INTERNET DRAFT Neil Haller
draft-haller-otp-04.txt Bellcore
October 23, 1995 Craig Metz
Kaman Sciences Corporation
A One-Time Password System
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1.0 ABSTRACT
This document describes a one-time password authentication system
(OTP). The OTP 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* 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 computing system. One-time password systems are
designed to counter this type of attack, called a "replay attack"
[4].
---------
* S/KEY is a trademark of Bellcore
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The authentication system described in this document uses a secret
pass-phase to generate a sequence of single use (one-time)
passwords. With this system, the user's secret pass-phrase never
crosses 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 against active
attacks where the potential intruder is able to intercept and modify
the packet stream [9].
3.0 INTRODUCTION
There are two entities in the operation of the OTP one-time password
system. The client must generate 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 client, 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. On each use, 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 client 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.
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- 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 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
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MUST support a pass-phrase with a maximum of at least a 63 pass-
phrase. 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 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.
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.
To achieve a uniform minimum pass-phrase complexity, all generators
MUST enforce the following rules for character-string pass-phrases:
Pass-phrases MUST be at least 10 characters in length, MUST contain
at least one upper case alphabetic, MUST contain at least one lower
case alphabetic, and MUST contain at least one non-blank non-
alphabetic.
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
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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 blanks 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
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
accept 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-
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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 at 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 RFC 1760 [5]. This dictionary is included in this
document as Appendix C.
To facilitate the implementation of smaller generators,
hexadecimal output is an acceptable alternative for the
presentation of the one-time password. All implementations of the
server software MUST accept case-insensitive hexadecimal as well
as six-word format. The hexadecimal digits may be separated by
white space so servers are REQUIRED to ignore all white space.
Examples of hexadecimal format are:
3503785b369cda8b
e5cc a1b8 7c13 096b
C7 48 90 F4 27 7B A1 CF
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.
In summary, all conforming servers MUST accept six-word input that
uses the Standard Dictionary (RFC 1760), 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 an 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
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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 client
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
client decreases by one each time, at some point the user must
reinitialize the system or be unable to authenticate.
Implementations MUST provide clients with a means of reinitializing
a sequence through explicit specification of the first one-time
password of a sequence. This allows a client to initialize without
making it necessary to send a secret pass-phrase over the network as
only the one-time password is sent. When the sequence of one-time
passwords is reinitialized, implementations MUST verify that the
seed is changed. Installations SHOULD discourage any operation that
sends the secret pass-phrase over a network 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
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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] R. L. Rivest, The MD4 Message-Digest Algorithm, "Request For
Comments (RFC) 1320", MIT and RSA Data Security, Inc., April
1992
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[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] Neil Haller & Ran Atkinson, On Internet Authentication,
"Request for Comments (RFC) 1704", Bellcore and Naval Research
Laboratory, October 1994
[5] Neil Haller, The S/KEY One-Time Password System, "Request for
Comments (RFC) 1760", Bellcore, February 1995
[6] R. L. Rivest, The MD5 Message-Digest Algorithm, "Request For
Comments (RFC) 1321", MIT and RSA Data Security, Inc., April
1992
[7] National Institute of Standards and Technology (NIST),
"Announcing the Secure Hash Standard", FIPS 180-1, U.S.
Department of Commerce, April 1995.
[8] International Standard - Information Processing -- ISO 7-bit
coded character set for information interchange (Invariant Code
Set), ISO-646, International Standards Organization, Geneva,
Switzerland, 1983
[9] Computer Emergency Response Team (CERT), "IP Spoofing and
Hijacked Terminal Connections", CA-95:01, January 1995.
Available via anonymous ftp from info.cert.org in
/pub/cert_advisories.
13.0 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
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
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Email: cmetz@cs.nrl.navy.mil
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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];
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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.
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( uppercase( W ) ) % 2048 == N
where
alg is the hash algorithm used (e.g. MD4, MD5, SHA).
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.
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Appendix C - Dictionary for Converting Between 6-Word and Binary Formats
This dictionary is from the module put.c.
{ "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",
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"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",
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INTERNET DRAFT A One Time Password System October 23, 1995
"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",
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INTERNET DRAFT A One Time Password System October 23, 1995
"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",
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INTERNET DRAFT A One Time Password System October 23, 1995
"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",
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INTERNET DRAFT A One Time Password System October 23, 1995
"WEAL", "WEAN", "WEAR", "WEED", "WEEK", "WEIR", "WELD", "WELL",
"WELT", "WENT", "WERE", "WERT", "WEST", "WHAM", "WHAT", "WHEE",
"WHEN", "WHET", "WHOA", "WHOM", "WICK", "WIFE", "WILD", "WILL",
"WIND", "WINE", "WING", "WINK", "WINO", "WIRE", "WISE", "WISH",
"WITH", "WOLF", "WONT", "WOOD", "WOOL", "WORD", "WORE", "WORK",
"WORM", "WORN", "WOVE", "WRIT", "WYNN", "YALE", "YANG", "YANK",
"YARD", "YARN", "YAWL", "YAWN", "YEAH", "YEAR", "YELL", "YOGA",
"YOKE" };
Haller & Metz [Page 18]
