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§RustCrypto: Digital Signature Algorithms

crate Docs Build Status Apache2/MIT licensed Rust Version Project Chat

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 1.60 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.

§License

Licensed under either of

at your option.

§Contribution

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 implementations of the same trait. These parameters are common in signature systems, notably ones which support different serializations of a signature (e.g. raw vs ASN.1).
  • Digital signatures are almost always larger than the present 32-entry trait impl limitation on array types, which complicates bounds for these types (particularly things like From or Borrow bounds).

§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:

  • 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§

Modules§

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

Structs§

Traits§

  • DigestSigner
    Sign the given prehashed message Digest using Self.
  • DigestVerifier
    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.
  • PrehashSignature
    Marker trait for Signature types computable as 𝐒(𝐇(𝒎)) i.e. ones which prehash a message to be signed as 𝐇(𝒎)
  • RandomizedDigestSigner
    Combination of DigestSigner and RandomizedSigner with support for computing a signature over a digest which requires entropy from an RNG.
  • RandomizedSigner
    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)

Type Aliases§