A network device includes a substring indicator memory and a signature search module for detecting a signature pattern within a stream of network traffic. The signature search module is configured to receive the stream of network traffic, identify a first subject substring from the stream of network traffic, generate a first substring indicator for the first subject substring using a first bitmask on the first subject substring, the first bitmask identifying a plurality of masked bits of the first subject substring, identify a first counter using the first substring indicator as an address into the substring indicator memory, and generate an indication that the signature pattern may be present in the stream of network traffic based on a value of the first counter.