* bellman: add VerificationError
This adds a distinct VerificationError type to the crate and changes
`verify_proof` to return `Result<(), VerificationError>` rather than
`Result<bool, SynthesisError>`. This is significantly safer, because it avoids
the need to mix pattern-matching logic with boolean logic (the cause of
RUSTSEC-2019-0004).
* Rename VerificationError variants per review comments.
* Add missing Clone impl to VerificationError.
Instead of imposing the requirement that bellman users explicitly
specify an engine for every proving system, we allow the Rust type
system to figure it out for us. An engine is specifically useful in
places where we require defined relationships between several types;
ff::ScalarEngine only has one type, and thus any usage of it can be
trivially replaced by an explicit Scalar type. This is also more
readable :)
The ff_derive, pairing, zcash_primitives::jubjub, and bellman dummy_engine
changes are minimally implemented on top of the existing *_assign()
functions.
Henry de Valence
Jack Grigg
Eirik Ogilvie-Wigley
Sean Bowe