 SHA-512/256

SHA-512/256 is a SHA-2 cryptographic hash function with a 256-bit digest. It uses the 1024-bit blocks and 64-bit word operations of SHA-512, a variant-specific hash initialization vector, and the first 256 bits of the final state. It is one member of SHA-512/t.^[ref]

Its initialization illustrates cryptographic domain separation. Let $v$ be the eight-word ordinary SHA-512 initial state. First form $\tilde{v}$ by applying bitwise exclusive or to each word with the hexadecimal word a5a5a5a5a5a5a5a5. Run the SHA-512 computation on the ASCII label SHA-512/256, using $\tilde{v}$ as its initial state. The full 512-bit result is the variant's fixed initial state $v^{(256)}$. This label computation is part of defining the function, not an extra message prefix chosen by its user.^[ref]

If ${SHA512}^{v^{(256)}}(m)$ denotes the full final state obtained by hashing $m$ with that initialization, thenThe output comprises four 64-bit words: 32 bytes or 64 hexadecimal digits. The initial state is public and fixed; it is not a cryptographic salt or a secret key.

For a computed example, the three ASCII bytes 61 62 63 give

53048e2681941ef99b2e29b76b4c7dabe4c2d0c634fc6d46e0e2f13107e7af23.

Ordinary SHA-512 of those bytes begins ddaf35a193617aba, while SHA-256 begins ba7816bf8f01cfea. Neither output implements SHA-512/256. In particular, the slash and output width do not mean “hash with ordinary SHA-512 and keep 256 bits”.

The padding also follows the SHA-512 family: a 128-bit length field and 1024-bit blocks. An input of 56 bytes therefore fits into one padded block here, while SHA-256 needs two of its smaller blocks. The identical 32-byte digest sizes say nothing about those different computations.