Ubuntu Manpage: otp - generate one-time key pads or password lists

NAME

       otp - generate one-time key pads or password lists

SYNOPSIS

       otp [ -cchars -ddigits -echars -lchars -msigfile -nnkeys -rseed -snchars -u -wlinelen
             outfile ]

DESCRIPTION

Systems which use passwords and/or encryption keys to authenticate an individual's
identity or protect against interception of communications achieve the highest degree of
security when each password or key is used only once. Spies are furnished ``one-time
pads'' containing pages of keys used to encrypt individual characters of secret messages,
then discarded. As long as the physical security of the two copies of a one-time pad is
assured and the keys on the pad are sufficiently random, security is absolute.

Swiss banks which accept electronic payment orders use a similar mechanism to verify the
identity of the issuer of an order. When a client authorises the bank to accept
electronic orders, the bank delivers, by registered mail, a list of individual session
passwords (usually numbers of 4 to 6 digits). The customer agrees to keep the list of
passwords physically secure, and to not hold the bank liable if the customer allows the
list to fall into unauthorised hands. Each time an order is given, in addition to the
regular user identity and password, the next key from the list must be entered, and then
crossed off by the user. The bank verifies the key against a copy of the list stored in
their own secure computer, and only if the key matches is the order accepted. Multiple
incorrect entries block electronic access to the customer's account until re-enabled by
the customer providing suitable verification that an unauthorised access attempt did not,
in fact, occur.

Note that even if the customer's entire communication session with the bank is
intercepted, the eavesdropper will not be able to subsequently issue orders in the
customer's name since the one-time password used in the compromised session will never be
used again, and provides no usable clue as to subsequent one-time passwords.

otp creates key and password lists for verification and security purposes in a variety of
formats. Keys can be of any length, consist of digits or letters (capital or lower case),
and alphabetic passwords can either be entirely random (most secure) or obey the digraph
statistics of English text (easier to remember when transcribing, but less secure).

For computer applications, for example one-time login passwords, otp can create a file
containing the MD5 signature of each of the generated keys. This permits the computer to
verify keys without the need to store the keys in plaintext. As noted below, this
improves security only if keys are sufficiently long to deter exhaustive search for their
signatures.

OPTIONS

-cchars Generate keys consisting of chars (default 8) capital letters. The keys
consist of random letters unless the -e option is also specified, in which
case they obey the digraph statistics of English text.

-ddigits Generate keys consisting of digits (default 8) decimal digits.

-echars Generate keys consisting of chars (default 8) letters which obey the digraph
statistics of English text. Such keys are usually easier to remember when
transcribing from a printed pad to a computer, but are less secure than
entirely random sequences of letters. Keys default to lower case letters;
specify the -c option along with -e if you prefer capital letters.

-lchars Generate keys consisting of chars (default 8) lower case letters. The keys
consist of random letters unless the -e option is also specified, in which
case they obey the digraph statistics of English text.

-msigfile A file sigfile is written which contains the MD5 signature of each of the
generated keys, with each 128 bit signature written as 32 hexadecimal digits
on a line by itself. Computer applications can use this signature file for
verification when the user supplies a key, rather than storing a copy of the
keys as plaintext. Note that if the MD5 signature file is compromised, short
keys are still vulnerable since their MD5 signatures can be found by
exhaustive search.

-nnkeys Generate nkeys keys. By default, 50 keys are generated.

-rseed The string seed is used to initialise the random number generator. Every run
of otp with the same seed will produce the same output. This is primarily
useful for testing, but it also allows users at different locations to produce
identical sheets given only a seed known to both. If you're about to use up
all the keys on a sheet, you can generate a new pair of sheets by using the
last key on the sheet as the seed for a new one. (This is not as secure as
physically exchanging a new pair of sheets, but if you're about to run out of
keys, it's better than nothing.) If the -r option is not specified, the
generator is initialised with a value derived from the date and time and
various system environment information; each run will produce a different
sheet.

-snchars Include a hyphen separator every nchars characters in the keys. Breaking up
long keys into segments with separators makes them easier to transcribe. By
default, a hyphen is inserted every 4 characters.

-u Print how-to-call information.

-wlinelen Format output so lines are less than or equal to linelen characters (unless
individual keys exceed the line length). The default line length is 79
characters.

FILES

       If no outfile is specified, output is written on standard output.

APPLICATION NOTES

       If you're using otp-generated keys for computer system passwords, it's wise to include one
       or  more  non-alphanumeric  characters and to mix upper and lower case letters; this makes
       your password much more difficult to guess by exhaustive  search.   For  example,  if  otp
       generated a password of jxuc-uiuf and you're using a system on which passwords are limited
       to 8 characters, you might actually use jXu&uIU= as your password.

       When using one-time keys to communicate with other people,  it's  often  a  good  idea  to
       supply  both  the  current session key and the key for the previous session.  It's easy to
       forget to cross off a key after using it; including the previous key makes  it  easier  to
       discover  if  this  has  happened  and  get  back  in  sync.  Similarly, in computer-based
       authentication systems it's a good idea to  respond  to  entry  of  an  incorrect  key  by
       prompting the user with the key from the last session.

       Pseudorandom  numbers  on  which  entries  in  the  pads  are  based  are generated by the
       exclusive-or of four concurrently-running BSD random() generators, each with 256 bytes  of
       state,  independently  seeded  from  4  byte  segments of the 16-byte MD5 signature of the
       original seed.

EXIT STATUS

       otp returns status 0 if processing was  completed  without  errors,  and  2  if  an  error
       prevented generation of output.

ACKNOWLEDGEMENTS

       The  English-digraph  frequency key generator is based on the ``mpw'' program developed at
       MIT, which was converted from Multics PL/I to C by Bill Sommerfeld, 4/21/86.  The original
       PL/I version was provided by Jerry Saltzer.

       The  implementation  of  MD5  message-digest algorithm is based on a public domain version
       written by Colin Plumb in 1993.  The algorithm is due to Ron  Rivest.   The  algorithm  is
       described in Internet RFC 1321.

AUTHOR

           John Walker
           <http://www.fourmilab.ch/>

       This  software  is  in the public domain.  Permission to use, copy, modify, and distribute
       this software and its documentation for any purpose and without  fee  is  hereby  granted,
       without  any  conditions  or  restrictions.   This  software is provided ``as is'' without
       express or implied warranty.