Deep packet inspection typically uses MD5 whitelists/blacklists or regular expressions to identify viruses, malware and certain internal files in network traffic. Fuzzy hashing, also referred to as context-triggered piecewise hashing, can be used to compare two files and determine their level of similarity. This chapter presents the stream fuzzy hash algorithm that can hash files on the fly regardless of whether the input is unordered, incomplete or has an initially-undetermined length. The algorithm, which can generate a signature of appropriate length using a one-way process, reduces the computational complexity from