Getting a Constant key size using a crypto-graphy Algorithm
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I am using Visual Studio 2010 and Sql server 2008

I am new to Cryptography.
I need to encrypted module names into a key. and after the key is given to the 
customer the encrypted data should be able to retrieve the module name from the key.

I have to make CD-key pattern of key. which has some data encrypted inside it.

I need to have a constant size of key.
What is the best algorithm for this kind of specification.

Please help me out.