validates the point of failure for signing tx with wrong key (#5601)

adds a test that signs a transaction with a different key than the key used to
generate proofs for the transaction

the test confirms that signing the transaction with the wrong key succeeds, but
the transaction does not verify

Author: hughy

ironfish-rust/src/transaction/tests.rs

@@ -812,6 +812,58 @@ fn test_sign_simple() {
     verify_transaction(&signed_transaction).expect("should be able to verify transaction");
 }
 
+#[test]
+fn test_sign_key_mismatch_failure() {
+    let spender_key = SaplingKey::generate_key();
+    let receiver_key = SaplingKey::generate_key();
+    let sender_key = SaplingKey::generate_key();
+
+    let in_note = Note::new(
+        spender_key.public_address(),
+        42,
+        "",
+        NATIVE_ASSET,
+        sender_key.public_address(),
+    );
+    let out_note = Note::new(
+        receiver_key.public_address(),
+        40,
+        "",
+        NATIVE_ASSET,
+        spender_key.public_address(),
+    );
+    let witness = make_fake_witness(&in_note);
+
+    // create transaction, add spend and output
+    let mut transaction = ProposedTransaction::new(TransactionVersion::latest());
+    transaction
+        .add_spend(in_note, &witness)
+        .expect("should be able to add a spend");
+    transaction
+        .add_output(out_note)
+        .expect("should be able to add an output");
+
+    // build transaction, generate proofs
+    let unsigned_transaction = transaction
+        .build(
+            spender_key.proof_authorizing_key,
+            spender_key.view_key().clone(),
+            spender_key.outgoing_view_key().clone(),
+            1,
+            Some(spender_key.public_address()),
+        )
+        .expect("should be able to build unsigned transaction");
+
+    // sign with different, mismatched key
+    let signer_key = SaplingKey::generate_key();
+    let signed_transaction = unsigned_transaction
+        .sign(&signer_key)
+        .expect("should be able to sign transaction");
+
+    // verify transaction
+    verify_transaction(&signed_transaction).expect_err("should not be able to verify transaction");
+}
+
 #[test]
 fn test_aggregate_signature_shares() {
     let spender_key = SaplingKey::generate_key();