Ciphertext-Only Attack
The ciphertext-only attack is the weakest attack model, where the adversary obtains nothing but ciphertexts; a cipher should be indistinguishable from a random permutation under it.
Purpose
It defines the most basic security requirement: nothing about the plaintext or key can be inferred from ciphertexts alone.
Mechanism
The adversary holds several ciphertexts C_i = E_K(P_i) without knowing P_i or K and tries to infer P_i or K. Modern ciphers under IND-CPA implicitly resist ciphertext-only attacks.
Use cases
- Cipher security model definitions
- Classical cryptanalysis
- Confidentiality baseline