ecdsa: add getrandom feature (#1107)

Adds a `getrandom` feature which allows infallible generation of
`SigningKey`s using the system's cryptographically secure RNG.

Proxies through to the newly added `getrandom` feature in the
`elliptic-curve` crate: RustCrypto/traits#2085

Author: tarcieri

.github/workflows/ecdsa.yml

@@ -40,7 +40,7 @@ jobs:
           toolchain: ${{ matrix.rust }}
           targets: ${{ matrix.target }}
       - uses: RustCrypto/actions/cargo-hack-install@master
-      - run: cargo hack build --target ${{ matrix.target }} --feature-powerset --exclude-features std
+      - run: cargo hack build --target ${{ matrix.target }} --feature-powerset --exclude-features std,getrandom
 
   test:
     runs-on: ubuntu-latest

Cargo.lock

@@ -43,6 +43,7 @@ algorithm = ["dep:rfc6979", "digest", "elliptic-curve/arithmetic", "hazmat"]
 dev = ["algorithm", "digest/dev", "elliptic-curve/dev"]
 der = ["dep:der"]
 digest = ["dep:digest", "elliptic-curve/digest", "signature/digest"]
+getrandom = ["elliptic-curve/getrandom"]
 hazmat = []
 pkcs8 = ["der", "digest", "elliptic-curve/pkcs8"]
 pem = ["elliptic-curve/pem", "pkcs8"]

ecdsa/Cargo.toml

@@ -18,7 +18,7 @@ use signature::{
     DigestSigner, MultipartSigner, RandomizedDigestSigner, RandomizedMultipartSigner,
     RandomizedSigner, Signer,
     hazmat::{PrehashSigner, RandomizedPrehashSigner},
-    rand_core::{CryptoRng, TryCryptoRng},
+    rand_core::TryCryptoRng,
 };
 
 #[cfg(feature = "der")]
@@ -85,11 +85,16 @@ where
     SignatureSize<C>: ArraySize,
 {
     /// Generate a cryptographically random [`SigningKey`].
-    pub fn random<R: CryptoRng + ?Sized>(rng: &mut R) -> Self {
-        NonZeroScalar::<C>::random(rng).into()
+    ///
+    /// # Panics
+    ///
+    /// If the system's cryptographically secure RNG has an internal error.
+    #[cfg(feature = "getrandom")]
+    pub fn generate() -> Self {
+        NonZeroScalar::<C>::generate().into()
     }
 
-    /// Generate a cryptographically random [`SigningKey`].
+    /// Generate a cryptographically random [`SigningKey`], returning underlying RNG errors.
     pub fn try_from_rng<R: TryCryptoRng + ?Sized>(
         rng: &mut R,
     ) -> core::result::Result<Self, R::Error> {