feat: derive Keystore public key from secret key

lib/bls.ex

@@ -54,6 +54,11 @@ defmodule Bls do
     :erlang.nif_error(:nif_not_loaded)
   end
 
+  @spec derive_pubkey(privkey()) :: {:ok, pubkey()} | {:error, any()}
+  def derive_pubkey(_private_key) do
+    :erlang.nif_error(:nif_not_loaded)
+  end
+
   ##### Helpers #####
   @doc """
   Same as ``Bls.verify``, but treats errors as invalid signatures.

lib/keystore.ex

@@ -23,8 +23,15 @@ defmodule Keystore do
     validate_empty_path!(decoded_json["path"])
 
     privkey = decrypt!(decoded_json["crypto"], password)
-    # TODO: derive from privkey and validate with this pubkey
+
     pubkey = Map.fetch!(decoded_json, "pubkey") |> parse_binary!()
+
+    {:ok, derived_pubkey} = Bls.derive_pubkey(privkey)
+
+    if derived_pubkey != pubkey do
+      raise("Keystore secret and public keys don't form a valid pair")
+    end
+
     {pubkey, privkey}
   end
 

native/bls_nif/src/lib.rs

@@ -161,6 +161,17 @@ fn key_validate<'env>(public_key: Binary) -> Result<bool, String> {
 
     Ok(true)
 }
+#[rustler::nif]
+fn derive_pubkey<'env>(env: Env<'env>, private_key: Binary) -> Result<Binary<'env>, String> {
+    let sk = match SecretKey::deserialize(private_key.as_slice()) {
+        Ok(sk) => sk,
+        Err(e) => return Err(format!("{:?}", e)),
+    };
+    let public_key = sk.public_key();
+    let public_key_bytes = public_key.serialize();
+
+    Ok(bytes_to_binary(env, &public_key_bytes))
+}
 
 rustler::init!(
     "Elixir.Bls",
@@ -172,6 +183,7 @@ rustler::init!(
         eth_fast_aggregate_verify,
         eth_aggregate_pubkeys,
         verify,
-        key_validate
+        key_validate,
+        derive_pubkey
     ]
 );

test/unit/bls_test.exs

@@ -17,4 +17,21 @@ defmodule BlsTest do
       assert Bls.key_validate(invalid_public_key) == {:error, "BlstError(BLST_BAD_ENCODING)"}
     end
   end
+
+  describe "Private to public key" do
+    test "return the correct public key for a private key" do
+      valid_public_key =
+        Base.decode16!(
+          "8abb15ca57942b6225af4710bbb74ce8466e99fdc2264d9ffd3b335c7396667e45f537ff1f75ed5afa00585db274f3b6",
+          case: :mixed
+        )
+
+      private_key =
+        Base.decode16!("18363054f52f3f1fdc9ae50d271de853c582c652ebe8dd0f261da3b00cd98984",
+          case: :mixed
+        )
+
+      assert Bls.derive_pubkey(private_key) == {:ok, valid_public_key}
+    end
+  end
 end