* Make basepoint table constants static references
This ensures they have a fixed address and aren't duplicated across
compilation units.
Since they were already always borrowed, this changes the static values
to be `&'static` addresses to ensure they're always borrowed rather than
potentially copied.
* rustfmt
For the field element types `FieldElement` and `Scalar`, use inherent
constants instead of (non-const) functions to return these constant
values.
It's likely the original functions predate support for inherent
constants, but now that they're available, they're a better fit for
these sort of constant values.
Crate features are intended to be additive, whereas only 1-of-N possible
backends can be selected.
Features can also be activated by transitive dependencies, which leads
to a problem of different dependences selecting conflicting backends.
Using `--cfg` instead moves all backend selection control to the
toplevel executable.
This commit switches to the following RUSTFLAGS to enable backends:
- `--cfg curve25519_dalek_backend="fiat"`: uses `fiat-crypto`
- `--cfg curve25519_dalek_backend="simd"`: uses nightly-only SIMD
`zeroize` is WASM-friendly as it has no dependencies on C compilers.
Instead uses Rust's own volatile write semantics and compiler fences to
ensure zeroization is not elided by the compiler.
This was more useful at the time when we were determining, e.g., optimal lookup
table sizes and could regenerate them more easily, but it came at a massive
complexity cost. It also meant that we were unable to implement backend
autoselection. This commit removes the `build.rs` entirely. In the future, a
different `build.rs` could be added that auto-selects a backend, but it seems
like the current default-u64 setup has been working fine.
The NAF computation can generate a 1 in the last digit (only) when s = 2^255-1,
so someone who manually constructed the value s = 2^255-1 and fed it into a NAF-using
computation could generate an incorrect result. Some version of this bug has
been present from the beginning of the library, but it has no security content,
because the NAF computations are not applied to secret data, and the error
occurs only on one value which is not constructed by any client caller.
When using Scalar::from_bits to manually create unreduced Scalars (e.g.,
X/Ed25519 keys with specified bit patterns), it's possible to construct Scalar
values that range up to 2^255-1. These shouldn't ever end up in a vartime
multiscalar mul call anyways, because it doesn't handle secret data, but it is
technically allowed by the type system and should be handled. When w=8, these
can generate terminal carries that can't be folded into the last digit, but
this can be handled by folding them into an extra digit instead.
This fixes a bug in the Pippenger implementation reported by Fernando Krell and
diagnosed by Oleg Andreev. The problem is that at the largest problem sizes
(using w=8), the signed digits fill the value range of an i8, and so doing
computation on them to calculate the bucket index can hit an overflow.
This was not caught in CI because the test suite didn't check all problem
sizes; tests for these sizes which expose this bug were added in the previous
commit.
Building the docs currently doesn't work, because rustdoc enables parts of the
code (to document them) which then don't check (because there are missing
exports or dependencies). This **should** fix the issue, but there's no way to
test without publishing a new version.
This doesn't (yet) give any speedup over the non-precomputed multiscalar
multiplication, and it's not clear that it's a good idea to commit to
supporting it in the future. Removing it means that it's not committed-to as
part of the public API, but the source is still there in the tree if we want to
revisit it later.
