web/src/decrypt/kwm.ts

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

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

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
}