curve25519-dalek

A pure-Rust implementation of group operations on Ristretto and Curve25519.

curve25519-dalek is a library providing group operations on the Edwards and Montgomery forms of Curve25519, and on the prime-order Ristretto group.

curve25519-dalek is not intended to provide implementations of any particular crypto protocol. Rather, implementations of those protocols (such as x25519-dalek and ed25519-dalek) should use curve25519-dalek as a library.

curve25519-dalek is intended to provide a clean and safe mid-level API for use implementing a wide range of ECC-based crypto protocols, such as key agreement, signatures, anonymous credentials, rangeproofs, and zero-knowledge proof systems.

In particular, curve25519-dalek implements Ristretto, which constructs a prime-order group from a non-prime-order Edwards curve. This provides the speed and safety benefits of Edwards curve arithmetic, without the pitfalls of cofactor-related abstraction mismatches.

Use

Stable

To import curve25519-dalek, add the following to the dependencies section of your project's Cargo.toml:

curve25519-dalek = "4"

Feature Flags

Feature	Default?	Description
alloc	✓	Enables Edwards and Ristretto multiscalar multiplication, batch scalar inversion, and batch Ristretto double-and-compress. Also enables zeroize.
zeroize	✓	Enables Zeroize for all scalar and curve point types.
precomputed-tables	✓	Includes precomputed basepoint multiplication tables. This speeds up EdwardsPoint::mul_base and RistrettoPoint::mul_base by ~4x, at the cost of ~30KB added to the code size.
rand_core		Enables Scalar::random and RistrettoPoint::random. This is an optional dependency whose version is not subject to SemVer. See below for more details.
digest		Enables RistrettoPoint::{from_hash, hash_from_bytes} and Scalar::{from_hash, hash_from_bytes}. This is an optional dependency whose version is not subject to SemVer. See below for more details.
serde		Enables serde serialization/deserialization for all the point and scalar types.
legacy_compatibility		Enables Scalar::from_bits, which allows the user to build unreduced scalars whose arithmetic is broken. Do not use this unless you know what you're doing.

To disable the default features when using curve25519-dalek as a dependency, add default-features = false to the dependency in your Cargo.toml. To disable it when running cargo, add the --no-default-features CLI flag.

Major Version API Changes

Breaking changes for each major version release can be found in CHANGELOG.md, under the "Breaking changes" subheader. The latest breaking changes in high level are below:

Breaking changes in 4.0.0

• Update the MSRV from 1.41 to 1.60
• Provide SemVer policy
• Make digest and rand_core optional features
• Remove std and nightly features
• Replace backend selection - See CHANGELOG.md and backends
• Replace methods Scalar::{zero, one} with constants Scalar::{ZERO, ONE}
• Scalar::from_canonical_bytes now returns CtOption
• Scalar::is_canonical now returns Choice
• Remove Scalar::from_bytes_clamped and Scalar::reduce
• Deprecate and feature-gate Scalar::from_bits behind legacy_compatibility
• Deprecate EdwardsPoint::hash_from_bytes and rename it EdwardsPoint::nonspec_map_to_curve
• Require including a new trait, use curve25519_dalek::traits::BasepointTable whenever using EdwardsBasepointTable or RistrettoBasepointTable

This release also does a lot of dependency updates and relaxations to unblock upstream build issues.

Backends

Curve arithmetic is implemented and used by one of the following backends:

Backend	Selection	Implementation	Bits / Word sizes
serial	Automatic	An optimized, non-parllel implementation	32 and 64
fiat	Manual	Formally verified field arithmetic from fiat-crypto	32 and 64
simd	Automatic	Intel AVX2 / AVX512 IFMA accelerated backend	64 only

At runtime, curve25519-dalek selects an arithmetic backend from the set of backends it was compiled to support. For Intel x86-64 targets, unless otherwise specified, it will build itself with simd support, and default to serial at runtime if the appropriate CPU features aren't detected. See SIMD backend for more details.

In the future, simd backend may be extended to cover more instruction sets. This change will be non-breaking as this is considered as implementation detail.

Manual Backend Override

You can force the crate to compile with specific backend support, e.g., serial for x86-64 targets to save code size, or fiat to force the runtime to use verified code. To do this, set the environment variable:

RUSTFLAGS='--cfg curve25519_dalek_backend="BACKEND"'

Equivalently, you can write to ~/.cargo/config:

[build]
rustflags = ['--cfg=curve25519_dalek_backend="BACKEND"']

More info here.

Note for contributors: The target backends are not entirely independent of each other. The SIMD backend directly depends on parts of the serial backend to function.

Bits / Word size

curve25519-dalek will automatically choose the word size for the fiat and serial backends, based on the build target. For example, building for a 64-bit machine, the default 64 bit word size is automatically chosen when either the serial or fiat backend is selected.

In some targets it might be required to override the word size for better performance. Backend word size can be overridden for serial and fiat by setting the environment variable:

RUSTFLAGS='--cfg curve25519_dalek_bits="SIZE"'

SIZE is 32 or 64. As in the above section, this can also be placed in ~/.cargo/config.

Note: The SIMD backend requires a word size of 64 bits. Attempting to set bits=32 and backend=simd will yield a compile error.

Cross-compilation

Because backend selection is done by target, cross-compiling will select the correct word size automatically. For example, if a x86-64 Linux machine runs the following commands, curve25519-dalek will be compiled with the 32-bit serial backend.

$ sudo apt install gcc-multilib # (or whatever package manager you use)
$ rustup target add i686-unknown-linux-gnu
$ cargo build --target i686-unknown-linux-gnu

SIMD backend

The specific SIMD backend (AVX512 / AVX2 / serial default) is selected automatically at runtime, depending on the currently available CPU features, and whether Rust nightly is being used for compilation. The precise conditions are specified below.

For a given CPU feature, you can also specify an appropriate -C target_feature to build a binary which assumes the required SIMD instructions are always available. Don't do this if you don't have a good reason.

Backend	RUSTFLAGS	Requires nightly?
avx2	-C target_feature=+avx2	no
avx512	-C target_feature=+avx512ifma,+avx512vl	yes

If compiled on a non-nightly compiler, curve25519-dalek will not include AVX512 code, and therefore will never select it at runtime.

Documentation

The semver-stable, public-facing curve25519-dalek API is documented here.

Building Docs Locally

The curve25519-dalek documentation requires a custom HTML header to include KaTeX for math support. Unfortunately cargo doc does not currently support this, but docs can be built using

make doc

for regular docs, and

make doc-internal

for docs that include private items.

Maintenance Policies

All on-by-default features of this library are covered by semantic versioning (SemVer). SemVer exemptions are outlined below for MSRV and public API.

Minimum Supported Rust Version

Releases	MSRV
4.x	1.60.0
3.x	1.41.0

From 4.x and on, MSRV changes will be accompanied by a minor version bump.

Public API SemVer Exemptions

Breaking changes to SemVer exempted components affecting the public API will be accompanied by some version bump. Below are the specific policies:

Releases	Public API Component(s)	Policy
4.x	Dependencies digest and rand_core	Minor SemVer bump

Safety

The curve25519-dalek types are designed to make illegal states unrepresentable. For example, any instance of an EdwardsPoint is guaranteed to hold a point on the Edwards curve, and any instance of a RistrettoPoint is guaranteed to hold a valid point in the Ristretto group.

All operations are implemented using constant-time logic (no secret-dependent branches, no secret-dependent memory accesses), unless specifically marked as being variable-time code. We believe that our constant-time logic is lowered to constant-time assembly, at least on x86_64 targets.

As an additional guard against possible future compiler optimizations, the subtle crate places an optimization barrier before every conditional move or assignment. More details can be found in the documentation for the subtle crate.

Some functionality (e.g., multiscalar multiplication or batch inversion) requires heap allocation for temporary buffers. All heap-allocated buffers of potentially secret data are explicitly zeroed before release.

However, we do not attempt to zero stack data, for two reasons. First, it's not possible to do so correctly: we don't have control over stack allocations, so there's no way to know how much data to wipe. Second, because curve25519-dalek provides a mid-level API, the correct place to start zeroing stack data is likely not at the entrypoints of curve25519-dalek functions, but at the entrypoints of functions in other crates.

The implementation is memory-safe, and contains no significant unsafe code. The SIMD backend uses unsafe internally to call SIMD intrinsics. These are marked unsafe only because invoking them on an inappropriate CPU would cause SIGILL, but the entire backend is only invoked when the appropriate CPU features are detected at runtime, or when the whole program is compiled with the appropriate target_features.

Performance

Benchmarks are run using criterion.rs:

cargo bench --features "rand_core"
export RUSTFLAGS='-C target_cpu=native'
cargo +nightly bench --features "rand_core"

Performance is a secondary goal behind correctness, safety, and clarity, but we aim to be competitive with other implementations.

FFI

Unfortunately, we have no plans to add FFI to curve25519-dalek directly. The reason is that we use Rust features to provide an API that maintains safety invariants, which are not possible to maintain across an FFI boundary. For instance, as described in the Safety section above, invalid points are impossible to construct, and this would not be the case if we exposed point operations over FFI.

However, curve25519-dalek is designed as a mid-level API, aimed at implementing other, higher-level primitives. Instead of providing FFI at the mid-level, our suggestion is to implement the higher-level primitive (a signature, PAKE, ZKP, etc) in Rust, using curve25519-dalek as a dependency, and have that crate provide a minimal, byte-buffer-oriented FFI specific to that primitive.

Contributing

Please see CONTRIBUTING.md.

About

SPOILER ALERT: The Twelfth Doctor's first encounter with the Daleks is in his second full episode, "Into the Dalek". A beleaguered ship of the "Combined Galactic Resistance" has discovered a broken Dalek that has turned "good", desiring to kill all other Daleks. The Doctor, Clara and a team of soldiers are miniaturized and enter the Dalek, which the Doctor names Rusty. They repair the damage, but accidentally restore it to its original nature, causing it to go on the rampage and alert the Dalek fleet to the whereabouts of the rebel ship. However, the Doctor manages to return Rusty to its previous state by linking his mind with the Dalek's: Rusty shares the Doctor's view of the universe's beauty, but also his deep hatred of the Daleks. Rusty destroys the other Daleks and departs the ship, determined to track down and bring an end to the Dalek race.

curve25519-dalek is authored by Isis Agora Lovecruft and Henry de Valence.

Portions of this library were originally a port of Adam Langley's Golang ed25519 library, which was in turn a port of the reference ref10 implementation. Most of this code, including the 32-bit field arithmetic, has since been rewritten.

The fast u32 and u64 scalar arithmetic was implemented by Andrew Moon, and the addition chain for scalar inversion was provided by Brian Smith. The optimised batch inversion was contributed by Sean Bowe and Daira Hopwood.

The no_std and zeroize support was contributed by Tony Arcieri.

The formally verified fiat_backend integrates Rust code generated by the Fiat Crypto project and was contributed by François Garillot.

Thanks also to Ashley Hauck, Lucas Salibian, Manish Goregaokar, Jack Grigg, Pratyush Mishra, Michael Rosenberg, @pinkforest, and countless others for their contributions.