ecdsa: Implement EncodePrivateKey for SigningKey (#523)

ecdsa/src/sign.rs

@@ -32,7 +32,10 @@ use crate::elliptic_curve::{
 };
 
 #[cfg(feature = "pem")]
-use core::str::FromStr;
+use {
+ crate::elliptic_curve::pkcs8::{EncodePrivateKey, SecretDocument},
+ core::str::FromStr,
+};
 
 /// ECDSA signing key. Generic over elliptic curves.
 ///
@@ -290,6 +293,22 @@ where
  }
 }
 
+#[cfg(feature = "pem")]
+#[cfg_attr(docsrs, doc(cfg(feature = "pem")))]
+impl<C> EncodePrivateKey for SigningKey<C>
+where
+ C: AssociatedOid + PrimeCurve + ProjectiveArithmetic,
+ AffinePoint<C>: FromEncodedPoint<C> + ToEncodedPoint<C>,
+ FieldSize<C>: sec1::ModulusSize,
+ Scalar<C>: Invert<Output = CtOption<Scalar<C>>> + Reduce<C::UInt> + SignPrimitive<C>,
+ SignatureSize<C>: ArrayLength<u8>,
+{
+ fn to_pkcs8_der(&self) -> pkcs8::Result<SecretDocument> {
+ let secret_key = SecretKey::new(self.inner.into());
+ secret_key.to_pkcs8_der()
+ }
+}
+
 #[cfg(feature = "pkcs8")]
 #[cfg_attr(docsrs, doc(cfg(feature = "pkcs8")))]
 impl<C> DecodePrivateKey for SigningKey<C>