Impl signature::Digest* traits for Ed25519ph (#270)

* Impl `signature::Digest*` traits for Ed25519ph

Adds the following trait impls:

- impl DigestSigner for SigningKey
- impl DigestVerifier for VerifyingKey

These traits can be used to create and verify Ed25519 signatures,
thunking to `SigningKey::sign_prehashed` and
`VerifyingKey::verify_prehashed` respectively.

* Add rustdoc comments for trait impls

Cargo.lock

@@ -301,6 +301,7 @@ dependencies = [
  "serde_bytes",
  "serde_json",
  "sha2",
+ "signature",
  "toml",
  "zeroize",
 ]
@@ -749,6 +750,9 @@ name = "signature"
 version = "2.0.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "8fe458c98333f9c8152221191a77e2a44e8325d0193484af2e9421a53019e57d"
+dependencies = [
+ "digest",
+]
 
 [[package]]
 name = "spki"

Cargo.toml

@@ -26,11 +26,14 @@ features = ["nightly", "batch", "pkcs8"]
 [dependencies]
 curve25519-dalek = { version = "=4.0.0-pre.5", default-features = false, features = ["digest"] }
 ed25519 = { version = "2", default-features = false }
+signature = { version = ">=2.0, <2.1", optional = true, default-features = false }
+sha2 = { version = "0.10", default-features = false }
+
+# optional features
 merlin = { version = "3", default-features = false, optional = true }
 rand_core = { version = "0.6.4", default-features = false, optional = true }
 serde = { version = "1.0", default-features = false, optional = true }
 serde_bytes = { version = "0.11", optional = true }
-sha2 = { version = "0.10", default-features = false }
 zeroize = { version = "1.5", default-features = false, optional = true }
 
 [dev-dependencies]
@@ -58,7 +61,7 @@ std = ["alloc", "ed25519/std", "serde?/std", "sha2/std"]
 asm = ["sha2/asm"]
 batch = ["alloc", "merlin", "rand_core"]
 fast = ["curve25519-dalek/precomputed-tables"]
-digest = []
+digest = ["signature/digest"]
 # This features turns off stricter checking for scalar malleability in signatures
 legacy_compatibility = []
 pkcs8 = ["ed25519/pkcs8"]

src/signing.rs

@@ -33,6 +33,9 @@ use curve25519_dalek::scalar::Scalar;
 
 use ed25519::signature::{KeypairRef, Signer, Verifier};
 
+#[cfg(feature = "digest")]
+use signature::DigestSigner;
+
 #[cfg(feature = "zeroize")]
 use zeroize::{Zeroize, ZeroizeOnDrop};
 
@@ -484,6 +487,17 @@ impl Signer<ed25519::Signature> for SigningKey {
     }
 }
 
+/// Equivalent to [`SigningKey::sign_prehashed`] with `context` set to [`None`].
+#[cfg(feature = "digest")]
+impl<D> DigestSigner<D, ed25519::Signature> for SigningKey
+where
+    D: Digest<OutputSize = U64>,
+{
+    fn try_sign_digest(&self, msg_digest: D) -> Result<ed25519::Signature, SignatureError> {
+        self.sign_prehashed(msg_digest, None)
+    }
+}
+
 impl Verifier<ed25519::Signature> for SigningKey {
     /// Verify a signature on a message with this signing key's public key.
     fn verify(&self, message: &[u8], signature: &ed25519::Signature) -> Result<(), SignatureError> {

src/verifying.rs

@@ -34,6 +34,9 @@ use serde::{Deserialize, Deserializer, Serialize, Serializer};
 #[cfg(feature = "serde")]
 use serde_bytes::{ByteBuf as SerdeByteBuf, Bytes as SerdeBytes};
 
+#[cfg(feature = "digest")]
+use signature::DigestVerifier;
+
 use crate::constants::*;
 use crate::errors::*;
 use crate::signature::*;
@@ -417,6 +420,21 @@ impl Verifier<ed25519::Signature> for VerifyingKey {
     }
 }
 
+/// Equivalent to [`VerifyingKey::verify_prehashed`] with `context` set to [`None`].
+#[cfg(feature = "digest")]
+impl<D> DigestVerifier<D, ed25519::Signature> for VerifyingKey
+where
+    D: Digest<OutputSize = U64>,
+{
+    fn verify_digest(
+        &self,
+        msg_digest: D,
+        signature: &ed25519::Signature,
+    ) -> Result<(), SignatureError> {
+        self.verify_prehashed(msg_digest, None, signature)
+    }
+}
+
 impl TryFrom<&[u8]> for VerifyingKey {
     type Error = SignatureError;