Indifferentiable Authenticated Encryption

We study Authenticated Encryption with Associated Data (AEAD) from the viewpoint of composition in arbitrary (single-stage) environments. We use the indifferentiability framework to formalize the intuition that a “good” AEAD scheme should have random ciphertexts subject to decryptability. Within this framework, we can then apply the indifferentiability composition theorem to show that such schemes offer extra safeguards wherever the relevant security properties are not known, or cannot be predicted in advance, as in general-purpose crypto libraries and standards. We show, on the negative side, that generic composition (in many of its configurations) and well-known classical and recent schemes fail to achieve indifferentiability. On the positive side, we give a provably indifferentiable Feistel-based construction, which reduces the round complexity from at least 6, needed for blockciphers, to only 3 for encryption. This result is not too far off the theoretical optimum as we give a lower bound that rules out the indifferentiability of any construction with less than 2 rounds

Programmable encryption and key-dependent messages

This is a preliminary version. The technical content is complete, but discussion and examples will be extended. Feedback is welcome. We present the notion of PROG-KDM security for public-key encryption schemes. This security notion captures both KDM security and revealing of secret keys (key corruptions) in a single definition. This is achieved by requiring the existence of a simulator that can program ciphertexts when a secret key is revealed, i.e., the simulator can delay the decision what plaintext is contained in what ciphertext to the moment where the ciphertext is opened. The definition is formulated in the random oracle model. We show that PROG-KDM security can be achieved by showing that a natural and practical construction in the ideal cipher model is PROG-KDM secure (hybri