As noted in the Decaf paper, mapping twice and adding the results ensures a
uniform distribution over the group. This changes our random point and
hash-to-point functions to do this, matching the Sage script.
A missing import of the Borrow trait caused the build to break with the
"yolocrypto" feature enabled; this was't caught by CI because the CI machine
that Travis used didn't have AVX2, so the code was never built.
This commit adds the missing import and changes `std` to `core` so that the
AVX2 backend builds with no_std, but this isn't tested and is, actually,
"yolocrypto".
This allows iterators returning either &Scalars or Scalars, so that it's
possible to use map() and friends to adjust scalars as they're being fed into
the multiscalar multiplication.
The `MontgomeryPoint` struct is now a point on the Kummer line of the Montgomery curve.
The `ProjectivePoint` struct is made private, since its only purpose is
internal to the Montgomery ladder.
The Montgomery ladder takes affine input, making it faster, and produces affine output.
The Edwards-Montgomery correspondence is simplified.
These were originally added in 32da4c7d50
to implement Scalar negation in terms of multiply-add.
But we have a full implementation of scalar arithmetic now, so it's not
necessary to keep the constants around.
The public-facing types with arithmetic operations are:
- `Scalar`s
- `ExtendedPoint`s
- `RistrettoPoint`s
For these types we define operators with all combinations of borrowed and
non-borrowed inputs, to avoid forcing API consumers to write extra ampersands.
Since all of the operations involved with these types are expensive relative to
the cost of an unnecessary copy, this isn't a big deal.
The `MontgomeryPoint` struct isn't included in the above because it's only
useful for scalar multiplication.
This commit is based on work by @UnlawfulMonad.
