dsa: impl ZeroizeOnDrop for SigningKey (#917)

Author: pqcfox

dsa/src/signing_key.rs

@@ -26,7 +26,7 @@ use signature::{
     hazmat::{PrehashSigner, RandomizedPrehashSigner},
     rand_core::TryCryptoRng,
 };
-use zeroize::{Zeroize, Zeroizing};
+use zeroize::{Zeroize, ZeroizeOnDrop, Zeroizing};
 
 /// DSA private key.
 ///
@@ -145,6 +145,8 @@ impl SigningKey {
     }
 }
 
+impl ZeroizeOnDrop for SigningKey {}
+
 impl Signer<Signature> for SigningKey {
     fn try_sign(&self, msg: &[u8]) -> Result<Signature, signature::Error> {
         let digest = sha2::Sha256::new_with_prefix(msg);