diff --git a/src/utils/cipher.ts b/src/utils/cipher.ts
index f75c1e3..3121e8d 100644
--- a/src/utils/cipher.ts
+++ b/src/utils/cipher.ts
@@ -1,28 +1,20 @@
-function decryptResIndex(encData: Uint8Array, key: string): Uint8Array {
+function processResIndex(data: Uint8Array, key: string, isEncrypt: boolean): Uint8Array {
   const keyBytes = Buffer.from(key, 'utf-8');
-  const keyLength = keyBytes.length;
-  const result = new Uint8Array(encData.length);
-  for (let i = 0; i < encData.length; i++) {
-    const encByte = encData[i]!;
-    const keyByte = keyBytes[i % keyLength]!;
-    result[i] = (encByte - keyByte + 256) % 256;
+  const result = new Uint8Array(data.length);
+
+  for (let i = 0; i < data.length; i++) {
+    const keyByte = keyBytes[i % keyBytes.length]!;
+    result[i] = isEncrypt ? (data[i]! + keyByte) % 256 : (data[i]! - keyByte + 256) % 256;
   }
   return result;
 }
 
+function decryptResIndex(encData: Uint8Array, key: string): Uint8Array {
+  return processResIndex(encData, key, false);
+}
+
 function encryptResIndex(plainData: Uint8Array, key: string): Uint8Array {
-  const keyBytes = Buffer.from(key, 'utf-8');
-  const keyLength = keyBytes.length;
-  const result = new Uint8Array(plainData.length);
-  for (let i = 0; i < plainData.length; i++) {
-    const plainByte = plainData[i]!;
-    const keyByte = keyBytes[i % keyLength]!;
-    result[i] = (plainByte + keyByte) % 256;
-  }
-  return result;
+  return processResIndex(plainData, key, true);
 }
 
-export default {
-  decryptResIndex,
-  encryptResIndex,
-};
+export default { decryptResIndex, encryptResIndex };