Crate signature

Expand description

RustCrypto: Digital Signature Algorithms

This crate contains traits which provide generic, object-safe APIs for generating and verifying digital signatures.

Used by the dsa, ecdsa, ed25519, and rsa crates maintained by the RustCrypto organization, as well as ed25519-dalek.

Documentation

Minimum Supported Rust Version

Rust 1.56 or higher.

Minimum supported Rust version can be changed in the future, but it will be done with a minor version bump.

SemVer Policy

All on-by-default features of this library are covered by SemVer
MSRV is considered exempt from SemVer as noted above
The derive feature is stable and covered by SemVer
The off-by-default features digest and rand_core are unstable features which are also considered exempt from SemVer as they correspond to pre-1.0 crates which are still subject to changes. Breaking changes to these features will, like MSRV, be done with a minor version bump.

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.

Design

This crate provides a common set of traits for signing and verifying digital signatures intended to be implemented by libraries which produce or contain implementations of digital signature algorithms, and used by libraries which want to produce or verify digital signatures while generically supporting any compatible backend.

Goals

The traits provided by this crate were designed with the following goals in mind:

Provide an easy-to-use, misuse resistant API optimized for consumers (as opposed to implementers) of its traits.
Support common type-safe wrappers around “bag-of-bytes” representations which can be directly parsed from or written to the “wire”.
Expose a trait/object-safe API where signers/verifiers spanning multiple homogeneous provider implementations can be seamlessly leveraged together in the same logical “keyring” so long as they operate on the same underlying signature type.
Allow one provider type to potentially implement support (including being generic over) several signature types.
Keep signature algorithm customizations / “knobs” out-of-band from the signing/verification APIs, ideally pushing such concerns into the type system so that algorithm mismatches are caught as type errors.
Opaque error type which minimizes information leaked from cryptographic failures, as “rich” error types in these scenarios are often a source of sidechannel information for attackers (e.g. BB’06)

Implementation

To accomplish the above goals, the Signer and Verifier traits provided by this are generic over a signature value, and use generic parameters rather than associated types. Notably, they use such a parameter for the return value, allowing it to be inferred by the type checker based on the desired signature type.

Alternatives considered

This crate is based on many years of exploration of how to encapsulate digital signature systems in the most flexible, developer-friendly way. During that time many design alternatives were explored, tradeoffs compared, and ultimately the provided API was selected.

The tradeoffs made in this API have all been to improve simplicity, ergonomics, type safety, and flexibility for consumers of the traits. At times, this has come at a cost to implementers. Below are some concerns we are cognizant of which were considered in the design of the API:

“Bag-of-bytes” serialization precludes signature providers from using their own internal representation of a signature, which can be helpful for many reasons (e.g. advanced signature system features like batch verification).
Associated types, rather than generic parameters of traits, could allow more customization of the types used by a particular signature system, e.g. using custom error types.

It may still make sense to continue to explore the above tradeoffs, but with a new set of traits which are intended to be implementor-friendly, rather than consumer friendly. The existing Signer and Verifier traits could have blanket impls for the “provider-friendly” traits. However, as noted above this is a design space easily explored after stabilizing the consumer-oriented traits, and thus we consider these more important.

That said, below are some caveats of trying to design such traits, and why we haven’t actively pursued them:

Generics in the return position are already used to select which trait impl to use, i.e. for a particular signature algorithm/system. Avoiding a unified, concrete signature type adds another dimension to complexity and compiler errors, and in our experience makes them unsuitable for this sort of API. We believe such an API is the natural one for signature systems, reflecting the natural way they are written absent a trait.
Associated types preclude multiple (or generic) implementations of the same trait. These parameters are common in signature systems, notably ones which support different digest algorithms.
Digital signatures are almost always larger than the present 32-entry trait impl limitation on array types, which complicates trait signatures for these types (particularly things like From or Borrow bounds). This may be more interesting to explore after const generics.

Unstable features

Despite being post-1.0, this crate includes off-by-default unstable optional features, each of which depends on a pre-1.0 crate.

These features are considered exempt from SemVer. See the SemVer policy above for more information.

The following unstable features are presently supported:

derive: for implementers of signature systems using DigestSigner and DigestVerifier, the derive feature can be used to derive Signer and Verifier traits which prehash the input message using the PrehashSignature::Digest algorithm for a given signature type. When the derive feature is enabled import the proc macros with use signature::{Signer, Verifier} and then add a derive(Signer) or derive(Verifier) attribute to the given digest signer/verifier type. Enabling this feature also enables digest support (see immediately below).
digest: enables the DigestSigner and DigestVerifier traits which are based on the Digest trait from the digest crate. These traits are used for representing signature systems based on the Fiat-Shamir heuristic which compute a random challenge value to sign by computing a cryptographically secure digest of the input message.
rand_core: enables the RandomizedSigner trait for signature systems which rely on a cryptographically secure random number generator for security.

NOTE: the async-signature crate contains experimental async support for Signer and DigestSigner.

Re-exports

pub use digest;

pub use rand_core;

Modules

hazmat

Hazardous Materials: low-level APIs which can be insecure if misused.

Structs

Error

Signature errors.

Traits

DigestSignerdigest

Sign the given prehashed message Digest using Self.

DigestVerifierdigest

Verify the provided signature for the given prehashed message Digest is authentic.

Keypair

Signing keypair with an associated verifying key.

KeypairRef

Signing keypair with an associated verifying key.

PrehashSignaturedigest

Marker trait for Signature types computable as 𝐒(𝐇(𝒎)) i.e. ones which prehash a message to be signed as 𝐇(𝒎)

RandomizedDigestSignerdigest and rand_core

Combination of DigestSigner and RandomizedSigner with support for computing a signature over a digest which requires entropy from an RNG.

RandomizedSignerrand_core

Sign the given message using the provided external randomness source.

SignatureEncoding

Support for decoding/encoding signatures as bytes.

Signer

Sign the provided message bytestring using Self (e.g. a cryptographic key or connection to an HSM), returning a digital signature.

SignerMut

Sign the provided message bytestring using &mut Self (e.g. an evolving cryptographic key such as a stateful hash-based signature), returning a digital signature.

Verifier

Verify the provided message bytestring using Self (e.g. a public key)