From ServeTrue Technologies:
MD5/SHA1 Hash Extractor can be used to form a "practically unique" key of any data. This tiny MD5/SHA1 extractor has a Windows GUI and can extract hashes out of files of terabytes size. When using a one-way hash function, one can compare a calculated message digest against the message digest that is decrypted with a public key to verify that the message hasn't been tampered with. This comparison is called a 'hashcheck'."