ECB Oracle

This takes much longer time than I would like to admit. It is known to me that ECB leads to poor diffusion of the plaintext - the classic example from the Linux penguin used in almost every cryptography book ever. The attack, however, is not exactly clear to me at the beginning. With fuzzy memory of how AES in ECB mode, I was thinking of a scheme that xor a plaintext block with a key block to generate the corresponding ciphertext block....