CRYPTOGRAM SOLVER by Michael Markov
For HP-75 with I/O ROM & Video monitor. 

Solving cryptograms is great fun.  Cryptograms can be found in
newspapers or in a variety of collections of pencil puzzles & word
games.  You are given sentences in which one set of letters is
substituted for another.  For example, "A BIG CAT" can become "M
HWX UMY".  You can break such codes by watching the frequency of
certain letters or the way they are grouped.  A single letter is
usually A or I.  THE, AND, IS, IT, OF and other common words occur
very often in the English language... 

Such simple substitution cryptograms can usually be solved with a
little trial and error, with a minimum of effort.  "Expert" solvers
keep lists of pattern words that can be easily recognized, such as
THAT & HIGH (4 letters, with same first and last character).  This
allows them to solve cryptograms more rapidly.  However, even
without pattern word lists, you should be able to break these
cyphers in a relatively short period of time, using paper and
pencil (and eraser!). 

The tedious part of cryptogram solving is the time you spend
erasing bad guesses and writing-in your newest guess.  For example,
if you decide that "M" stands for "I", you look-up every M in the
cryptogram and write an "I" underneath.  Then, when you find out
that this is a bad guess, you must erase the "I" and then
eventually replace it with a better guess.  Also, when you enter a
guess value, you should erase prior guesses that may have used the
same plain text character, since "H" and "M" cannot both stand for
"A". 

The "DECRYPT" programs was designed to do this work automatically.
When you see the prompt "Replace :  ", you respond by keying in two
strings.  The first string should be a letter or word from the
message you wish to break, and the second should the letter or word
you think it could be.  Press [RTN].  A few moments later, the
video monitor will display the changes you wanted made, for the
entire message.  You can now study the screen and make more
guesses, until you have solved the cryptograms or until you want to
stop. 

When you wish to stop, enter a "+" in response to the "Replace :  "
prompt.  A new menu allows you to make a printed listing of your
solution ("P"), go on to another cryptogram ("A") or quit ("Q").
It should be noted that your current guesses are saved in line # 0
of the text file that holds the cipher text you are solving.  This
means that you can quit at any time, and continue solving the
cryptogram at a latter date, without having to go back to the
beginning. 

















"DECRYPT" should therefore relieve the solver of most of the tedium
associated with solving cryptograms.  This leaves you with the fun
part of the game -- making smart guesses.  Also, since you may need
a letter frequency count when you solve the tougher cryptograms,
"DECRYPT" gives you a frequency count that tells you how many times
any one given character occurs in the cryptogram. 

GENERAL COMMENTS: 
Simple substitution cyphers offer almost no security whatsoever,
even when words are run together, or when letters are given in
groups of 5 (the usual practise), with no reguard to word division
or punctuation.  See Elementary Cryptanalysis by Helen Fouche
Gaines (Dover publications) for a good introduction to the art of
solving various kinds of cyphers. 

The HP-75, especially when enhanced with the many keywords provided
by the I/O ROM, is an excellent tool for cryptanalysts.  It can
carry-out extensive statistical analysis of cypher text in very
little time.  This capability could be used to help solve some very
sophisticated cyphers. 

It should be possible to set up a routine that looks-up pattern
words stored in text files, or, possibly, in some special purpose
mass storage files... 
HAPPY PROGRAMMING!