 SHA-2

SHA-2 is a family of cryptographic hash functions including SHA-224, SHA-256, SHA-384, SHA-512, SHA-512/224 and SHA-512/256. NIST specifies these algorithms in FIPS 180-4. Their digest lengths are respectively 224, 256, 384, 512, 224 and 256 bits; the algorithm name therefore matters as well as the output size.^[ref]

The SHA-224/SHA-256 branch uses 32-bit words and 512-bit message blocks. The SHA-384/SHA-512 branch and its specified truncation variants use 64-bit words and 1024-bit message blocks. Different initial values are part of the definitions. In particular, SHA-512/256 is not simply the first 256 bits of an ordinary SHA-512 digest.^[ref]

For a computed example, hashing the three ASCII bytes 61 62 63 with SHA-512/256 produces

53048e2681941ef99b2e29b76b4c7dabe4c2d0c634fc6d46e0e2f13107e7af23.

Taking the first 256 bits of the SHA-512 digest of those same bytes instead gives

ddaf35a193617abacc417349ae20413112e6fa4e89a97ea20a9eeee64b55d39a.

Both outputs occupy 32 bytes and require 64 hexadecimal digits, yet they differ. An interface identifying its hash only as a 256-bit digest leaves the selected algorithm ambiguous.

A digest is deterministic and does not conceal low-entropy inputs by itself. Nor does an unkeyed digest establish who supplied a message: a sender-authentication mechanism such as HMAC uses a cryptographic key and an appropriate security definition. Preimage resistance, second-preimage resistance and collision resistance are distinct properties; their meanings should not be replaced by the mere observation that two displayed examples have different outputs.