Expose secret_key method for ecdsa::Keypair and eth::Keypair (#1628)

* Expose `secret_key` method for `ecdsa::Keypair` and `eth::Keypair`

* cargo fmt

* fix eth::secret_key

---------

Co-authored-by: James Wilson <james@jsdw.me>

signer/src/ecdsa.rs

@@ -157,6 +157,11 @@ impl Keypair {
         PublicKey(self.0.public_key().serialize())
     }
 
+    /// Obtain the [`SecretKey`] part of this key pair. This should be kept secret.
+    pub fn secret_key(&self) -> SecretKeyBytes {
+        *self.0.secret_key().as_ref()
+    }
+
     /// Sign some message. These bytes can be used directly in a Substrate `MultiSignature::Ecdsa(..)`.
     pub fn sign(&self, message: &[u8]) -> Signature {
         self.sign_prehashed(&sp_crypto_hashing::blake2_256(message))

signer/src/eth.rs

@@ -97,6 +97,11 @@ impl Keypair {
             .map_err(|_| Error::InvalidSeed)
     }
 
+    /// Obtain the [`ecdsa::SecretKeyBytes`] of this keypair.
+    pub fn secret_key(&self) -> SecretKeyBytes {
+        self.0.secret_key()
+    }
+
     /// Obtain the [`eth::PublicKey`] of this keypair.
     pub fn public_key(&self) -> PublicKey {
         let uncompressed = self.0 .0.public_key().serialize_uncompressed();