How do I break a Symmetric Encryption scheme where the Keyphrase is simply added to the Cleartext?

The encryption scheme takes the ASCII values of the characters in the Keyphrase and the Cleartext and adds them sequentially. So if the Cleartext is BAT, and the Keyphrase is CAT, the resulting word would be:

chr(asc("B")+asc("C")) & _
chr(asc("A")+asc("A")) & _
chr(asc("T")+asc("T"))

or the string "…‚¨".

This is an exceedingly simple method of encryption which impresses me as being just as simple to break, and I'm going to be asked to demonstrate why this method of encryption should be abandoned. I'm not an expert on Cryptography however, and I fear I may not be able to research or develop a "crack" in time (outside of Brute Force - which won't be acceptable).

Does anyone know a method outside of Brute Force that can be used to determine the Keyphrase and the Cleartext contained in the encrypted data? Perhaps quickly and easily? What is it?

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