Merge pull request '优化 QMCCache 解密过程' (#6) from jixunmoe/um-web:refactor/optimise-qm-cache-decode into master

Reviewed-on: https://git.unlock-music.dev/um/web/pulls/6

src/decrypt/__test__/QmcCache.test.ts

@@ -0,0 +1,20 @@
+import { DecryptBuffer as DecryptQmcCacheBuffer } from '../qmccache';
+import fs from 'fs';
+
+const expectedBuffer = fs.readFileSync(__dirname + '/fixture/qmc_cache_expected.bin');
+
+const createInputBuffer = () => {
+  const buffer = Buffer.alloc(256);
+  for (let i = buffer.byteLength; i >= 0; i--) {
+    buffer[i] = i;
+  }
+  return buffer;
+};
+
+describe('decrypt/qmccache', () => {
+  it('should decrypt specified buffer correctly', () => {
+    const input = createInputBuffer();
+    DecryptQmcCacheBuffer(input);
+    expect(input).toEqual(expectedBuffer);
+  });
+});

src/decrypt/qmccache.ts

@@ -13,16 +13,18 @@ import { DecryptResult } from '@/decrypt/entity';
 
 import { parseBlob as metaParseBlob } from 'music-metadata-browser';
 
-export async function Decrypt(file: Blob, raw_filename: string, _: string): Promise<DecryptResult> {
-  const buffer = new Uint8Array(await GetArrayBuffer(file));
-  let length = buffer.length;
+export function DecryptBuffer(buffer: Uint8Array | Buffer) {
+  let length = buffer.byteLength;
   for (let i = 0; i < length; i++) {
-    buffer[i] ^= 0xf4;
-    if (buffer[i] <= 0x3f) buffer[i] = buffer[i] * 4;
-    else if (buffer[i] <= 0x7f) buffer[i] = (buffer[i] - 0x40) * 4 + 1;
-    else if (buffer[i] <= 0xbf) buffer[i] = (buffer[i] - 0x80) * 4 + 2;
-    else buffer[i] = (buffer[i] - 0xc0) * 4 + 3;
+    let byte = buffer[i] ^ 0xf4; // xor 0xf4
+    byte = ((byte & 0b0011_1111) << 2) | (byte >> 6); // rol 2
+    buffer[i] = byte;
+  }
 }
+
+export async function Decrypt(file: Blob, raw_filename: string, _: string): Promise<DecryptResult> {
+  const buffer = new Uint8Array(await GetArrayBuffer(file));
+  DecryptBuffer(buffer);
   let ext = SniffAudioExt(buffer, '');
   const newName = SplitFilename(raw_filename);
   let audioBlob: Blob;