PGP word list

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The PGP Word List ('Pretty Good Privacy word list', also called a biometric word list for reasons explained below) is a list of words for conveying data bytes in a clear unambiguous way via a voice channel. They are analogous in purpose to the NATO phonetic alphabet used by pilots, except a longer list of words is used, each word corresponding to one of the 256 unique numeric byte values.

History and structure[edit]

The PGP Word List list was designed in 1995 by Patrick Juola, a computational linguist, and Philip Zimmermann, creator of PGP.[1][2] The words were carefully chosen for their phonetic distinctiveness, using genetic algorithms to select lists of words that had optimum separations in phoneme space. The candidate word lists were randomly drawn from Grady Ward's Moby Pronunciator list as raw material for the search, successively refined by the genetic algorithms. The automated search converged to an optimized solution in about 40 hours on a DEC Alpha, a particularly fast machine in that era.

The Zimmermann/Juola list was originally designed to be used in PGPfone, a secure VoIP application, to allow the two parties to verbally compare a short authentication string to detect a man-in-the-middle attack (MiTM). It was called a biometric word list because the authentication depended on the two human users recognizing each other's distinct voices as they read and compared the words over the voice channel, binding the identity of the speaker with the words, which helped protect against the MiTM attack. The list can be used in many other situations where a biometric binding of identity is not needed, so calling it a biometric word list may be imprecise. Later, it was used in PGP to compare and verify PGP public key fingerprints over a voice channel. This is known in PGP applications as the "biometric" representation. When it was applied to PGP, the list of words was further refined, with contributions by Jon Callas. More recently, it has been used in Zfone and the ZRTP protocol, the successor to PGPfone.

The list is actually composed of two lists, each containing 256 phonetically distinct words, in which each word represents a different byte value between 0 and 255. Two lists are used because reading aloud long random sequences of human words usually risks three kinds of errors: 1) transposition of two consecutive words, 2) duplicate words, or 3) omitted words. To detect all three kinds of errors, the two lists are used alternately for the even-offset bytes and the odd-offset bytes in the byte sequence. Each byte value is actually represented by two different words, depending on whether that byte appears at an even or an odd offset from the beginning of the byte sequence. The two lists are readily distinguished by the number of syllables; the even list has words of two syllables, the odd list has three. The two lists have a maximum word length of 9 and 11 letters, respectively. Using a two-list scheme was suggested by Zhahai Stewart.

Hex Even Word Odd Word
00 aardvark adroitness
01 absurd adviser
02 accrue aftermath
03 acme aggregate
04 adrift alkali
05 adult almighty
06 afflict amulet
07 ahead amusement
08 aimless antenna
09 Algol applicant
0A allow Apollo
0B alone armistice
0C ammo article
0D ancient asteroid
0E apple Atlantic
0F artist atmosphere
10 assume autopsy
11 Athens Babylon
12 atlas backwater
13 Aztec barbecue
14 baboon belowground
15 backfield bifocals
16 backward bodyguard
17 banjo bookseller
18 beaming borderline
19 bedlamp bottomless
1A beehive Bradbury
1B beeswax bravado
1C befriend Brazilian
1D Belfast breakaway
1E berserk Burlington
1F billiard businessman
20 bison butterfat
21 blackjack Camelot
22 blockade candidate
23 blowtorch cannonball
24 bluebird Capricorn
25 bombast caravan
26 bookshelf caretaker
27 brackish celebrate
28 breadline cellulose
29 breakup certify
2A brickyard chambermaid
2B briefcase Cherokee
2C Burbank Chicago
2D button clergyman
2E buzzard coherence
2F cement combustion
30 chairlift commando
31 chatter company
32 checkup component
33 chisel concurrent
34 choking confidence
35 chopper conformist
36 Christmas congregate
37 clamshell consensus
38 classic consulting
39 classroom corporate
3A cleanup corrosion
3B clockwork councilman
3C cobra crossover
3D commence crucifix
3E concert cumbersome
3F cowbell customer
Hex Even Word Odd Word
40 crackdown Dakota
41 cranky decadence
42 crowfoot December
43 crucial decimal
44 crumpled designing
45 crusade detector
46 cubic detergent
47 dashboard determine
48 deadbolt dictator
49 deckhand dinosaur
4A dogsled direction
4B dragnet disable
4C drainage disbelief
4D dreadful disruptive
4E drifter distortion
4F dropper document
50 drumbeat embezzle
51 drunken enchanting
52 Dupont enrollment
53 dwelling enterprise
54 eating equation
55 edict equipment
56 egghead escapade
57 eightball Eskimo
58 endorse everyday
59 endow examine
5A enlist existence
5B erase exodus
5C escape fascinate
5D exceed filament
5E eyeglass finicky
5F eyetooth forever
60 facial fortitude
61 fallout frequency
62 flagpole gadgetry
63 flatfoot Galveston
64 flytrap getaway
65 fracture glossary
66 framework gossamer
67 freedom graduate
68 frighten gravity
69 gazelle guitarist
6A Geiger hamburger
6B glitter Hamilton
6C glucose handiwork
6D goggles hazardous
6E goldfish headwaters
6F gremlin hemisphere
70 guidance hesitate
71 hamlet hideaway
72 highchair holiness
73 hockey hurricane
74 indoors hydraulic
75 indulge impartial
76 inverse impetus
77 involve inception
78 island indigo
79 jawbone inertia
7A keyboard infancy
7B kickoff inferno
7C kiwi informant
7D klaxon insincere
7E locale insurgent
7F lockup integrate
Hex Even Word Odd Word
80 merit intention
81 minnow inventive
82 miser Istanbul
83 Mohawk Jamaica
84 mural Jupiter
85 music leprosy
86 necklace letterhead
87 Neptune liberty
88 newborn maritime
89 nightbird matchmaker
8A Oakland maverick
8B obtuse Medusa
8C offload megaton
8D optic microscope
8E orca microwave
8F payday midsummer
90 peachy millionaire
91 pheasant miracle
92 physique misnomer
93 playhouse molasses
94 Pluto molecule
95 preclude Montana
96 prefer monument
97 preshrunk mosquito
98 printer narrative
99 prowler nebula
9A pupil newsletter
9B puppy Norwegian
9C python October
9D quadrant Ohio
9E quiver onlooker
9F quota opulent
A0 ragtime Orlando
A1 ratchet outfielder
A2 rebirth Pacific
A3 reform pandemic
A4 regain Pandora
A5 reindeer paperweight
A6 rematch paragon
A7 repay paragraph
A8 retouch paramount
A9 revenge passenger
AA reward pedigree
AB rhythm Pegasus
AC ribcage penetrate
AD ringbolt perceptive
AE robust performance
AF rocker pharmacy
B0 ruffled phonetic
B1 sailboat photograph
B2 sawdust pioneer
B3 scallion pocketful
B4 scenic politeness
B5 scorecard positive
B6 Scotland potato
B7 seabird processor
B8 select provincial
B9 sentence proximate
BA shadow puberty
BB shamrock publisher
BC showgirl pyramid
BD skullcap quantity
BE skydive racketeer
BF slingshot rebellion
Hex Even Word Odd Word
C0 slowdown recipe
C1 snapline recover
C2 snapshot repellent
C3 snowcap replica
C4 snowslide reproduce
C5 solo resistor
C6 southward responsive
C7 soybean retraction
C8 spaniel retrieval
C9 spearhead retrospect
CA spellbind revenue
CB spheroid revival
CC spigot revolver
CD spindle sandalwood
CE spyglass sardonic
CF stagehand Saturday
D0 stagnate savagery
D1 stairway scavenger
D2 standard sensation
D3 stapler sociable
D4 steamship souvenir
D5 sterling specialist
D6 stockman speculate
D7 stopwatch stethoscope
D8 stormy stupendous
D9 sugar supportive
DA surmount surrender
DB suspense suspicious
DC sweatband sympathy
DD swelter tambourine
DE tactics telephone
DF talon therapist
E0 tapeworm tobacco
E1 tempest tolerance
E2 tiger tomorrow
E3 tissue torpedo
E4 tonic tradition
E5 topmost travesty
E6 tracker trombonist
E7 transit truncated
E8 trauma typewriter
E9 treadmill ultimate
EA Trojan undaunted
EB trouble underfoot
EC tumor unicorn
ED tunnel unify
EE tycoon universe
EF uncut unravel
F0 unearth upcoming
F1 unwind vacancy
F2 uproot vagabond
F3 upset vertigo
F4 upshot Virginia
F5 vapor visitor
F6 village vocalist
F7 virus voyager
F8 Vulcan warranty
F9 waffle Waterloo
FA wallet whimsical
FB watchword Wichita
FC wayside Wilmington
FD willow Wyoming
FE woodlark yesteryear
FF Zulu Yucatán

Examples[edit]

Each byte in a bytestring is encoded as a single word. A sequence of bytes is rendered in network byte order, from left to right. For example, the leftmost (i.e. byte 0) is considered "even" and is encoded using the PGP Even Word table. The next byte to the right (i.e. byte 1) is considered "odd" and is encoded using the PGP Odd Word table. This process repeats until all bytes are encoded. Thus, "E582" produces "topmost Istanbul", whereas "82E5" produces "miser travesty".

A PGP public key fingerprint that displayed in hexadecimal as

E582 94F2 E9A2 2748 6E8B
061B 31CC 528F D7FA 3F19

would display in PGP Words (the "biometric" fingerprint) as

topmost Istanbul Pluto vagabond
treadmill Pacific brackish dictator
goldfish Medusa afflict bravado
chatter revolver Dupont midsummer
stopwatch whimsical cowbell bottomless

The order of bytes in a bytestring depends on Endianness.

Other word lists for data[edit]

There are several other word lists for conveying data in a clear unambiguous way via a voice channel:

  • the NATO phonetic alphabet maps individual letters and digits to individual words
  • the S/KEY system maps 64 bit numbers to 6 short words of 1 to 4 characters each from a publicly accessible 2048-word dictionary. The same dictionary is used in RFC 2289.
  • the Diceware system maps five base-6 random digits (almost 13 bits of entropy) to a word from a dictionary of 7,776 unique words.
  • FIPS 181: Automated Password Generator converts random numbers into somewhat pronounceable "words".
  • mnemonic encoding converts 32 bits of data into 3 words from a vocabulary of 1626 words.[3]
  • what3words encodes geographic coordinates in 3 dictionary words.

References[edit]

This article incorporates material that is copyrighted by PGP Corporation and has been licensed under the GNU Free Documentation License. (per Jon Callas, CTO, CSO PGP Corporation, 4-Jan-2007)
  1. ^ Juola, Patrick; Zimmermann, Philip (1996). "Whole-Word Phonetic Distances and the PGPfone Alphabet". Proceedings of the International Conference of Spoken Language Processing (ICSLP-96). 
  2. ^ Juola, Patrick (1996). "Isolated Word Confusion Metrics and the PGPfone Alphabet". Proceedings of New Methods in Language Processing 2 (Ankara, Turkey: Oxford University, Dept. of Experimental Psychology). 
  3. ^ mnemonic encoding and updated code[dead link]