signature: use &mut impl CryptoRngCore RNG arguments (#1147)

`rand_core` v0.6.4 added an auto-impl'd `CryptoRngCore` marker trait for
types which impl `CryptoRng + RngCore` which is slightly more convenient
and less verbose.

This commit changes to using `&mut impl CryptoRngCore` as proposed
in #1087. This hopefully strikes a balance between least surprise and
minimal required syntax, namely &mut references are reusable and don't
require knowledge of the blanket impl of `RngCore` for `&mut R: RngCore`

signature/Cargo.toml

@@ -14,7 +14,7 @@ categories    = ["cryptography", "no-std"]
 
 [dependencies]
 digest = { version = "0.10.3", optional = true, default-features = false }
-rand_core = { version = "0.6", optional = true, default-features = false }
+rand_core = { version = "0.6.4", optional = true, default-features = false }
 derive = { package = "signature_derive", version = "=2.0.0-pre.0", optional = true, path = "derive" }
 
 [dev-dependencies]

signature/src/signer.rs

@@ -6,7 +6,7 @@ use crate::error::Error;
 use crate::digest::Digest;
 
 #[cfg(feature = "rand-preview")]
-use crate::rand_core::{CryptoRng, RngCore};
+use crate::rand_core::CryptoRngCore;
 
 /// Sign the provided message bytestring using `Self` (e.g. a cryptographic key
 /// or connection to an HSM), returning a digital signature.
@@ -87,7 +87,7 @@ pub trait DigestSigner<D: Digest, S> {
 #[cfg_attr(docsrs, doc(cfg(feature = "rand-preview")))]
 pub trait RandomizedSigner<S> {
     /// Sign the given message and return a digital signature
-    fn sign_with_rng(&self, rng: impl CryptoRng + RngCore, msg: &[u8]) -> S {
+    fn sign_with_rng(&self, rng: &mut impl CryptoRngCore, msg: &[u8]) -> S {
         self.try_sign_with_rng(rng, msg)
             .expect("signature operation failed")
     }
@@ -97,7 +97,7 @@ pub trait RandomizedSigner<S> {
     ///
     /// The main intended use case for signing errors is when communicating
     /// with external signers, e.g. cloud KMS, HSMs, or other hardware tokens.
-    fn try_sign_with_rng(&self, rng: impl CryptoRng + RngCore, msg: &[u8]) -> Result<S, Error>;
+    fn try_sign_with_rng(&self, rng: &mut impl CryptoRngCore, msg: &[u8]) -> Result<S, Error>;
 }
 
 /// Combination of [`DigestSigner`] and [`RandomizedSigner`] with support for
@@ -109,16 +109,13 @@ pub trait RandomizedDigestSigner<D: Digest, S> {
     /// Sign the given prehashed message `Digest`, returning a signature.
     ///
     /// Panics in the event of a signing error.
-    fn sign_digest_with_rng(&self, rng: impl CryptoRng + RngCore, digest: D) -> S {
+    fn sign_digest_with_rng(&self, rng: &mut impl CryptoRngCore, digest: D) -> S {
         self.try_sign_digest_with_rng(rng, digest)
             .expect("signature operation failed")
     }
 
     /// Attempt to sign the given prehashed message `Digest`, returning a
     /// digital signature on success, or an error if something went wrong.
-    fn try_sign_digest_with_rng(
-        &self,
-        rng: impl CryptoRng + RngCore,
-        digest: D,
-    ) -> Result<S, Error>;
+    fn try_sign_digest_with_rng(&self, rng: &mut impl CryptoRngCore, digest: D)
+        -> Result<S, Error>;
 }