Speed up AEAD on wrong prekey (#1369)

src/libspark/aead.cpp

@@ -48,15 +48,15 @@ AEADEncryptedData AEAD::encrypt(const GroupElement& prekey, const std::string ad
 // NOTE: This uses a fixed zero nonce, which is safe when used in Spark as directed
 // It is NOT safe in general to do this!
 CDataStream AEAD::decrypt_and_verify(const GroupElement& prekey, const std::string additional_data, AEADEncryptedData& data) {
-	// Derive the key and commitment
-	std::vector<unsigned char> key = SparkUtils::kdf_aead(prekey);
-	std::vector<unsigned char> key_commitment = SparkUtils::commit_aead(prekey);
-
 	// Assert that the key commitment is valid
+	std::vector<unsigned char> key_commitment = SparkUtils::commit_aead(prekey);
 	if (key_commitment != data.key_commitment) {
 		throw std::runtime_error("Bad AEAD key commitment");
 	}
 
+	// Derive the key
+	std::vector<unsigned char> key = SparkUtils::kdf_aead(prekey);
+
 	// Set up the result
 	CDataStream result(SER_NETWORK, PROTOCOL_VERSION);