web/src/decrypt/kwm.ts

import {
  AudioMimeType,
  BytesHasPrefix,
  GetArrayBuffer,
  GetCoverFromFile,
  GetMetaFromFile,
  SniffAudioExt,
} from '@/decrypt/utils';
import { Decrypt as RawDecrypt } from '@/decrypt/raw';

import { parseBlob as metaParseBlob } from 'music-metadata-browser';
import { DecryptResult } from '@/decrypt/entity';

//prettier-ignore
const MagicHeader = [
  0x79, 0x65, 0x65, 0x6C, 0x69, 0x6F, 0x6E, 0x2D,
  0x6B, 0x75, 0x77, 0x6F, 0x2D, 0x74, 0x6D, 0x65,
]
const PreDefinedKey = 'MoOtOiTvINGwd2E6n0E1i7L5t2IoOoNk';

export async function Decrypt(file: File, raw_filename: string, _: string): Promise<DecryptResult> {
  const oriData = new Uint8Array(await GetArrayBuffer(file));
  if (!BytesHasPrefix(oriData, MagicHeader)) {
    if (SniffAudioExt(oriData) === 'aac') {
      return await RawDecrypt(file, raw_filename, 'aac', false);
    }
    throw Error('not a valid kwm file');
  }

  let fileKey = oriData.slice(0x18, 0x20);
  let mask = createMaskFromKey(fileKey);
  let audioData = oriData.slice(0x400);
  let lenAudioData = audioData.length;
  for (let cur = 0; cur < lenAudioData; ++cur) audioData[cur] ^= mask[cur % 0x20];

  const ext = SniffAudioExt(audioData);
  const mime = AudioMimeType[ext];
  let musicBlob = new Blob([audioData], { type: mime });

  const musicMeta = await metaParseBlob(musicBlob);
  const { title, artist } = GetMetaFromFile(raw_filename, musicMeta.common.title, musicMeta.common.artist);
  return {
    album: musicMeta.common.album,
    picture: GetCoverFromFile(musicMeta),
    file: URL.createObjectURL(musicBlob),
    blob: musicBlob,
    mime,
    title,
    artist,
    ext,
  };
}

function createMaskFromKey(keyBytes: Uint8Array): Uint8Array {
  let keyView = new DataView(keyBytes.buffer);
  let keyStr = keyView.getBigUint64(0, true).toString();
  let keyStrTrim = trimKey(keyStr);
  let key = new Uint8Array(32);
  for (let i = 0; i < 32; i++) {
    key[i] = PreDefinedKey.charCodeAt(i) ^ keyStrTrim.charCodeAt(i);
  }
  return key;
}

function trimKey(keyRaw: string): string {
  let lenRaw = keyRaw.length;
  let out = keyRaw;
  if (lenRaw > 32) {
    out = keyRaw.slice(0, 32);
  } else if (lenRaw < 32) {
    out = keyRaw.padEnd(32, keyRaw);
  }
  return out;
}