Uses o1js’s newly released bitwise operations gadgets to optimize the SHA256 circuit implementation by operating on the 32-bit words (32 field elements) as a single field element utilizing native binary gadgets.

The repository serves as a proof of concept for the development of the SHA256 hashing algorithm using the o1js SDK during ZK-HACK Istanbul. The o1js SDK provides robust native binary functions, facilitating the construction of an efficient SHA2 circuit for zero-knowledge applications on the Mina blockchain.

Optimizations:
In general, SHA2 circuits face inefficiencies as binaries (0 or 1) consume the space of a complete field element. Performing multiple binary operations on binaries represented as field elements makes the SHA2 algorithm a constraint-heavy circuit. To address this, the o1js SDK aims to optimize the circuit implementation by treating 32-bit words (32 field elements) as a single field element, especially given that SHA256 construction (Merkle–Damgård) operates on 32-bit words.

How SHA-256 Works:
SHA-2 algorithms can be delineated into two stages: preprocessing and hash computation. During preprocessing, the process involves padding a message, parsing the padded message into m-bit blocks, and establishing initialization values for use in the subsequent hash computation. The hash computation generates a message schedule from the padded message, utilizing this schedule along with functions, constants, and word operations to iteratively produce a series of hash values. The final hash value, derived through the hash computation, determines the message digest.

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Further information here >

Github https://github.com/Shigoto-dev19/sha256-o1js