one-time
   password and permit the secure changing of the user's secret pass-
   phrase.

   An S/KEY system client passes the user's secret pass-phrase through
   multiple applications of a secure hash function to produce a one-time
   password.  On each use, the number of applications is reduced by one.
   Thus a unique sequence of passwords is generated.  The S/KEY system
   host verifies the one-time password by making one pass though the
   secure hash function and comparing the result with the previous one-
   time password.  This technique was first suggested by Leslie Lamport
   [1].

Secure Hash Function

   A secure hash function is a function that is easy to compute in the
   forward direction, but computationally infeasible to invert.  The
   S/KEY system is based on the MD4 Message Digest algorithm designed by
   Ronald Rivest [2].  Since the S/KEY authentication system went into
   use, the MD5 Message Digest was released.  We have chosen to continue
   to use MD4 due the large number of client programs that have been
   distributed.  Some sites have generated functionally similar systems
   based on MD5.  Clearly clients and hosts must use the same secure
   hash function to interoperate.

   The S/KEY system one-time passwords are 64 bits in length.  This is
   believed to be long enough to be secure and short enough to be
   manually entered (see below, Form of Passwords) when necessary.

   The S/KEY system applies the secure hash function multiple times,
   producing a 64 bit final output.  MD4 accepts an arbitrary number of
   bits as input and produces a 128 bit output.  The S/KEY secure hash
   function consists of applying MD4 to a 64 bit input and folding the
   output of MD4 with exclusive or to produce a 64 bit output.

Generation of One-Time Passwords

   This section describes the computation of the S/KEY one-time
   passwords.  It consists of a preparatory step in which all inputs are
   combined, a generation step where the secure hash function is applied
   multiple times, and an output function where the 64 bit one-time

Haller                                                          [Page 2]
RFC 1760           The S/KEY One-Time Password System      February 1995

   password is displayed in readable form.

   The client's secret pass phrase may be of any length and should be
   more than eight characters.  As the S/KEY secure hash function
   described above accepts a 64 bit input, a preparatory step is needed.
   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 a client to use the same secret pass phrase on multiple
   machines (using different seeds) and to safely recycle secret
   passwords by changing the seed.  (For ease in parsing, the seed may
   not contain any blanks, and should consist of strictly alphanumeric
   characters.) The result of the concatenation is passed through MD4,
   and then reduced to 64 bits by exclusive-OR of the two 8-byte halves.

   The following code fragment uses the MD4 implementation defined in
   RFC 1320 [2] and defines the preparatory step:

         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];

   A sequence of one-time passwords is produced by applying the secure
   hash function multiple times to the output of the preparatory step
   (called S).  That is, the first one-time password is produced by
   passing S through the secure hash function a number of times (N)
   specified by the user.  The next one-time password 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 any succeeding password because doing so would
   require inverting the hash function.

Form of Passwords

   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 S/KEY system one-time password calculator programs
   insert this password into the input stream, others make it available
   for system cut and paste.  Some arrangements require the one-time
   password to be entered manually. The S/KEY system is designed to
   facilitate this manual entry without impeding automatic methods.  The
   one-time password is therefore converted to, and accepted as, a
   sequence of six short (1 to 4 letter) English words.  Each word is
   chosen from a dictionary of 2048 words; at 11 bits per word, all

Haller                                                          [Page 3]
RFC 1760           The S/KEY One-Time Password System      February 1995

   one-time passwords may be encoded.  Interoperability requires at all
   S/KEY system hosts and calculators use the same dictionary.  The
   standard dictionary is attached to this RFC.

Verification of One-Time Passwords

   A function on the host system that requires S/KEY authentication is
   expected to issue an S/KEY challenge.  This challenge give the client
   the current S/KEY parameters - the sequence number and seed.  It is
   important that the S/KEY challenge be in a standard format so that
   automated clients (see below) can recognize the challenge and extract
   the parameters.  The format of the challenge is:

        s/key sequence_integer seed

   The three tokens are separated by single space characters.  The
   challenge is terminated by a blank or a newline.

   Given the parameters and the secret pass phrase, the client can
   compute (or lookup) the one time password.  It then passes it to the
   host system where it can be verified.

   The host system has a file (on the UNIX reference implementation it
   is /etc/skeykeys) containing, for each user, the one-time password
   from the last successful login, or it may be initialized with the
   first one-time password of the sequence using the keyinit command
   (this command name may be implementation dependent).  To verify an
   authentication attempt, it passes the transmitted one-time password
   through the secure hash function one time.  If the result of this
   operation matches the stored previous one-time password, the
   authentication is successful and the accepted one-time password is
   stored for future use.

   Because the number of hash function applications executed by the
   client decreases by one each time, at some point the user must
   reinitialize the system of be unable to login again.  This is done by
   using the keyinit command which allows the changing of the secret
   pass phrase, the iteration count, and the seed. A frequent technique
   is to increment a trailing digit(s) of the seed and to reset the
   iteration count (to something in range of 500-1000).

Clients

   Several programs are available to calculate S/KEY one time passwords.
   Included in the reference implementation are command line interfaces
   for UNIX and PC systems (key), TSR interfaces for PCs (ctkey,
   termkey, and popkey), and GUI interfaces for Macintosh and Windows
   (keyapp and un-named Macintosh interface).

Haller                                                          [Page 4]
RFC 1760           The S/KEY One-Time Password System      February 1995

   The most basic calculator is the key command whose format is:

        key [-n count] sequence seed

   The optional count is used to display more than a single one time
   password.  This is useful to create a paper list of one time
   passwords.

   The most automated calculator is the termkey program that runs as a
   Terminate and Stay Resident (TSR) program on a PC.  It scans the
   screen to find the S/KEY parameters, prompts for the secret pass
   phrase, and stuffs the one time password into the keyboard buffer.

Acknowledgements

   The idea behind S/KEY authentication was first proposed by Leslie
   Lamport [1].  The specific system described was proposed by Phil
   Karn, who also wrote most of the reference implementation.

References

   [1] Lamport, L., "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] Haller, N., "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, Bell Communications Research and Naval Research Laboratory,
       October 1994

Haller                                                          [Page 5]
RFC 1760           The S/KEY One-Time Password System      February 1995

Security Considerations

   This entire document is about Security Considerations.

Author's Address

   Neil Haller
   Bellcore
   MRE 2Q-280
   445 South Street
   Morristown, NJ, 07960-6438, USA

   Phone: +1 201 829-4478
   Fax:  +1 201 829-2504
   EMail: nmh@bellcore.com

Haller                                                          [Page 6]
RFC 1760           The S/KEY One-Time Password System      February 1995

Dictionary for Converting Between S/KEY 6-Word and Binary Formats

   This dictionary is from the module put.c.  The code for this module,
   and an implementation of the entire S/KEY One Time Password System is
   available by anonymous ftp from ftp.bellcore.com in the directory
   pub/nmh/skey.

{        "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",
"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",

Haller                                                          [Page 7]
RFC 1760           The S/KEY One-Time Password System      February 1995

"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",
"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",

Haller                                                          [Page 8]
RFC 1760           The S/KEY One-Time Password System      February 1995

"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",
"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",

Haller                                                          [Page 9]
RFC 1760           The S/KEY One-Time Password System      February 1995

"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",
"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",

Haller                                                         [Page 10]
RFC 1760           The S/KEY One-Time Password System      February 1995

"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",
"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",

Haller                                                         [Page 11]
RFC 1760           The S/KEY One-Time Password System      February 1995

"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",
"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                                                         [Page 12]