File using same key on XOR cipher. Using CBC
The "XOR cipher" described does not encrypt more than the first block, even if you do not reuse keys. The subsequent blocks can be "decrypted" by the attacker simply by undoing the XOR – there is no secret involved.

Decrypting the first block and finding the key does require more than one message. It is a case of the many-time pad and can be solved either using a known plaintext or by crib-dragging as described in the link above.