ecdsa: SecretKey updates

ecdsa/src/lib.rs

@@ -53,17 +53,23 @@ pub mod sign;
 pub mod verify;
 
 // Re-export the `elliptic-curve` crate (and select types)
-pub use elliptic_curve::{self, generic_array, sec1::EncodedPoint, weierstrass::Curve, SecretKey};
+pub use elliptic_curve::{self, generic_array, sec1::EncodedPoint, weierstrass::Curve};
 
 // Re-export the `signature` crate (and select types)
 pub use signature::{self, Error};
 
 #[cfg(feature = "sign")]
+#[cfg_attr(docsrs, doc(cfg(feature = "sign")))]
 pub use sign::SigningKey;
 
 #[cfg(feature = "verify")]
+#[cfg_attr(docsrs, doc(cfg(feature = "verify")))]
 pub use verify::VerifyKey;
 
+#[cfg(feature = "zeroize")]
+#[cfg_attr(docsrs, doc(cfg(feature = "zeroize")))]
+pub use elliptic_curve::SecretKey;
+
 use core::{
     convert::{TryFrom, TryInto},
     fmt::{self, Debug},

ecdsa/src/sign.rs

@@ -57,12 +57,6 @@ where
             .ok_or_else(Error::new)
     }
 
-    /// Create a new signing key from a [`SecretKey`].
-    // TODO(tarcieri): infallible `From` conversion from a secret key
-    pub fn from_secret_key(secret_key: &SecretKey<C>) -> Result<Self, Error> {
-        Self::new(secret_key.as_bytes())
-    }
-
     /// Get the [`VerifyKey`] which corresponds to this [`SigningKey`]
     #[cfg(feature = "verify")]
     #[cfg_attr(docsrs, doc(cfg(feature = "verify")))]
@@ -78,6 +72,19 @@ where
     }
 }
 
+impl<C> From<&SecretKey<C>> for SigningKey<C>
+where
+    C: Curve + Arithmetic,
+    C::Scalar: FromDigest<C> + Invert<Output = C::Scalar> + SignPrimitive<C> + Zeroize,
+    SignatureSize<C>: ArrayLength<u8>,
+{
+    fn from(secret_key: &SecretKey<C>) -> Self {
+        Self {
+            secret_scalar: *secret_key.secret_scalar(),
+        }
+    }
+}
+
 impl<C, D> DigestSigner<D, Signature<C>> for SigningKey<C>
 where
     C: Curve + Arithmetic,