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Hashing

SSZ utilizes the SHA-256 hash function.

The standard specification for SHA-256 can be found in FIPS 180-4.

Hashing primitive for binary trees.

H(a: bytes32, b: bytes32) -> SHA_256(a ++ b)

Where ++ is concatenation, i.e. tightly packing a and b into 64 bytes. And SHA_256 is run on the standard unmodified pre-state, and returns the digest after writing and processing the above 64 bytes.

Zero-hashes

A common occurrence in merkleization in SSZ is H(X, H(0, 0)), H(X, H(H(0, 0), H(0, 0))), H(X, H(H(H(...

The right-hand side here would be costly to merkleize leaf by leaf, but is efficiently precomputed, and referred to as a "zero hash" of some order N, starting from 0 being a bare zeroed bytes32:

Z[0]: 000000....   # a zeroed bytes32
Z[1]: H(Z[0], Z[0])
Z[2]: H(Z[1], Z[1])
Z[2]: H(Z[1], Z[1])
Z[3]: H(Z[2], Z[2])
...

Alternatives

Other hash-functions have been considered for specific use cases, but are actively avoided for higher compatibility with other platforms, and more consistency.

  • 256 bits SHA-3 and Keccak were considered but dropped in favor of SHA-256 support and compatibility.
  • "fast-SHA256" as described in BIP-98, optimized for two 32 byte inputs. But still a draft, and even more compatibility concerns.
  • A S[T/N]ARK-friendly hash function, the aim is to migrate in a future Eth2 deployment phase.