Multi-Circuit Digest

The ability of digesting dynamically sized payloads using SHA algorithm is provided by the excellent Mina Attestations library. We are using a fork to expose some of the needed functionality, using the same underlying code.

SHA digesting a long payload like LDS or a x509 certificate do not fit into a single circuit. Therefore, we use a scheme of Update & Validate to calculate the digest value.

Poseidon-digest is way cheaper than SHA-digest in circuits.

Note: we are carrying a carry field so that we can plug this in our pipeline, carrying the previous leaf's output.

Digest State Data Type

Update

For each update step, we are digesting a chunk of values, which is a couple of SHA blocks, at the same time with both SHA and Poseidon algorithms. Between each update, the digest state is always fed to the circuit.

Only the current state and the chunk is fed to a update circuit, not the whole payload.

Private Inputs

where iteration holds the chunk to be digested

Public Output

Validate

We give the whole payload as well as the latest digest state which contains both digest algorithms state inside, to the circuit.

The circuit Poseidon-digests the whole payload again to check if the previous steps processed the correct payload chunks in correct order. If all is well, it returns the SHA-digest output.

This step happens inside the "caller" circuit since on its own does not mean much, only meaningful alongside other validations from the application logic.

Required Inputs