web/src/decrypt/qmc_wasm.ts

import QMCCryptoModule from '@jixun/qmc2-crypto/QMC2-wasm-bundle';

// 检测文件末端使用的缓冲区大小
const DETECTION_SIZE = 40;

// 每次处理 2M 的数据
const DECRYPTION_BUF_SIZE = 2 * 1024 * 1024;

function MergeUint8Array(array: Uint8Array[]): Uint8Array {
  let length = 0;
  array.forEach(item => {
    length += item.length;
  });

  let mergedArray = new Uint8Array(length);
  let offset = 0;
  array.forEach(item => {
    mergedArray.set(item, offset);
    offset += item.length;
  });

  return mergedArray;
}

/**
 * 解密一个 QMC2 加密的文件。
 *
 * 如果检测并解密成功，返回解密后的 Uint8Array 数据。
 * @param  {ArrayBuffer} mggBlob 读入的文件 Blob
 * @return {Promise<Uint8Array|false>}
 */
export async function DecryptQMCWasm(mggBlob: ArrayBuffer) {
  // 初始化模组
  const QMCCrypto = await QMCCryptoModule();

  // 申请内存块，并文件末端数据到 WASM 的内存堆
  const detectionBuf = new Uint8Array(mggBlob.slice(-DETECTION_SIZE));
  const pDetectionBuf = QMCCrypto._malloc(detectionBuf.length);
  QMCCrypto.writeArrayToMemory(detectionBuf, pDetectionBuf);

  // 检测结果内存块
  const pDetectionResult = QMCCrypto._malloc(QMCCrypto.sizeof_qmc_detection());

  // 进行检测
  const detectOK = QMCCrypto.detectKeyEndPosition(
    pDetectionResult,
    pDetectionBuf,
    detectionBuf.length
  );

  // 提取结构体内容：
  // (pos: i32; len: i32; error: char[??])
  const position = QMCCrypto.getValue(pDetectionResult, "i32");
  const len = QMCCrypto.getValue(pDetectionResult + 4, "i32");

  // 释放内存
  QMCCrypto._free(pDetectionBuf);
  QMCCrypto._free(pDetectionResult);

  if (!detectOK) {
    return false;
  }

  // 计算解密后文件的大小。
  // 之前得到的 position 为相对当前检测数据起点的偏移。
  const decryptedSize = mggBlob.byteLength - DETECTION_SIZE + position;

  // 提取嵌入到文件的 EKey
  const ekey = new Uint8Array(
    mggBlob.slice(decryptedSize, decryptedSize + len)
  );

  // 解码 UTF-8 数据到 string
  const decoder = new TextDecoder();
  const ekey_b64 = decoder.decode(ekey);

  // 初始化加密与缓冲区
  const hCrypto = QMCCrypto.createInstWidthEKey(ekey_b64);
  const buf = QMCCrypto._malloc(DECRYPTION_BUF_SIZE);

  const decryptedParts = [];
  let offset = 0;
  let bytesToDecrypt = decryptedSize;
  while (bytesToDecrypt > 0) {
    const blockSize = Math.min(bytesToDecrypt, DECRYPTION_BUF_SIZE);

    // 解密一些片段
    const blockData = new Uint8Array(
      mggBlob.slice(offset, offset + blockSize)
    );
    QMCCrypto.writeArrayToMemory(blockData, buf);
    QMCCrypto.decryptStream(hCrypto, buf, offset, blockSize);
    decryptedParts.push(QMCCrypto.HEAPU8.slice(buf, buf + blockSize));

    offset += blockSize;
    bytesToDecrypt -= blockSize;
  }
  QMCCrypto._free(buf);
  hCrypto.delete();

  return MergeUint8Array(decryptedParts);
}