SHA-1 status
Collision found in 2017 (SHAttered). Deprecated. Still used in legacy systems (Git commit IDs — deliberately, no security).
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SHA-256
Merkle-Damgård construction. 256-bit output. 64 rounds. Standard for TLS certs, JWT, most modern uses.
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SHA-3 (Keccak)
Sponge construction — different from Merkle-Damgård. Absorbs input into state, squeezes output. Resistant to length-extension attacks.