About

A sha1 (Secure Hash Algorithm) is an hash function that generates 40-character string (message_digest) composed of hexadecimal characters (0–9 and a–f).

In a file system, it's calculated based on the contents of a file or directory structure (ie several files)

(Secure Hash Algorithm-1) produces a 160-bit digest.

An SHA-1 hash is:

• a 40 hexadecimal character string.
• ie 160 bits. (ie 1 hexa goes from 0 to 4 bit to code 0 to 16 character)

Security

sha1 is not a secure hash algorithm. Ref

Library

See Hash - Secure Hash Algorithm (SHA)