Encryption Simulatability Reconsidered

The notion of encryption simulatability was proposed by Dent to help proving plaintext awareness, and it was claimed that a hybrid encryption scheme composed of a simulatable KEM and a simulatable DEM is simulatable. Here we prove the simulatability of IND-CCA2 secure probabilistic symmetric encryption scheme with every string in its ciphertext space being a valid ciphertext. Moreover, for such a DEM and a simulatable KEM with sparse valid ciphertext space, we show that the resulting hybrid encryption scheme is not simulatable. However, if both the KEM and DEM have sparse valid ciphertext space, or every string in the ciphertext space of the hybrid encryption scheme is a valid ciphertext, then the hybrid encryption scheme is simulatable. Thus the proof for the plaintext awareness of the Cramer-Shoup hybrid encryption scheme provided by Dent is still valid.

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