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<h1>Network format</h1>

<p>eAmuse packets are sent and received over HTTP (no S), with requests being in the body of <code>POST</code> requests,
    and replies being in the, well, reply.</p>
<p>The packets are typically both encrypted and compressed. The compression format used is indicated by the
    <code>X-Compress</code> header, and valid values are
</p>
<ul>
    <li><code>none</code></li>
    <li><code>lz77</code></li>
</ul>
<details>
    <summary>Source code details</summary>
    <figure>
        <img src="./images/lz77.png">
        <figcaption><code>libavs-win32-ea3.dll:0x1000fa29</code></figcaption>
    </figure>
</details>
<p>Encryption is performed <b>after</b> compression, and uses RC4. RC4 is symmetric, so decryption is performed the same
    as encryption. That is, <code>packet = encrypt(compress(data))</code> and
    <code>data = decompress(decrypt(data))</code>.
</p>

<h2 id="keys">Encryption keys</h2>
<p>Encryption is not performed using a single static key. Instead, each request and response has its own key that is
    generated.</p>
<p>These keys are generated baesd on the <code>X-Eamuse-Info</code> header.</p>
<p>This header loosely follows the format <code>1-[0-9a-f]{8}-[0-9a-f]{4}</code>. This corresponds to
    <code>[version]-[serial]-[salt]</code>. <b>TODO: Confirm this</b>
</p>
<p>Our per-packet key is then generated using <code>md5(serial | salt | KEY)</code>. Identifying <code>KEY</code> is
    left as an exercise for the reader, however should not be especially challenging. <span style="color: #fff">Check
        the page source if you're stuck.</span></p>
<!-- It's 69d74627d985ee2187161570d08d93b12455035b6df0d8205df5, if you were wondering. libavs-win32-ea3.dll:0x10054160 -->

<h2 id="lz77">LZ77</h2>
<p>Packets are compressed using lzss. The compressed data structure is a repeating cycle of an 8 bit flags byte,
    followed by 8 values. Each value is either a single literal byte, if the corresponding bit in the preceeding flag is
    high, or is a two byte lookup into the window.</p>
<p>The lookup bytes are structured as <code>pppppppp ppppllll</code> where <code>p</code> is a 12 bit index in the
    window, and <code>l</code> is a 4 bit integer that determines how many times to repeat the value located at that
    index in the window.</p>

<p>The exact algorithm used for compression is not especially important, as long as it follows this format. One can
    feasibly perform no compression at all, and instead insert <code>0xFF</code> every 8 bytes (starting at index 0), to
    indicate that all values are literals. While obviously poor for compression, this is an easy way to test without
    first implementing a compressor.</p>

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