43 lines
2.8 KiB
HTML
43 lines
2.8 KiB
HTML
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<!DOCTYPE html>
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<html lang="en">
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<head>
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<meta charset="UTF-8">
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<meta http-equiv="X-UA-Compatible" content="IE=edge">
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<meta name="viewport" content="width=device-width, initial-scale=1.0">
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<title>Transport | eAmuse API</title>
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<link rel="stylesheet" href="styles.css">
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</head>
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<body>
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<table>
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<tr>
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<td><a href=".">Contents</a></td>
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<td><a href="./transport.html">Transport layer</a></td>
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<td><a href="./packet.html">Packet format</a></td>
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</tr>
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</table>
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<h1>Network format</h1>
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<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>
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<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>
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<ul>
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<li><code>none</code></li>
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<li><code>lz77</code></li>
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</ul>
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<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>
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<h2 id="keys">Encryption keys</h2>
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<p>Encryption is not performed using a single static key. Instead, each request and response has its own key that is generated.</p>
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<p>These keys are generated baesd on the <code>X-Eamuse-Info</code> header.</p>
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<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>
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<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.</p>
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<h2 id="lz77">LZ77</h2>
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<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>
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<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>
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<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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</body>
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</html>
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