A system and method for relating files in a distributed data storage environment allows for positive identification of membership of a file within a group, even in a loosely coupled environment where files are not available for comparison in real time. In disclosed embodiments, base files of a client are stored on a server and are accompanied by tokens uniquely identifying the base files. The tokens are generated on the client and may be derived from the contents of the base file using a digital signature. Each file transmitted to the server is accompanied with a token. Incremental backups may be used, and may employ file differencing. Accordingly, sub-files related to the base files may be transmitted to the server for backup. The sub-files are related to their respective base files using the tokens and are cross-linked to the base files so that any sub-files can be retrieved together with the base file from which the sub-file was derived.