Title: Substitution Ciphers
Author: hexxeh - hexxeh@hotmail.com
Target Level: Beginners
Date: 28th of July 2002
Note: It may help if you read my crypt.txt before this.

.:[Introduction]:.
Substitution cipher's must of encountered you one time in your life.
It's when a character(s) from the plaintext is substituted with another
character(s) in ciphertext. The cryptanalysis can become very simple, all
you have to do is substitute a character from the ciphertext back to a
character in plaintext. This type of cryptography dates back a long time
ago and can possibly be accomplished by hand. Computers only make it easier.

.:[Listings]:.
There are a total of five classical substitution ciphers and they are as follows:

*) Simple substitution cipher - Sometimes called a monoalphabetic cipher,
                                is one in which *each* character in plaintext is
                                substituted with another in ciphertext.

*) Polyalphabetic substitution cipher - This kind of substitution cipher consists
                                        of running more than one simple substitution cipher
                                        throughout their cryptographic algorithm.

*) Homophonic substitution cipher - Similar to the simple substitution cipher, but
                                    one character of plaintext can map more than one
                                    character in ciphertext. For example, "E" can
                                    correspond to either "132" or "22" or "53" etc.
                                    These kind of cipher's *usually* require a user
                                    inputted key that will salt and determine which
                                    specific characters to correspond to in ciphertext.

*) Polygram substitution cipher - This is when blocks of characters are enciphered or
                                  deciphered. For example, lets say we have a string of
                                  "errokeys" and run it against a polygram cipher then
                                  depending on the application itself, it may encipher
                                  the first 4 characters, then the other 4 characters.
                                  They also commonly work in mathematical patterns.

*) Running-key cipher - Is when one file is used to encrypt another. So basically,
                        a key(keyspace of a length of the file) is used to encrypt
                        the plaintext.

Now I'll tackle them a bit.

.:[Simple substitution ciphers]:.
An example of one of these is the famous rot13, that's an alternate of Caesar's
Cipher of rot3("A" would be "D", "B" would be "D"). These nowadays are not used
for security purposes, it is recognizable to most knowledgable computer users.
It's offenly used in public forums, where a poster would want to talk about
something that may be offensive to other viewers. Simple substitution cipher's
are really not hard to break anyway, considering if they only substituted
characters, the cipher does not hide the underlying frequencies of the different
letters of the plaintext.

.:[Homophonic substitution ciphers]:.
Homophonic is superior to the simple substitution ciphers and are much more
complicated to break, but still do not interrupt all of the stats properties
of the plaintext language. A ciphertext only attack will do the trick on a computer.

.:[Polyalphabetic substitution]:.
These can possibly be easy to break using computers. There's nothing to really
know about them really, their just a combination of simple substitution ciphers.
Sorry! :-)

.:[Polygram substitution ciphers]:.
These can be quite interesting at times. The methods used to determine a pattern
can be very complicated or simple at times. The most common mathematical pattern
determination I've been taught of is the ascii decimal relational determination.
But that's something else that I can't go into in this text. Transposition ciphers
intend to use polygramic substitution ciphers.

.:[Conclusion]:.
I hope this text has helped you understand the fundamentals of substitutions
substitution. If you have the time, please let me know what you think about it.

.:[End]:.