Move to SHA256

SHA256 is de facto blockchain standard. Standardisation of the hash function is a prerequisite for [full standardisation of BLS12-381 signatures](https://github.com/ethereum/eth2.0-specs/issues/605). Blockchain projects are likely to provide a cheap SHA256 opcods/precompile, and unlikely to provide a Keccak256 equivelent. (Even WASM-enabled blockchains are likely to provide a SHA256 opcode/precompile since WASM does *not* natively support optimised SHA256 CPU instructions.) With Ethereum 2.0 embracing SHA256 the wider industry is more likely to converge towards a unified cross-blockchain communication scheme via Merkle receipts.

There are no security blockers with SHA256 (see comments by Dan Boneh [here](https://github.com/ethereum/eth2.0-specs/issues/612#issuecomment-470452562)).
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Justin 2019-03-15 10:51:08 +00:00 committed by GitHub
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@ -660,7 +660,7 @@ def xor(bytes1: Bytes32, bytes2: Bytes32) -> Bytes32:
### `hash`
The hash function is denoted by `hash`. In Phase 0 the beacon chain is deployed with the same hash function as Ethereum 1.0, i.e. Keccak-256 (also incorrectly known as SHA3).
The `hash` function is SHA256.
Note: We aim to migrate to a S[T/N]ARK-friendly hash function in a future Ethereum 2.0 deployment phase.