Abstract. Bit-splitting breaks the problem of monitoring traffic payloads to detect the occurrence of suspicious patterns into several parallel components, each of which searches for a particular bit pattern. We analyze bit-splitting as applied to Aho-Corasick style string matching. The problem can be viewed as the recovery of a special class of regular languages over product alphabets from a collection of homomorphic images. We use this characterization to prove correctness and to give space bounds. In particular we show that the NFA to DFA conversion of the Aho-Corasick type machine used for bit-splitting incurs only linear overhead.