The C/C++ source code (both encryption & decryption) has to be
(important) as short as possible, ready to compile & execute. There is
not need for a GUI or a documentation, a command prompt interaction is
all that is needed.

Monoalphabetic Substitution cipher
Affine cipher
Homophonic Substitution cipher

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Clarification of Question by emidekol-ga on 26 May 2005 07:15 PDT

Just to clear up that by decryption I don't mean cracking a cipher.

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Clarification of Question by emidekol-ga on 26 May 2005 07:28 PDT

You can either post the code itself or the links you found.

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Clarification of Question by emidekol-ga on 26 May 2005 22:38 PDT

I am only interested in the 3 ciphers I mentioned above. Also from now
on, I will accept JAVA code as well, provided that it's short/simple &
ready to execute (I am not very good with JAVA).

Here are a few open source encryption projects, and include sourcecode

http://www.openssh.org/
http://www.openpgp.net/
http://www.openssl.org/

You will be able to find many more open source code here,

http://sourceforge.net/search/ Search for encryption

Thanks for the question,
Let me know if I may provide more information,
Jason

Thanx okitekmyth-ga but I wasn't able to find exactly what I am looking for..

Monoalphabetic Substitution cypher is a standard
introduction-to-programming task. 
http://users.powernet.co.uk/eton/crypto/ has an interesting decription
software.
http://www.free2code.net/plugins/articles/read.php?id=7

This book seems to be referenced over and over again:  Bruce Schneier.
Applied Cryptography: Protocols, Algorithms, and Source Code in C (2nd
ed). John Wiley & Sons, 1996. ISBN: 0471117099

Still, 6 dollars is a bit low for this question as it's requiring at
least three sources and, frankly, they don't just pop up unless you go
through some Computer Science syllabi.

This is a free comment.

The one must-have symmetric algorithm is here.
http://fp.gladman.plus.com/cryptography_technology/rijndael/