Which can be used to compress two proofs into one. Each inner proof can be either
- an "EVM root" proof (which typically proves one transaction, though it could be 0 or more)
- another aggregation proof
Reverts the degree adjustment part of #436. As @jimpo pointed out, the adjustment complicates security by allowing rational functions of the form `poly(x) / x`.
A tight degree bound shouldn't be necessary. Ultimately we want to check that some witness function `f(x)` exists satisfying (simplified) `c(f(x)) = Z_H(x) q(x)`. We only need `f(x)` to be low-degree because that allows us to use polynomial identity testing. With PIT we don't care about exact degree bounds; a negligible degree change will have a negligible effect on PIT soundness.
The goal here is to end up with a single "root" circuit representing any EVM proof. I.e. it must verify each STARK, but be general enough to work with any combination of STARK sizes (within some range of sizes that we chose to support). This root circuit can then be plugged into our aggregation circuit.
In particular, for each STARK, and for each initial `degree_bits` (within a range that we choose to support), this adds a "shrinking chain" of circuits. Such a chain shrinks a STARK proof from that initial `degree_bits` down to a constant, `THRESHOLD_DEGREE_BITS`.
The root circuit then combines these shrunk-to-constant proofs for each table. It's similar to `RecursiveAllProof::verify_circuit`; I adapted the code from there and I think we can remove it after. The main difference is that now instead of having one verification key per STARK, we have several possible VKs, one per initial `degree_bits`. We bake the list of possible VKs into the root circuit, and have the prover indicate the index of the VK they're actually using.
This also partially removes the default feature of CTLs. So far we've used filters instead of defaults. Until now it was easy to keep supporting defaults just in case, but here maintaining support would require some more work. E.g. we couldn't use `exp_u64` any more, since the size delta is now dynamic, it can't be hardcoded. If there are no concerns, I'll fully remove the feature after.
Dmitry Vagner