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RFC1938 - A One-Time Password System

王朝other·作者佚名  2008-05-31
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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

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:

- MUST

This word or the adjective "REQUIRED" means that the item is an

absolute requirement of the specification.

- 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.

- 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

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.

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.

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

described in RFC1760 [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 (RFC1760 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.

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

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.

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, RFC1320",

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",

RFC1704, Bellcore and Naval Research Laboratory, October 1994.

[5] Haller, N., "The S/KEY One-Time Password System", RFC1760,

Bellcore, February 1995.

[6] Rivest, R., "The MD5 Message-Digest Algorithm", RFC1321,

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.

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

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.

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",

"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",

"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",

"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",

"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",

"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",

"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" };

 
 
 
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