On-Line Ciphers and the Hash-CBC constructions

Abstract We initiate a study of on-line ciphers. These are ciphers that can take input plaintexts oflarge and varying lengths and will output the ith block of the ciphertext after having processedonly the first i blocks of the plaintext. Such ciphers permit length-preserving encryption of adata stream with only a single pass through the data. We provide security definitions for this primitive and study its basic properties. We then provide attacks on some possible candidates,including CBC with fixed IV. We then provide two constructions, HCBC1 and HCBC2, basedon a given block cipher E and a family of computationally AXU functions. HCBC1 is provensecure against chosen-plaintext attacks assuming that E is a PRP secure against chosen-plaintextattacks, while HCBC2 is proven secure against chosen-ciphertext attacks assuming that E is aPRP secure against chosen-ciphertext attacks