k256: improve documentation for sha256 feature (#197)

k256/src/ecdsa.rs

@@ -14,9 +14,23 @@
 //!   [`VerifyKey`] types which natively implement ECDSA/secp256k1 signing and
 //!   verification.
 //!
+//! Additionally, this crate contains support for computing ECDSA signatures
+//! using either the SHA-256 (standard) or Keccak-256 (Ethereum) digest
+//! functions, which are gated under the following Cargo features:
+//!
+//! - `sha256`: compute signatures using NIST's standard SHA-256 digest
+//!   function. Unless you are computing signatures for Ethereum, this is
+//!   almost certainly what you want.
+//! - `keccak256`: compute signatures using the Keccak-256 digest function,
+//!   an incompatible variant of the SHA-3 algorithm used exclusively by
+//!   Ethereum.
+//!
+//! Most users of this library who want to sign/verify signatures will want to
+//! enable the `ecdsa` and `sha256` Cargo features.
+//!
 //! ## Signing/Verification Example
 //!
-//! This example requires the `ecdsa` Cargo feature is enabled:
+//! This example requires the `ecdsa` and `sha256` Cargo features are enabled:
 //!
 //! ```
 //! # #[cfg(feature = "ecdsa")]

k256/src/ecdsa/sign.rs

@@ -66,7 +66,7 @@ impl From<&SecretKey> for SigningKey {
     }
 }
 
-#[cfg(feature = "sha256")]
+#[cfg(any(feature = "keccak256", feature = "sha256"))]
 impl<S> signature::Signer<S> for SigningKey
 where
     S: PrehashSignature,
@@ -77,7 +77,7 @@ where
     }
 }
 
-#[cfg(feature = "sha256")]
+#[cfg(any(feature = "keccak256", feature = "sha256"))]
 impl<S> RandomizedSigner<S> for SigningKey
 where
     S: PrehashSignature,