Let's suppose I encrypt two blocks of 1MiB with AES-256 in CBC mode, each one using different keys, XOR the resulting ciphertexts and send this XORed block to an adversary. Remembering that the adversary will not have access to resulting ciphertexts, only the XOR of them.
What could he/she do with the XORed block? Can he/she find the keys I used, or even the plaintext of one of the ciphertexts in less than 2^256 tries (256-bits).
This question could sound strange but it's part of one of my researches.
PS: Sorry for my English.