说明：  Freeware implementation of the Secure Hashing Standard, defined in FIPS PUB 180-1, defines the Secure Hashing Algorithm (SHA-1). SHA-1 can be used to produce a message digest for a given message. Essentially, this is a 160-bit number that represents the message. In theory, no two messages would ever share the same message digest. What this means is that the message digest can serve as a fingerprint for a file or other source of data. SHA-1 is used by Digital Signature Standard (DSS), which is a standard used for digitally signing documents or other data. This is the C++ and the C the implementations of SHA-1. This is the same source base from which the code in RFC 3174 was based.