Change Scalar::non_adjacent_form() to take a width parameter.
This rewrite also makes it faster, although it's probably a ways off
from optimal. I don't know how much it matters.
TODO: write up description of why this computes the same thing.
Thanks to @oleganza for pointing out an error reading bits across words
in an earlier version of this code.
This change required some work, because the to-be-stabilized SIMD functions
don't allow non-constant `imm8`s. Previously, the `stdsimd` functions had a
constifying macro that ensured that the immediates were known. The dalek code
used this to build helper functions which would be inlined into different
places where the immediates were known. Unfortunately, since constexprs aren't
fully supported in Rust yet, this is done by a hidden compiler attribute, and
there's no way to propagate "constness".
To deal with this, some of the functions are specialized (e.g.,
`square_and_negate_D` instead of taking a mask), and others use an enum.
The serial (`u32`/`u64`) implementations use a multiple curve models, passing
between extended and projective coordinates when performing addition and
doubling (respectively). But the AVX2 backend doesn't, so in order to write a
single scalar mult implementation, we have to either abstract over the curve
models or have two implementations.
A generic solution is possible but extremely unreadable: the scalar mul
implementation would be parameterized over the point types used by the serial
implementations, with many where clauses describing how the types relate. The
AVX2 types could then be substituted in the appropriate places.
Instead we just duplicate the code into the `avx2` backend.
This should contain generic implementations of scalar multiplication algorithms
that can be used with multiple backends. The goal is to move the existing
scalar multiplication code into this submodule, then call it from the
user-facing API. This can also contain code for things we can't do now, like
multiscalar multiplication with precomputation.
Since Criterion can only benchmark public API, these changes just drop
all internal benchmarks (e.g., benchmarks for field operations). But
those are usually microbenchmarks whose meaning is kind of questionable
anyways, so I don't think this is a big loss.
The `bench` feature disappears, since Criterion works on stable Rust.
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".
