SHA1 Hash

SHA-1 (Secure Hash Algorithm 1) was published by the National Institute of Standards and Technology (NIST) in 1995 as part of the SHA family. It produces a 160-bit (20-byte) digest, rendered as 40 hexadecimal characters. For many years, SHA-1 was the workhorse for digital signatures, certificate verification, and version control systems (e.g., Git). Its longer digest compared to MD5 offered stronger collision resistance—until cryptographic advances rendered SHA-1 insecure.

SHA-1 processes input in 512-bit blocks, like MD5, but uses a more complex message schedule and five 32-bit registers instead of four. The algorithm spans 80 rounds, mixing the message and internal state through nonlinear functions and constants derived from the square roots of small prime numbers.

In 2005, cryptanalysts demonstrated theoretical collision weaknesses, and by 2017 Google and CWI Amsterdam produced a practical SHA-1 collision ("SHAttered"). As a result, major browsers, certificate authorities, and software projects deprecated SHA-1, migrating to SHA-256 or stronger.

1. **Padding**: Append one ‘1’ bit, enough ‘0’ bits to reach 448 mod 512, then 64-bit length. 2. **Initialize**: Set five 32-bit registers (H0–H4) to fixed constants. 3. **Message Schedule**: Expand each 16-word block to 80 words using bitwise operations and rotations. 4. **Compression**: For 80 rounds, compute a temporary value based on nonlinear functions (Ch, Parity, Maj), the schedule word, and constants; update registers accordingly. 5. **Finalization**: Add the working registers back to the intermediate hash state to form the final 160-bit digest.

SHA-1 accepts any byte sequence as input. The output is a fixed 160-bit value, typically shown as a 40-character hex string. For "hello", SHA-1 yields “aaf4c61ddcc5e8a2dabede0f3b482cd9aea9434d”.

SHA-1 is slower than MD5 but still processes hundreds of MB/s on optimized hardware. However, due to collision attacks, it’s no longer considered secure. NIST formally disallowed SHA-1 for digital signatures after 2010 and for certificate signing after 2015. All new applications should use SHA-256 or SHA-3 instead.

While SHA-1 should not be used for new security functions, legacy systems and certain hash-based data structures may still rely on SHA-1. Version control systems like Git continue to use SHA-1 for object naming, though some projects are migrating to SHA-256 repos.

To upgrade from SHA-1, select a secure alternative: SHA-256, SHA-3, or BLAKE2. For HMAC, use HMAC-SHA256. When backward compatibility is needed, implement dual-hashing (compute both SHA-1 and SHA-256) and phase out SHA-1 usage over time.

— Use streaming APIs for large inputs. — Validate outputs against known test vectors from FIPS PUB 180-4. — Avoid homegrown implementations; rely on vetted cryptographic libraries.

`hello` → `aaf4c61ddcc5e8a2dabede0f3b482cd9aea9434d`