Let's say I have
- two keys K1 and K2.
- two messages M1 and M2 of the same length.
- Cipher (E,D)
- 3 ciphertexts: C11, C12,C22 where
Cij = E(Ki, Mj)
In situation where attacker knows K2 and all the ciphertexts (which also gives him M2), can I choose a cipher to guarantee that attacker won't be able to compute neither the K1 key, nor first message.
I see three possible options:
- Attacker won't be able to compute anything no matter what cipher was chosen [if cipher itself is "secure"].
- Attacker will be able to compute either K1 or M1 for some ciphers.
- Attacker will be able to compute either K1 or M1 for all ciphers.
Which one is correct?
P.S.: Saying "all ciphers" I mean "all more or less secure ciphers".
P.S. 2: I suspect that it is not the first option because at least for stream cipher attacker can easily compute the pseudorandom sequence for K1 if he knows M2 and C12. Then he will be able to decode C11 using this sequence. Is it true for other ciphers?