225 lines
8.4 KiB
HTML
225 lines
8.4 KiB
HTML
{% extends "sega.html" %} {% block title %}AlphADVD{% endblock %} {% block body %}
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<h1>AlphaDVD (aka αDVD)</h1>
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<p>
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αDVD is the custom copy-protection SEGA employ for update DVDs. It is handled by
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<code>mxAuthDisc.exe</code> on Ring systems. Is is present on DVR-* discs, typically the first in a multi-DVD
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install process.
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</p>
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<p>
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In order to understand αDVD, it's important to first have a basic understanding of how data is stored on a
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DVD. Rather than like random access storage, where the data stream can be moddeled as a large addressable series of
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bytes, DVDs are more akin to HDDs in their division into sectors. Unlike HDDs, however, there is no prescribed order
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for the sectors! Each sector of data on disc is prefixed by a header identifying that sector, and notably including
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its sector number. When a DVD reader is asked to read a specific sector, it spins the disc until it reads the
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appropriate header, then returns the data following that header. There is importantly nothing here that would stop a
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disc from containing multiple sectors with the same sector number in their prefix!
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</p>
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<p>
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DVD readers will return the first sector that matches the requested sector number, so if we know where the different
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duplicates are on disc we can seek to a known sector a short distance before the instance of the duplicate we wish
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to aquire, then ask the DVD reader to read the duplicated sector. Depending on where we first seek, we will receive
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different data back.
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</p>
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<p>
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αDVD utilises this, with 6 duplicated sectors, each with three distinct copies. When authenticating a disc,
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only one of these 6 duplicates will be checked, however which is checked is random, so in practice all 6 should be
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present lest the disc sporadically fails the authentication. This is similar to a copy-protection scheme called
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TAGES, however more advanced. All three instances <b>must</b> be present, so it is impossible to create a single
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flat image that passes authentication!
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</p>
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<p>
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As well as this more hardware based authentication, there is a level of encryption applied to the disc headers too.
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This is however much easeier to work with. Each disc has a header in sector 16, sector 1, or sector 17 (checked in
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that order). There is no indication which sector contains the header, so in turn each sector is read and decryption
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is attempted. We can then validate the header magic number.
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</p>
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<h2>αDVD Encryption</h2>
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<p>
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αDVD encryption is a basic XOR cipher, where the text is XORed with a key, repeating the key as needed. The
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key is always 32768 (8000<sub>h</sub>) bytes, and is unmodified during this process.
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</p>
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<p>
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Keys are derrived based on a key expansion algorithm that takes as input an unsigned short (16 bit) seed. This
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algorithm is fundamentally an LSFR-based PRNG, used to generate a stream of bytes, which becomes the key. The
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<code>& 0xffffffff</code> is not strictly necessary but is convenient in python in order to avoid
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<code>seed</code> blowing up in size.
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</p>
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<pre>
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{%highlight "python"%}
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def amAuthDiskInitKey(seed):
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key = bytearray(0x8000)
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seed <<= 1
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for i in range(len(key)):
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x = seed
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for _ in range(8):
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x |= ((((seed >> 4) ^ x) >> 11) & 1)
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seed <<= 1
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x <<= 1
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key[i] = (x >> 1) & 0xff
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seed = x & 0xffffffff
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return key
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{% endhighlight %}</pre
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>
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<h2>αDVD Headers</h2>
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<p>
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Now that we know how to decrypt data on αDVDs we can search for the header. The header will always be
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encrypted with a fixed key with seed <code>5369</code>. The header is a sequence of 53 bytes, located at offset 318
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if it is in sector 16, and offset 508 if it is in sector 1 or 17.
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</p>
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<table>
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<thead>
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<tr>
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<td colspan="17">Header Offset</td>
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</tr>
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<tr>
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<td></td>
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<td>0</td>
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<td>1</td>
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<td>2</td>
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<td>3</td>
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<td>4</td>
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<td>5</td>
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<td>6</td>
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<td>7</td>
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<td>8</td>
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<td>9</td>
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<td>A</td>
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<td>B</td>
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<td>C</td>
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<td>D</td>
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<td>E</td>
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<td>F</td>
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</tr>
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</thead>
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<tbody>
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<tr>
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<td><b>0</b></td>
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<td colspan="4">Magic = F1FFFF1F<sub>h</sub></td>
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<td colspan="2"></td>
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<td colspan="2">Auth sector 1</td>
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<td colspan="2">Auth sector 2</td>
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<td colspan="2">Auth sector 3</td>
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<td colspan="6"></td>
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</tr>
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<tr>
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<td><b>1</b></td>
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<td colspan="2">Data Offset</td>
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<td colspan="4"></td>
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<td colspan="2">??</td>
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<td colspan="20">DVD Name</td>
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</tr>
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<tr>
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<td><b>2</b></td>
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<td colspan="2">Key seed</td>
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<td colspan="2">Dummy number</td>
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<td colspan="3"></td>
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</tr>
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</tbody>
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</table>
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<p>
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To validate the decryption of a header, both the magic number and the DVD name are checked. The DVD name must start
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with <code>SEGA_DVD</code>.
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</p>
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<p>
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The key seed present is the header is used to generate a new key that will be used to decrypt the authentication
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sectors.
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</p>
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<h2>The Authentication Sectors</h2>
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<p>
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The three sector addresses in the header are now used to perform a series of seeks and reads. We seek the drive by
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requesting a read of 16 sectors, but disregarding the returned data. The first step is to choose the authentication
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sector we wish to read. The six duplicates are present using the following offsets:
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</p>
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<ul>
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<li>140<sub>h</sub></li>
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<li>150<sub>h</sub></li>
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<li>160<sub>h</sub></li>
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<li>170<sub>h</sub></li>
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<li>180<sub>h</sub></li>
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<li>190<sub>h</sub></li>
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</ul>
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<p>We will refer to our chosen offset, from this list, as <code>n</code>.</p>
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<ul>
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<li>Seek to <code>[Auth sector 1] - 16</code></li>
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<li>Read(16) <code>([Auth sector 1] + (n - 1)) & 0xFFFFFFF0</code></li>
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<li>Seek to <code>[Auth sector 1] - 16</code></li>
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<li>Seek to <code>[Auth sector 1] + [Auth sector 2] - 8</code></li>
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<li>Read(16) <code>([Auth sector 1] + (n - 1)) & 0xFFFFFFF0</code></li>
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<li>Seek to <code>[Auth sector 1] + [Auth sector 3] + 8</code></li>
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<li>Read(16) <code>([Auth sector 1] + (n - 1)) & 0xFFFFFFF0</code></li>
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</ul>
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<p>
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Each of the three reads are decrypted using the key we generated earlier, and are authentication block 1, 2, and 3
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respectively. The actual data is at offset 31228 in these 16-sector blocks, and follows the following structure:
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</p>
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<table>
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<thead>
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<tr>
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<td colspan="17">Header Offset</td>
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</tr>
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<tr>
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<td></td>
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<td>0</td>
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<td>1</td>
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<td>2</td>
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<td>3</td>
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<td>4</td>
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<td>5</td>
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<td>6</td>
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<td>7</td>
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<td>8</td>
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<td>9</td>
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<td>A</td>
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<td>B</td>
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<td>C</td>
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<td>D</td>
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<td>E</td>
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<td>F</td>
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</tr>
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</thead>
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<tbody>
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<tr>
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<td><b>0</b></td>
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<td colspan="4">Magic = F1FFFF1F<sub>h</sub></td>
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<td colspan="4">Num magic</td>
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<td colspan="4"><code>n</code></td>
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<td colspan="4"></td>
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</tr>
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</tbody>
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</table>
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<p>
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Num magic will be F1FFFF1F<sub>h</sub> in the first sector, F2FFFF2F<sub>h</sub> in the second, and F3FFFF3F<sub
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>h</sub
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>
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in the third.
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</p>
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<p>
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There is, however, one extra curveball. One of these three sectors is a dummy sector that contains nonsensiacal data
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(in practice this appears to just be nulls). This is the sector indicated by the lower byte of the
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<code>Dummy number</code> field in the alpha header. It is essential that this header is <b>not</b> valid.
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</p>
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<p>
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Assuming we pass these checks, αDVD authentication succeeded. The disc will now be be read as usual, applying
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the data offset from the alpha header before any operations. Coindidentally if an ISO image has been made of an
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alphaDVD (which will be unable to pass authentication anyway), all sectors preceeding this offset can be stripped,
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and the ISO now matches that of a non-alpha disc.
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</p>
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{% endblock %}
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