Ciphertext indistinguishability is property of randomised encryption schemes where it is computationally infeasible to tell if two ciphertexts are encryptions of the same plaintext.

Ciphertext indistinguishability is a property of randomised encryption schemes where each possible plaintext has a large number of possible encryptions. For indistinguishability, it should be computationally infeasible to tell if two encryptions were created from the same plaintext. More specifically, given a plaintext and a choice of two ciphertexts (one of which is an encryption of the plaintext and the other which is not) it should be computationally infeasible to guess the correct ciphertext with probability significantly bigger than 1/2.