Design of a Synchronous Stream Cipher from Hash Functions

We consider a simple and secure way to realize a synchronous stream cipher from iterated hash functions. It is similar to the OFB mode where the underlying block cipher algorithm is replaced with the keyed hash function, adopting the secret suffix method[20]. We analyzed the key, the keystream and the necessary properties to assume from the underlying hash function for the stream cipher to be considered secure. Motivated by our analysis we conjecture that the most efficient way to break the proposed stream cipher is to break the hash function or through exhaustive search for the keyspace K of k bits, that requires O(2^k) operations.