Unforgeable Encryption and Chosen Ciphertext Secure Modes of Operation

We find certain neglected issues in the study of private-key encryption schemes. For one, private-key encryption is generally held to the same standard of security as public-key encryption (i.e., indistinguishability) even though usage of the two is very different. Secondly, though the importance of secure encryption of single blocks is well known, the security of modes of encryption (used to encrypt multiple blocks) is often ignored. With this in mind, we present definitions of a new notion of security for private-key encryption called encryption unforgeability which captures an adversary's inability to generate valid ciphertexts. We show applications of this definition to authentication protocols and adaptive chosen ciphertext security. Additionally, we present and analyze a new mode of encryption, RPC (for Related Plaintext Chaining), which is unforgeable in the strongest sense of the above definition. This gives the first mode provably secure against chosen ciphertext attacks. Although RPC is slightly less efficient than, say, CBC mode (requiring about 33% more block cipher applications and having ciphertext expansion of the same amount when using a block cipher with 128-bit blocksize), it has highly parallelizable encryption and decryption operations.