ff_derive still implements Ord and PartialOrd for the fields it
implements, because pairing::bls12_381 internally assumes that those are
implemented. Once we delete that implementation, we will remove the Ord
and PartialOrd implementations from ff_derive.
This enables generic code to reliably operate on the bits of an encoded
field element, by converting them to and from a known (little)
endianness.
The BitAnd and Shr bounds on PrimeField are now removed, as users can
perform these operations themselves as needed.
The sqrt() function is now part of the Field trait. ff_derive returns an
error on fields for which it does not support generating a square root
function.
Note that Fq6 and Fq12 in pairing::bls12_381 leave the function
unimplemented. They will be dropped once the migration to the bls12_381
crate is complete. The equivalent structs in that crate are not exposed.
The trait is implemented by default for u8 and u64, allowing pow_vartime
to be used with both the byte encoding and limb representation of field
elements.
WARNING: THIS IS NOT ACTUALLY CONSTANT TIME YET!
The jubjub and bls12_381 crates will replace our constant-time usages,
but we NEED to fix ff_derive because other users will expect it to
implement the Field trait correctly.
The ff_derive, pairing, zcash_primitives::jubjub, and bellman dummy_engine
changes are minimally implemented on top of the existing *_assign()
functions.
