crypto: Add ECDH trait and `MemorySigner` impl

Signed-off-by: Alexis Sellier <alexis@radicle.xyz>

radicle-crypto/src/lib.rs

@@ -26,6 +26,9 @@ pub struct Verified;
 #[derive(Debug, Copy, Clone, PartialEq, Eq)]
 pub struct Unverified;
 
+/// Output of a Diffie-Hellman key exchange.
+pub type SharedSecret = [u8; 32];
+
 /// Error returned if signing fails, eg. due to an HSM or KMS.
 #[derive(Debug, Error)]
 #[error(transparent)]
@@ -69,6 +72,13 @@ where
     }
 }
 
+/// A signer that can perform Elliptic-curve Diffie–Hellman.
+pub trait Ecdh: Signer {
+    /// Perform an ECDH key exchange. Takes the counter-party's public key,
+    /// and returns a computed shared secret.
+    fn ecdh(&self, other: &PublicKey) -> Result<SharedSecret, Error>;
+}
+
 /// Cryptographic signature.
 #[derive(PartialEq, Eq, Hash, Copy, Clone, Serialize, Deserialize)]
 #[serde(into = "String", try_from = "String")]

radicle-crypto/src/ssh/keystore.rs

@@ -5,7 +5,7 @@ use std::{fs, io};
 use thiserror::Error;
 use zeroize::Zeroizing;
 
-use crate::{keypair, PublicKey, SecretKey, Signature, Signer, SignerError};
+use crate::{keypair, Ecdh, PublicKey, SecretKey, SharedSecret, Signature, Signer, SignerError};
 
 /// A secret key passphrase.
 pub type Passphrase = Zeroizing<String>;
@@ -139,6 +139,16 @@ impl Signer for MemorySigner {
     }
 }
 
+impl Ecdh for MemorySigner {
+    fn ecdh(&self, other: &PublicKey) -> Result<SharedSecret, ed25519_compact::Error> {
+        let pk = ed25519_compact::x25519::PublicKey::from_ed25519(other)?;
+        let sk = ed25519_compact::x25519::SecretKey::from_ed25519(&self.secret)?;
+        let ss = pk.dh(&sk)?;
+
+        Ok(*ss)
+    }
+}
+
 impl MemorySigner {
     /// Load this signer from a keystore, given a secret key passphrase.
     pub fn load(keystore: &Keystore, passphrase: Passphrase) -> Result<Self, MemorySignerError> {