tthsum generates or checks TTH checksums (root of the THEX hash tree). The Merkle Hash Tree, invented by Ralph Merkle, is a hash construct that exhibits desirable properties for verifying the integrity of files and file subranges in an incremental or out-of-order fashion. tthsum uses the Tiger hash algorithm for both the internal and the leaf nodes.
The specification of the THEX algorithm is at: http://www.open-content.net/specs/draft-jchapweske-thex-02.html
The specification of the Tiger hash algorithm is at: http://www.cs.technion.ac.il/~biham/Reports/Tiger/