ASC-1: An Authenticated Encryption Stream Cipher

The goal of the modes of operation for authenticated encryption is to achieve faster encryption and message authentication by performing both the encryption and the message authentication in a single pass as opposed to the traditional encrypt-then-mac approach, which requires two passes. Unfortunately, the use of a block cipher as a building block limits the performance of the authenticated encryption schemes to at most one message block per block cipher evaluation.

In this paper, we propose the authenticated encryption scheme ASC-1 (Authenticating Stream Cipher One). Similarly to LEX, ASC-1 uses leak extraction from different AES rounds to compute the key material that is XOR-ed with the message to compute the ciphertext. Unlike LEX, the ASC-1 operates in a CFB fashion to compute an authentication tag over the encrypted message. We argue that ASC-1 is secure by reducing its (

IND-CCA

,

INT-CTXT

) security to the problem of distinguishing the case when the round keys are uniformly random from the case when the round keys are generated by a key scheduling algorithm.