Question about decryption I have a question about decryption.

Let's say I have a bunch of files that are encrypted. Someone gains access to those files, and finds out that he already has one of them, unencrypted.

My question is, can he use that file to make a comparison with the encrypted version of the file to find the key, therefor be able to decrypt all the files?
One would certainly hope not; if he can, then we would consider the encryption method "broken".

One of the things we expect from any encryption method is the resistance to a "known plaintext attack" (of which your question is an example); that is, even if he is given a number of plaintext/ciphertext pairs, he still is unable to decrypt another ciphertext that is encrypted with the same key.

Actually, we go father than that; we also ask encryption methods to be resistant to "chosen plaintext attacks"; that is, even if the attacker can ask that messages of his choosing be encrypted (and he then gets the resulting ciphertext), he still is unable to decrypt a target ciphertext.