The Tools/Checksum menu contains several popular hash functions. Hash function is a one-way function converting arbitrary length data to a fixed length value (a typical size is 128-256 bits). Depending on the context, this resulting value is called checksum, or hash, or sometimes message digest. An important property of a good hash function is that a small change of input data causes the function to produce a completely different hash value. This property makes hash functions an excellent tool for ensuring that the data have not been changed - any change in the data block can be easily detected by comparing the expected and the actual checksums.
In order to compute a checksum select a data block (if there is no selection the whole file will be processed) and select the desired hash function. The function process the selected block (or the whole file) and places the hash value into the clipboard. Take notice that the clipboard will contain both the binary value and the textual representation, so it is important to which pane you are pasting.