Zchunk is a format for compressing files in a way that is well-suited for synchronizing two copies of a file over a network; as well as a program that performs this syncronization. Arguably this problem was already solved by the older Rsync system, but due to the widespread adoption of HTTP for non-Web communications purposes, it cannot always be used. Zchunk first splits the file into a number of "chunks"; then compresses these chunks individually, so that, after a comparison has been made between the copies of the receiver and sender, only the changed chunks need to be downloaded. Unlike some similar formats, Zchunk concatenates all the chunks together, along with a header, into a single file.
Version 1 Zchunk files (as of 2020 the only version that exists) begin with
00 5A 43 4B 31.