New Paradigms for Constructing Symmetric Encryption Schemes Secure against Chosen-Ciphertext Attack

Abstract. The paradigms currently used to realize symmetric encryption schemes secure against adaptive chosen ciphertext attack (CCA) try to make it infeasible for an attacker to forge “valid” ciphertexts. This is achieved by either encoding the plaintext with some redundancy before encrypting or by appending a MAC to the ciphertext. We suggest schemes which are provably secure against CCA, and yet every string is a “valid” ciphertext. Consequently, our schemes have a smaller ciphertext expansion than any other scheme known to be secure against CCA. Our most efficient scheme is based on a novel use of “variable-length” pseudorandom functions and can be efficiently implemented using block ciphers. We relate the difficulty of breaking our schemes to that of breaking the underlying primitives in a precise and quantitative way.

• Type: Conference
• Year: 2000
• Where: CRYPTO
• Authors: Anand Desai
• Keywords: Chosen Ciphertext Attack, CRYPTO 2000, Cryptology, Provably Secure, Symmetric Encryption Schemes