This section describes what is SHA1 (Secure Hash Algorithm 1) - a message digest algorithm which takes as input a message of arbitrary length and produces as output a 160-bit 'fingerprint'.
SHA1 (Secure Hash Algorithm 1) is message-digest algorithm, which takes
an input message of any length < 2^64 bits and produces a 160-bit output
as the message digest.
Based on the SHA1 RFC document,
the SHA-1 is called secure because it is computationally infeasible
to find a message which corresponds to a given message digest, or to
find two different messages which produce the same message digest.
Any change to a message in transit will, with very high probability,
result in a different message digest, and the signature will fail to
The original specification of the algorithm was published in 1993 as the Secure Hash Standard,
FIPS PUB 180, by US government standards agency NIST (National Institute of Standards and Technology).
This version is now often referred to as "SHA0".
SHA-0 was withdrawn by the NSA shortly after publication and was superseded by the revised version,
published in 1995 in FIPS PUB 180-1 and commonly referred to as "SHA1".