fix(identity): correctly follow extract-expand for HKDF

As @mxinden pointed out in https://github.com/libp2p/rust-libp2p/pull/4554#discussion_r1344747938, we were not correctly following the HKDF steps of extract and expand.

Pull-Request: #4589.

Cargo.lock

@@ -2634,7 +2634,7 @@ dependencies = [
 
 [[package]]
 name = "libp2p-identity"
-version = "0.2.4"
+version = "0.2.5"
 dependencies = [
  "asn1_der",
  "base64 0.21.4",

Cargo.toml

@@ -82,7 +82,7 @@ libp2p-dns = { version = "0.40.1", path = "transports/dns" }
 libp2p-floodsub = { version = "0.43.0", path = "protocols/floodsub" }
 libp2p-gossipsub = { version = "0.45.1", path = "protocols/gossipsub" }
 libp2p-identify = { version = "0.43.1", path = "protocols/identify" }
-libp2p-identity = { version = "0.2.4" }
+libp2p-identity = { version = "0.2.5" }
 libp2p-kad = { version = "0.44.6", path = "protocols/kad" }
 libp2p-mdns = { version = "0.44.0", path = "protocols/mdns" }
 libp2p-memory-connection-limits = { version = "0.1.0", path = "misc/memory-connection-limits" }

identity/CHANGELOG.md

@@ -1,3 +1,8 @@
+## 0.2.5
+
+- Fix usage of HKDF within `Keypair::derive_secret`.
+  See [PR 4554](https://github.com/libp2p/rust-libp2p/pull/4554).
+
 ## 0.2.4
 
 - Implement `Keypair::derive_secret`, to deterministically derive a new secret from the embedded secret key.

identity/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "libp2p-identity"
-version = "0.2.4"
+version = "0.2.5"
 edition = "2021"
 description = "Data structures and algorithms for identifying peers in libp2p."
 rust-version = { workspace = true }

identity/src/keypair.rs

@@ -358,20 +358,30 @@ impl Keypair {
     /// let new_key = key.derive_secret(b"my encryption key").expect("can derive secret for ed25519");
     /// # }
     /// ```
-    #[allow(unused_variables, unreachable_code)]
+    ///
+    #[cfg(any(
+        feature = "ecdsa",
+        feature = "secp256k1",
+        feature = "ed25519",
+        feature = "rsa"
+    ))]
     pub fn derive_secret(&self, domain: &[u8]) -> Option<[u8; 32]> {
-        #[cfg(any(
-            feature = "ecdsa",
-            feature = "secp256k1",
-            feature = "ed25519",
-            feature = "rsa"
-        ))]
-        return Some(
-            hkdf::Hkdf::<sha2::Sha256>::extract(None, &[domain, &self.secret()?].concat())
-                .0
-                .into(),
-        );
+        let mut okm = [0u8; 32];
+        hkdf::Hkdf::<sha2::Sha256>::new(None, &self.secret()?)
+            .expand(domain, &mut okm)
+            .expect("okm.len() == 32");
 
+        Some(okm)
+    }
+
+    // We build docs with all features so this doesn't need to have any docs.
+    #[cfg(not(any(
+        feature = "ecdsa",
+        feature = "secp256k1",
+        feature = "ed25519",
+        feature = "rsa"
+    )))]
+    pub fn derive_secret(&self, _: &[u8]) -> Option<[u8; 32]> {
         None
     }
 

identity/tests/keypair_api.rs

@@ -10,4 +10,5 @@ fn using_keypair(kp: Keypair) {
     let _ = kp.to_protobuf_encoding();
     let _ = kp.sign(&[]);
     let _ = kp.public();
+    let _: Option<[u8; 32]> = kp.derive_secret(b"foobar");
 }