web/src/decrypt/kwm.js

import {
    AudioMimeType,
    DetectAudioExt,
    GetArrayBuffer,
    GetFileInfo,
    GetMetaCoverURL,
    IsBytesEqual
} from "./util";

const musicMetadata = require("music-metadata-browser");
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, raw_filename, raw_ext) {
    const oriData = new Uint8Array(await GetArrayBuffer(file));
    if (!IsBytesEqual(MagicHeader, oriData.slice(0, 0x10)))
        return {status: false, message: "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 = DetectAudioExt(audioData, "mp3");
    const mime = AudioMimeType[ext];
    let musicBlob = new Blob([audioData], {type: mime});

    const musicMeta = await musicMetadata.parseBlob(musicBlob);
    const info = GetFileInfo(musicMeta.common.artist, musicMeta.common.title, raw_filename);

    const imgUrl = GetMetaCoverURL(musicMeta);

    return {
        status: true,
        title: info.title,
        artist: info.artist,
        ext: ext,
        album: musicMeta.common.album,
        picture: imgUrl,
        file: URL.createObjectURL(musicBlob),
        mime: mime
    }
}


function createMaskFromKey(keyBytes) {
    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[i].charCodeAt() ^ keyStrTrim[i].charCodeAt()
    }
    return key
}


function trimKey(keyRaw) {
    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
}