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SHA1 (Secure Hash Algorithm 1) is a cryptographic hash function that processes input data and generates a fixed-length 160-bit (20-byte) hash value, commonly referred to as a message digest. It was developed by the NSA and published by NIST as part of the Digital Signature Algorithm.

To ensure readability and usability in text-based environments, the 160-bit output is typically represented as a sequence of 40 hexadecimal digits. Each byte of the binary hash is converted into two hexadecimal characters, making the hash easier to store, compare, and transmit - for example, da39a3ee5e6b4b0d3255bfef95601890afd80709.

SHA1 was widely used in applications such as SSL certificates, code signing, and file integrity checks. However, due to advances in computational power and cryptanalysis techniques, SHA1 is no longer considered secure against collision attacks, where two different inputs yield the same hash output. As a result, major organizations and standards bodies have deprecated its use in favor of more secure alternatives like SHA-256 and SHA-3.

Despite its cryptographic obsolescence, SHA1 is still encountered in legacy systems and historical datasets. When security is not a critical factor, it may still serve in roles such as fingerprinting, indexing, or internal data validation where performance and compatibility are prioritized over resistance to attacks.