Use &Digest in ECDSA to avoid copies

Modified digest_scalar, sign_digest, and verification to pass digest::Digest
by reference (&Digest) instead of by value, preventing implicit copies via
Digest's Copy trait implementation, thus causing 64 byte saving in copying for each &h.

Author: dare3path

src/ec/suite_b/ecdsa/digest_scalar.rs

@@ -42,7 +42,7 @@ use crate::{digest, ec::suite_b::ops::*};
 /// right will give a value less than 2**255, which is less than `n`. The
 /// analogous argument applies for P-384. However, it does *not* apply in
 /// general; for example, it doesn't apply to P-521.
-pub(super) fn digest_scalar(n: &Modulus<N>, msg: digest::Digest) -> Scalar {
+pub(super) fn digest_scalar(n: &Modulus<N>, msg: &digest::Digest) -> Scalar {
     digest_scalar_(n, msg.as_ref())
 }
 

src/ec/suite_b/ecdsa/signing.rs

@@ -189,7 +189,7 @@ impl EcdsaKeyPair {
             rng,
         };
 
-        self.sign_digest(h, &nonce_rng, cpu)
+        self.sign_digest(&h, &nonce_rng, cpu)
     }
 
     #[cfg(test)]
@@ -201,14 +201,14 @@ impl EcdsaKeyPair {
         // Step 4 (out of order).
         let h = digest::digest(self.alg.digest_alg, message);
 
-        self.sign_digest(h, rng, cpu::features())
+        self.sign_digest(&h, rng, cpu::features())
     }
 
     /// Returns the signature of message digest `h` using a "random" nonce
     /// generated by `rng`.
     fn sign_digest(
         &self,
-        h: digest::Digest,
+        h: &digest::Digest,
         rng: &dyn rand::SecureRandom,
         cpu: cpu::Features,
     ) -> Result<signature::Signature, error::Unspecified> {

src/ec/suite_b/ecdsa/verification.rs

@@ -65,7 +65,7 @@ impl signature::VerificationAlgorithm for EcdsaVerificationAlgorithm {
             // NSA Guide Step 3: "Convert the bit string H to an integer e as
             // described in Appendix B.2."
             let n = &self.ops.scalar_ops.scalar_modulus(cpu);
-            digest_scalar(n, h)
+            digest_scalar(n, &h)
         };
 
         self.verify_digest(public_key, e, signature)