Encrypting and decrypting the Text Content

Question

Hi All, Presently i am working on C#.net. In my application, i am giving input of text content of any length. but i want encrypting text should be of 20 length. This encrypting text should be get back to my original text when i decrypt it. Note: Special charactes like +, /, = should not come in the encrypted text. I will be thankful, if any one can help me out in this regard. Thanks in Advance, Vasu.