(a xor x) xor x == a
Yeah but you don't have "a", you only have the encrypted data...
This situation is not so far fetched, if you just XOR password with data:
Source data: 010203040506000000000000000001020304050 Password: (XOR) 70617373776f726470617373776f72647061737 (password repeated over and over) Result: (XOR-encrypted) 71637077726a726470617373776f73667365727 Similarities: 7 6 7 7 7 6 726470617373776f7 6 7 6 7 7 Oops, password in clear text: r d p a s s w o
So if you are going to use an encryption algorithm based on XOR, you need proper "random" data to XOR with. Anything else will be exposed.
EDIT: (But I probably misunderstood your question, as I saw you already got your answer.)