Allow optional fee payer public key for zkapp command signature verification

Author: querolita

src/mina-signer/src/sign-zkapp-command.ts

@@ -77,19 +77,39 @@ function signZkappCommand(
   return ZkappCommand.toJSON(zkappCommand);
 }
 
+/**
+ * Verifies the signature of a zkApp command JSON object.
+ * 
+ * This function verifies the signatures of the fee payer and any account
+ * updates within the command that require signatures and are owned by the
+ * same public key. 
+ *
+ * @param zkappCommand_ - The zkApp command in JSON format, after signatures.
+ * @param publicKeyBase58 - The Base58-encoded public key used for verification.
+ * @param networkId - The network identifier that determines the signature domain.
+ * @param feePayerPublicKeyBase58 - Optional Base58-encoded public key of the fee
+ *                            payer, required if the provided public key does not
+ *                            match the fee payer's public key.
+ * @returns True if the signature is valid, false otherwise.
+ *
+ * @warning To verify the zkApp command signature, the public key must match the
+ * fee payer's public key, or the parameter `feePayerPublicKey` must be provided.
+ */
 function verifyZkappCommandSignature(
   zkappCommand_: Json.ZkappCommand,
   publicKeyBase58: string,
-  networkId: NetworkId
+  networkId: NetworkId,
+  feePayerPublicKeyBase58?: string
 ) {
   let zkappCommand = ZkappCommand.fromJSON(zkappCommand_);
 
   let { commitment, fullCommitment } = transactionCommitments(zkappCommand, networkId);
   let publicKey = PublicKey.fromBase58(publicKeyBase58);
 
-  // verify fee payer signature
+  // verify fee payer signature when public keys match
+  let feePayerPublicKey = feePayerPublicKeyBase58 ? PublicKey.fromBase58(feePayerPublicKeyBase58) : publicKey;
   let signature = Signature.fromBase58(zkappCommand.feePayer.authorization);
-  let ok = verifyFieldElement(signature, fullCommitment, publicKey, networkId);
+  let ok = verifyFieldElement(signature, fullCommitment, feePayerPublicKey, networkId) && PublicKey.equal(zkappCommand.feePayer.body.publicKey, feePayerPublicKey);
   if (!ok) return false;
 
   // verify other signatures for the same public key