1. The NIST Hash Competition
Following theoretical concerns regarding SHA-1 and MD5 in the mid-2000s, NIST launched a 5-year public competition to select a next-generation standard. In 2012, Keccak was selected as the winner and standardized in 2015 as FIPS 202 (SHA-3).
2. The Sponge Construction
Unlike SHA-1 and SHA-2, which use the Merkle-Damgård construction, SHA-3 uses a sponge construction over a 1600-bit state array. It operates in two phases:
• Absorbing Phase: Input message blocks are XORed into a subset of the state, followed by iterative state permutation rounds (Keccak-f[1600]).
• Squeezing Phase: Output bits are read from the state until the desired digest length (224, 256, 384, or 512 bits) is reached.
3. Immunity to Length-Extension Attacks
Because a portion of the sponge state (the capacity) is never exposed in the output digest, SHA-3 is natively immune to length-extension attacks without requiring HMAC-style outer hashing.