c2pa_raw_crypto

Raw cryptographic signing and validation primitives for C2PA.

IMPORTANT: This crate is an implemementation detail for the c2pa crate and not generally designed for independent use.

This crate provides the RawSigner and RawSignatureValidator traits together with built-in implementations of the digital signature algorithms required by the C2PA specification. It deliberately stays narrow: it knows nothing about COSE framing, RFC 3161 time stamping, or OCSP — those concerns are handled by the calling code (today, the c2pa crate).

Cryptography backend / Crate features

Crate capabilities are available via Cargo feature flags:

• rust_native_crypto (enabled by default) — pure-Rust crates.
• openssl — a vendored OpenSSL implementation. Unsupported on all wasm32 targets, including browser Wasm and WASI; enabling it there is a compile-time error.
• serde (enabled by default) — Serialize and Deserialize support for SigningAlg.

Disable default features and enable only what you need to omit serde, for example --no-default-features --features rust_native_crypto.

On non-wasm32 targets, rust_native_crypto takes precedence. If both features end up enabled (which can happen through Cargo feature unification in a workspace), the rust-native backend is selected at runtime and the OpenSSL backend, while compiled, is not used. This is not considered an error.

On wasm32, openssl is prohibited even when rust_native_crypto is also enabled. OpenSSL relies on C-language facilities unavailable on these targets. It is not silently ignored: --all-features is therefore also unsupported on wasm32. Use default features or --no-default-features --features rust_native_crypto, and ensure no dependency enables this crate's openssl feature through Cargo feature unification.

Feature selection	Non-wasm32 platforms	Browser Wasm / WASI
Default features	Rust-native backend	Rust-native backend
--no-default-features	No built-in backend	No built-in backend
--no-default-features --features rust_native_crypto	Rust-native backend	Rust-native backend
--no-default-features --features openssl	OpenSSL backend	Compile-time error
--all-features	Rust-native backend (OpenSSL also compiled)	Compile-time error

Enabling neither is allowed (e.g. when only the type definitions are needed, or when signing is delegated to a remote service and no validation is performed); in that case the built-in signer/validator constructors report an error / return None at runtime.

RustCrypto dependency compatibility

ECDSA uses the stable ecdsa 0.17 / p256, p384, and p521 0.14 family together: signature 3, elliptic-curve 0.14, digest / sha2 0.11, and PKCS#8 0.11 / SPKI 0.8. Import key-decoding and signature traits from that family, not from the older RSA dependencies. This replaces the standalone ecdsa update proposed in #26: ecdsa 0.17 traits and signature types cannot be used with the 0.13 curves. Dependabot groups ECDSA and the three curves so updates can be reviewed together.

Ed25519 uses ed25519-dalek 3 with signature 3, digest / sha2 0.11, and PKCS#8 0.11 / SPKI 0.8, matching the newer ECDSA dependency family.

RSA remains on stable rsa 0.9, pkcs1 0.7, and rand 0.8; RSA 0.10 is still prerelease. RSA uses signature 2, digest / sha2 0.10, and PKCS#8 0.10 / SPKI 0.7. The shared byte-oriented key parsers also retain PKCS#8 0.10 / SPKI 0.7. These generations coexist intentionally, with bounded duplicate exceptions in deny.toml; no prereleases or unrelated dependency majors are needed for the ECDSA migration.

The public APIs and key/signature formats are unchanged: PEM PKCS#8 private keys, DER SPKI public keys, fixed-width P1363 signing output (64, 96, and 132 bytes), and DER or P1363 verification input. Both low-S and high-S NIST signatures remain accepted. P-521 now uses the same zeroize-on-drop signing key and direct key decoding as the other curves, without an intermediate private-key byte copy. Its upstream implementation now uses deterministic RFC6979 signing, as P-256 and P-384 already did; callers must not rely on fresh randomness making repeated ECDSA signatures distinct.

The ECDSA/curve releases and PKCS#8 0.11 / SPKI 0.8 / getrandom 0.4 declare Rust 1.85. The resolved crypto-bigint 0.7.6 and sha2 0.11.1 require Rust 1.87 and 1.88 respectively, also below this crate's unchanged Rust 1.96.0 requirement. Browser Wasm enables getrandom 0.4's wasm_js feature alongside getrandom 0.2's js feature for RSA; this needs no custom RUSTFLAGS. These JavaScript features are scoped to wasm32-unknown-unknown, not WASI. WASI continues to use system randomness, and the supported targets and backend precedence are unchanged.

Upstream migration references: ECDSA 0.17 changelog, P-521 0.14 changelog, and getrandom 0.4 target support.

Contributions and feedback

We welcome contributions to this project. For information on contributing, providing feedback, and about ongoing work, see Contributing.

Requirements

The toolkit requires Rust version 1.96.0 or newer. When a newer version of Rust becomes required, a new minor version of this crate will be released.

Supported platforms

This crate's CI workflow builds and tests against the platforms that the c2pa-rs support tiers cover (Tiers 1A, 1B, and 2). Unlike c2pa-rs, this crate does not split those platforms across separate tier workflows — because it evolves more slowly, every platform below is exercised on every commit.

The crate is built and tested on the following platforms:

• Windows (x86_64-pc-windows-msvc and aarch64-pc-windows-msvc)

  • Only the MSVC build chain is supported on Windows. As discussed in #155, we would welcome a PR to enable GNU build chain support on Windows.

• macOS on Apple silicon (aarch64-apple-darwin)

• Linux on x86 (x86_64-unknown-linux-gnu) and ARM v8 (aarch64-unknown-linux-gnu)

• Wasm in the browser (wasm32-unknown-unknown)

• WASI (wasm32-wasip2)

The crate is also verified on the following platforms. CI runs the full test suite on the iOS simulator and confirms that the remaining targets build, but does not run tests on physical devices:

• iOS (tested on the aarch64-apple-ios-sim simulator; aarch64-apple-ios and x86_64-apple-ios are build-verified)

• Android (aarch64-linux-android, armv7-linux-androideabi, i686-linux-android, and x86_64-linux-android are build-verified)

License

The c2pa_raw_crypto crate is distributed under the terms of both the MIT license and the Apache License (Version 2.0).

See LICENSE-APACHE and LICENSE-MIT.

Note that some components and dependent crates are licensed under different terms; please check the license terms for each crate and component for details.