keyfork-derive-openpgp: fix encryption keys

keyfork-derive-openpgp/src/lib.rs

@@ -21,6 +21,9 @@ pub enum Error {
     #[error("{0}")]
     Anyhow(#[from] anyhow::Error),
 
+    #[error("Key configured with both encryption and non-encryption key flags: {0:?}")]
+    InvalidKeyFlags(KeyFlags),
+
     #[error("Incorrect derived data: {0}")]
     IncorrectDerivedData(#[from] TryFromDerivationResponseError),
 
@@ -79,13 +82,32 @@ pub fn derive(data: DerivationResponse, keys: &[KeyFlags], userid: UserID) -> Re
         // Generate subkey
         let index = u32::try_from(index)?;
         let derived_key = xprv.derive_child(&DerivationIndex::new(index, true)?)?;
-        let subkey = Key::from(
+        let is_enc =
+            subkey_flags.for_transport_encryption() || subkey_flags.for_storage_encryption();
+        let is_non_enc = subkey_flags.for_certification()
+            || subkey_flags.for_signing()
+            || subkey_flags.for_authentication();
+        let subkey = if is_enc && is_non_enc {
+            return Err(Error::InvalidKeyFlags(subkey_flags.clone()));
+        } else if is_enc {
+            Key::from(
+                Key4::<_, SubordinateRole>::import_secret_cv25519(
+                    &PrivateKey::to_bytes(derived_key.private_key()),
+                    None,
+                    None,
+                    epoch,
+                )
+                .expect("keyforkd gave invalid cv25519 key"),
+            )
+        } else {
+            Key::from(
                 Key4::<_, SubordinateRole>::import_secret_ed25519(
                     &PrivateKey::to_bytes(derived_key.private_key()),
                     epoch,
                 )
-            .unwrap(),
+                .expect("keyforkd gave invalid ed25519 key"),
-        );
+            )
+        };
 
         // As per OpenPGP spec, signing keys must backsig the primary key
         let builder = if subkey_flags.for_signing() {