Each backend can now be selected by an individual feature:
- `u32_backend` for `backend::u32`;
- `u64_backend` for `backend::u64`;
- `avx2_backend` for `backend::avx2`;
The `u64_backend` is selected by default, since most people use X64 and we have
no way to select based on target (see discussion in #126). However, these
changes mean that it is possible to select the backend explicitly, and if we
had the ability to select target-default features, we could do so easily.
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.
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.
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.
This commit defines a Scalar to hold an integer representing an element of
Z/lZ. Applications like X/Ed25519 that care about the bit-patterns of the
scalars they use can set a specific bit-pattern using the `from_bits`
constructor. Applications that want to treat scalars as integers mod l can use
the `from_bytes_mod_order` constructor. Either way, the constructor ensures
that the integer representing each Scalar is bounded by 2^255 so that the high
bit is set. This means that any Scalar object is always safe to use for scalar
multiplication, while maintaining compatibility with both the Ristretto
use-case and the X/Ed25519 usecase.
