A Convention for Human-Readable 128-bit Keys
draft-mcdonald-readable-keys-00

Network Working Group                                        D. McDonald
INTERNET-DRAFT                                                       NRL
draft-mcdonald-readable-keys-00.txt                       20 August 1994

              A Convention for Human-Readable 128-bit Keys

Status of this Memo

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Introduction

   The Internet community has begun to address matters of security.
   Recent standards, including version 2 of SNMP [GM93], and version 6
   of IP [Atk94] have explicit requirements for an authentication
   mechanism.  Both require used of a keyed message-digest algorithm,
   MD5 [Riv92].  Both require a key size of 128-bits.  A 128-bit key,
   while sufficiently strong, is hard for most people to read, remember,
   and type in.

A Solution Already Exists

   The S/Key(tm) one-time password system [Hal94] uses MD4 (and now MD5,
   as well) to compute one-time passwords.  It takes the 128-bit result
   of MD4 and collapses it to a 64-bit result.  Despite the size
   reduction, 64-bit one-time passwords are still difficult for ordinary
   people to remember and enter.  The authors of S/Key devised a system
   to make the 64-bit one-time password easy for people to enter.

   Their idea was to transform the password into a string of small
   English words.  English words are significantly easier for people to

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   both remember and type.  The authors of S/Key started with a
   dictionary of 2048 English words, ranging in length from one to four
   characters.  The space covered by a 64-bit key (2^64) could be
   covered by six words from this dictionary (2^66) with room remaining
   for parity.  For example, an S/Key one-time password of hex value:

                            EB33 F77E E73D 4053

   would become the following six English words:

                       TIDE ITCH SLOW REIN RULE MOT

The Proposal

   The code (see Appendix A) which S/Key uses to convert 64-bit numbers
   to six English words contains two primitives which perform
   conversions either way.  The primitive btoe(char *engout,char *c)
   takes a 64-bit quantity referenced by c and places English words in
   the string referenced by engout.  The primitive etob(char *out,char
   *e) performs the opposite with an input string of English words
   referenced by e, and by placing the 64-bit result into the buffer
   referenced by out.

   The aforementioned primitives can be applied to both halves of a
   128-bit key, or both halves of a string of twelve English words.  Two
   new primitives (see Appendix B), key2eng(char *engout,char *key) and
   eng2key(char *keyout,char *eng) serve as wrappers which call the
   S/Key primitives twice, once for each half of the 128-bit key or
   string of twelve words.

   For example, the 128-bit key of:

                  CCAC 2AED 5910 56BE 4F90 FD44 1C53 4766

   would become

         RASH BUSH MILK LOOK BAD BRIM AVID GAFF BAIT ROT POD LOVE

   Likewise, a user should be able to type in

          TROD MUTE TAIL WARM CHAR KONG HAAG CITY BORE O TEAL AWL

   as a key, and the machine should make the translation to:

                  EFF8 1F9B FBC6 5350 920C DD74 16DE 8009

   If this proposal is to work, it is critical that the dictionary of
   English words does not change with different implementations.  A

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   freely redistributable reference implementation is given in
   Appendices A and B.

Security Considerations

   This document recommends a method of representing 128-bit keys using
   strings of English words.  Since the strings of English words are
   easy to remember, people may potentially construct easy-to-guess
   strings of English words.  With easy-to-guess strings comes the
   possibility of a sentential equivalent of a dictionary attack.  In
   order to maximize the strength of any authentication mechanism that
   uses 128-bit keys, the keys must be sufficiently obscure.  In
   particular, people should avoid the temptation to devise sentences.

References

   [Atk94]  Atkinson, Randall, "IPv6 Authentication Header", Internet-
   Draft, August 1994.

   [GM93]  Galvin, J. and McCloghrie, K., "Security Protocols for
   version 2 of the Simple Network Management Protocol (SNMPv2)",
   RFC1446, Internet Architecture Board, April 1993.

   [Hal94]  Haller, Neil M., "The S/Key(tm) One-Time Password System",
   Proceedings of the Symposium on Network & Distributed Systems
   Security, Internet Society, San Diego, February 1994.

   [Riv92]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC1321,
   Internet Architecture Board, April 1992.

Author's Address

   Daniel L. McDonald
   United States Naval Research Laboratory
   Code 5544
   4555 Overlook Ave. SW
   Washington, DC 20375

   Phone:  (202) 404-7122

   E-mail:  danmcd@itd.nrl.navy.mil

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Appendix A - Source for S/Key 8-bytes to/from Words Routines (put.c)

/* This code originally appeared in the source for S/Key, available in the
 * directory
 *  ftp://thumper.bellcore.com/pub/nmh
 *
 * It has been modified only to remove explicit S/Key references.
 */

#include <stdio.h>
#include <string.h>
#include <assert.h>
#include <ctype.h>

#ifdef __STDC__
#define __ARGS(x) x
#else
#define __ARGS(x) ()
#endif

static unsigned long extract __ARGS((char *s,int start,int length));
static void standard __ARGS((char *word));
static void insert __ARGS((char *s, int x, int start, int length));
static int wsrch __ARGS((char *w,int low,int high));

/* Dictionary for integer-word translations */
char Wp[2048][4] = {
"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",

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"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",
"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",

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"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",
"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",

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"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", "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",

McDonald                                                        [Page 7]
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"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", "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",

McDonald                                                        [Page 8]
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"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"
};

/* Encode 8 bytes in 'c' as a string of English words.
 * Returns a pointer to a static buffer
 */
char *
btoe(engout,c)
char *c, *engout;
{
        char cp[9];     /* add in room for the parity 2 bits*/
        int p,i ;

        engout[0] = '\0';
        memcpy(cp, c,8);
        /* compute parity */
        for(p = 0,i = 0; i < 64;i += 2)
                p += extract(cp,i,2);

        cp[8] = (char)p << 6;
        strncat(engout,&Wp[extract(cp, 0,11)][0],4);
        strcat(engout," ");
        strncat(engout,&Wp[extract(cp,11,11)][0],4);
        strcat(engout," ");
        strncat(engout,&Wp[extract(cp,22,11)][0],4);
        strcat(engout," ");
        strncat(engout,&Wp[extract(cp,33,11)][0],4);
        strcat(engout," ");
        strncat(engout,&Wp[extract(cp,44,11)][0],4);
        strcat(engout," ");
        strncat(engout,&Wp[extract(cp,55,11)][0],4);
#ifdef  notdef
        printf("engout is %s\n\r",engout);
#endif
        return(engout);
}

/* convert English to binary
 * returns 1 OK - all good words and parity is OK
 *         0 word not in data base
 *        -1 badly formed in put ie > 4 char word
 *        -2 words OK but parity is wrong
 */
int
etob(out, e)
char *out;

McDonald                                                        [Page 9]
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char *e;
{
        char *word;
        int i, p, v,l, low,high;
        char b[9];
        char input[36];

        if(e == NULL)
                return -1;

        strncpy(input,e,sizeof(input));
        memset(b, 0, sizeof(b));
        memset(out, 0, 8);
        for(i=0,p=0;i<6;i++,p+=11){
                if((word = strtok(i == 0 ? input : NULL," ")) == NULL)
                        return -1;
                l = strlen(word);
                if(l > 4 || l < 1){
                        return -1;
                } else if(l < 4){
                        low = 0;
                        high = 570;
                } else {
                        low = 571;
                        high = 2047;
                }
                standard(word);
                if( (v = wsrch(word,low,high)) < 0 )
                        return 0;
                insert(b,v,p,11);
        }

        /* now check the parity of what we got */
        for(p = 0, i = 0; i < 64; i +=2)
                p += extract(b, i, 2);

        if( (p & 3) != extract(b, 64,2) )
                return -2;

        memcpy(out,b,8);

        return 1;
}
/* Display 8 bytes as a series of 16-bit hex digits */
char *
put8(out,s)
char *out;
char *s;

McDonald                                                       [Page 10]
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{
        sprintf(out,"%02X%02X %02X%02X %02X%02X %02X%02X",
                s[0] & 0xff,s[1] & 0xff,s[2] & 0xff,
                s[3] & 0xff,s[4] & 0xff,s[5] & 0xff,
                s[6] & 0xff,s[7] & 0xff);
        return out;
}
#ifdef  notdef
/* Encode 8 bytes in 'cp' as stream of ascii letters.
 * Provided as a possible alternative to btoe()
 */
char *
btoc(cp)
char *cp;
{
        int i;
        static char out[31];

        /* code out put by characters 6 bits each added to 0x21 (!)*/
        for(i=0;i <= 10;i++){
                /* last one is only 4 bits not 6*/
                out[i] = '!'+ extract(cp,6*i,i >= 10 ? 4:6);
        }
        out[i] = '\0';
        return(out);
}
#endif

/* Internal subroutines for word encoding/decoding */

/* Dictionary binary search */
static int
wsrch(w,low,high)
char *w;
int low, high;
{
        int i,j;

        for(;;){
                i = (low + high)/2;
                if((j = strncmp(w,Wp[i],4)) == 0)
                        return i;       /* Found it */
                if(high == low+1){
                        /* Avoid effects of integer truncation in /2 */
                        if(strncmp(w,Wp[high],4) == 0)
                                return high;
                        else
                                return -1;

McDonald                                                       [Page 11]
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                }
                if(low >= high)
                        return -1;      /* I don't *think* this can happen...*/
                if(j < 0)
                        high = i;       /* Search lower half */
                else
                        low = i;        /* Search upper half */
        }
}
static void
insert(s, x, start, length)
char *s;
int x;
int  start, length;
{
        unsigned char cl;
        unsigned char cc;
        unsigned char cr;
        unsigned long y;
        int shift;

        assert(length <= 11);
        assert(start >= 0);
        assert(length >= 0);
        assert(start +length <= 66);

        shift = ((8  -(( start + length) % 8))%8);
        y = (long) x << shift;
        cl = (y >> 16) & 0xff;
        cc = (y >> 8) & 0xff;
        cr = y & 0xff;
        if(shift + length > 16){
                s[start /8] |= cl;
                s[start/8 +1] |= cc;
                s[start/8 +2] |= cr;
        } else if(shift +length > 8){
                s[start/8] |= cc;
                s[start/8 + 1] |= cr;
        } else {
                s[start/8] |= cr;
        }
}

static void
standard(word)
register char *word;
{
        while(*word){

McDonald                                                       [Page 12]
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                if(!isascii(*word))
                        break;
                if(islower(*word))
                        *word = toupper(*word);
                if(*word == '1')
                        *word = 'L';
                if(*word == '0')
                        *word = 'O';
                if(*word == '5')
                        *word = 'S';
                word++;
        }
}

/* Extract 'length' bits from the char array 's' starting with bit 'start' */
static unsigned long
extract(s, start, length)
char *s;
int start, length;
{
        unsigned char cl;
        unsigned char cc;
        unsigned char cr;
        unsigned long x;

        assert(length <= 11);
        assert(start >= 0);
        assert(length >= 0);
        assert(start +length <= 66);

        cl = s[start/8];
        cc = s[start/8 +1];
        cr = s[start/8 +2];
        x = ((long)(cl<<8 | cc) <<8  | cr) ;
        x = x >> (24 - (length + (start %8)));
        x =( x & (0xffff >> (16-length) )   );
        return(x);
}

McDonald                                                       [Page 13]
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Appendix B - Source for 128-bit key to/from English words (convert.c)

/* convert.c  --  Wrapper to S/Key binary-to-English routines.
                  Daniel L. McDonald  --  U. S. Naval Research Laboratory. */

#include <string.h>

/* eng2key() assumes words must be separated by spaces only.

   eng2key() returns

   1 if succeeded
   0 if word not in dictionary
   -1 if badly formed string
   -2 if words are okay but parity is wrong.
   (see etob() in S/Key)
*/

int eng2key(keyout,eng)
char *keyout,*eng;
{
  int rc=0,state=1;
  char *eng2;

  /* Find pointer to word 7. */

  for (eng2 = eng; rc<7 && (*(++eng2) != '\0'); )
    if (*eng2 != ' ')
      {
        rc += state;
        state = 0;
      }
    else state=1;

  if ( (rc = etob(keyout,eng)) != 1)
    return rc;

  rc = etob(keyout+8,eng2);

  return rc;
}

/* key2eng() assumes string referenced by engout has at least 60 characters
   (4*12 + 11 spaces + '\0') of space.

   key2eng() returns pointer to engout.

*/

McDonald                                                       [Page 14]
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char *key2eng(engout,key)
char *engout,*key;
{
  btoe(engout,key);
  strcat(engout," ");
  btoe(engout+strlen(engout),key+8);
}

McDonald                                                       [Page 15]