SHA-1 (Secure Hash Algorithm) is a cryptographic hash function with a
160 bit output.
SHA1() computes the SHA-1 message digest of the n
bytes at d and places it in md (which must have space for
SHA_DIGEST_LENGTH == 20 bytes of output). If md is NULL, the digest
is placed in a static array.
The following functions may be used if the message is not completely
stored in memory:
SHA1_Init() initializes a SHA_CTX structure.
SHA1_Update() can be called repeatedly with chunks of the message to
be hashed (len bytes at data).
SHA1_Final() places the message digest in md, which must have space
for SHA_DIGEST_LENGTH == 20 bytes of output, and erases the SHA_CTX.
Applications should use the higher level functions
etc. instead of calling the hash functions directly.
The predecessor of SHA-1, SHA, is also implemented, but it should be
used only when backward compatibility is required.