For the "decryption" you will want tr. A couple of comments are in order:

• If this is for your work, you should be aware that a simple substitution is an extremely easy code to break, so you are not providing much security at all by this mechanism.
• If this is for a homework assignment, you will probably find the monks more helpful if you state that up front, and demonstrate some effort at solving the problem yourself.

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I'd like to be able to assign to an luser