COM 1205 Assignment #2.
Assigned:  Tuesday, 25 January 2000
Due:       Tuesday, 1 February 2000

Acting as the software developer who is to design the program
whose requirements are specified below, write a professional
electronic memo to the author of these requirements in which
you request any clarifications or further details that you may
need before you can design the program.  Submit your memo
before 5 pm on the due date by sending it to will@ccs.neu.edu
with a subject line of

    assignment #2

and a body that consists of

    1.  Your name
    2.  Any remarks that you wish to make to the instructor.
    3.  A line consisting of exactly 50 hyphens.
    4.  The text of your memo, formatted so that any email
        program can be used to read it.

--------------------------------------------------

Requirements for a simple cryptanalysis program.

The input to this program consists of a cryptogram, represented
as a string consisting of the 26 alphabetic characters of
English, spaces, and newlines.  The cryptogram may consist of
five-letter groups separated by whitespace, or it may be divided
into words as in a recreational cryptogram.  The program should
assume that this cryptogram was obtained by enciphering an
English plaintext using some kind of cipher that was popular
during the nineteenth century but has been made obsolete by
computers and by more powerful methods of cryptanalysis.
Examples of these obsolete ciphers include single or double
("US Army") columnar transposition ciphers, simple substitution
ciphers, periodic substitution ciphers (e.g. Vigenere/Beaufort),
the Playfair digram cipher, and combinations of transposition
and substitution ciphers.

Auxiliary inputs may also be provided, in the form of words or
phrases that are considered likely to occur within the original
plaintext.  These inputs will also be represented as strings.

Ideally, the output of the program should consist of the type
of cipher that was used, the cipher key, and the plaintext.  If
the program is unable to recover the original plaintext, then it
should output as much of the plaintext as it is able to recover,
using periods to take the place of unknown letters.  If there
are several plausible versions of the plaintext, then each
version should be output, in decreasing order of plausibility.

If the program is unable to recover the plaintext, but can
provide some insight into the type of cipher and/or the nature
of its key, then the output should consist of that information.

There are many algorithms and heuristics available for use in
this program.  See for example the following introductory texts:

    William F. Friedman, Elements of Cryptanalysis, Aegean Press.
    Helen F Gaines, Elementary Cryptanalysis, Dover Books, 1956
        (originally published in 1939).

For a more complete list of references, see

    http://www.faqs.org/ftp/faqs/cryptography-faq/part10
    http://www.aegeanparkpress.com/desc.html#C