* Add `basepoint-tables` crate feature
Feature-gates the inclusion of basepoint tables under a
`basepoint-tables` feature, with the goal of reducing code size for e.g.
embedded applications.
* Add `mul_base` method to `EdwardsPoint` and `RistrettoPoint`
Provides fixed-base scalar multiplication which optionally uses
precomputed basepoint tables when the `basepoint-tables` feature is
enabled, providing 4X better performance.
Falls back on variable-base scalar multiplication in the event the
feature is disabled.
Co-authored-by: Michael Rosenberg <michael@mrosenberg.pub>
* 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
Previously `alloc` implicitly activated `zeroize` via `zeroize/alloc`.
This commit switches to weak feature activation as added in Rust 1.60,
only activating `zeroize/alloc` if the `zeroize` dependency is
explicitly activated (which it is by default).
* Make `zeroize` an optional dependency
The `zeroize` crate provides a defense against memory read oracles which
typically arise from memory unsafety.
Pure Rust programs may not benefit from `zeroize`, and in certain cases
the unsafe code used by `zeroize` may be more concerning.
This commit makes `zeroize` into an optional feature so users may elect
to disable it if they so desire.
* Added zeroize feature flag to README
Co-authored-by: Michael Rosenberg <michael@mrosenberg.pub>
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
As suggested in #453 it is sometimes feasible to
select the backend bits via an override.
This change provides `cfg(curve25519_dalek_bits)`
to override the bits used in serial or fiat target backend.
As proposed in #414, this commit changes the backend selection approach,
introspecting `target_pointer_width` to select `u32_backend` vs
`u64_backend` (or `fiat_u32_backend`/`fiat_u64_backend` if the
`fiat_backend` feature is enabled).
This helps eliminate the use of non-additive features, and also the
rather confusing errors that happen if multiple backends are selected
(i.e. thousands of lines of rustc errors).
The selection logic checks if `target_pointer_width = "64"` and uses the
64-bit backend, or falls back to the 32-bit backend otherwise. This
means the crate will always have a valid backend regardless of the
pointer width, although there may be odd edge cases for exotic platforms
which would optimally use the 64-bit backend but have a non-"64" target
pointer width for whatever reason. We can handle those cases as they
come up.
Renames fiat backend directory to fiat_u64 and does the additional plumbing required to make fiat_{u32, u64}_backend equal alternatives.
Adds a few comments.
This implementation:
- is agnostic on the hash used to pick a field element, even though SHA512 is commonly used,
- follows https://tools.ietf.org/id/draft-irtf-cfrg-hash-to-curve-10.html closely
- tests the outputs of the function using libsignal's implementation.
`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.
