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POTips for successful decryption
 

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    <p>I have an executable that decrypts binary data input by user. My task is restoring the decryption algorithm without seeing the source code of the executable. I've already done about 50 attempts of decrypting, and judging by results the algorithm is obviously easy (XOR-like, but changing a byte of the input data usually affects two bytes of the output), and length of the key is definitely 16 bytes, but I still cannot restore it.</p> <p>So, my question is: Are there any tips of successful restoring the decryption algorithm? Some guidelines or any other help will be greatly appreciated.</p> <p>Here are some real examples, I can see the regularity there, but I cannot understand the whole algorithm. If I enter more than 16 bytes (e.g. 17 zero bytes), the output byte sequence repeats. The left part is what I enter, the right is what I get (everything is hex):</p> <pre><code>00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 -&gt; 73 36 15 B9 95 5D 8E EA 01 72 1C F4 FA 76 AA 65 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 01 -&gt; 94 36 15 B9 95 5D 8E EA 01 72 1C F4 FA 76 AA 66 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 10 -&gt; 93 36 15 B9 95 5D 8E EA 01 72 1C F4 FA 76 AA 75 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 11 -&gt; 83 36 15 B9 95 5D 8E EA 01 72 1C F4 FA 76 AA 76 00 00 00 00 00 00 00 00 00 00 00 00 00 00 01 00 -&gt; 83 36 15 B9 95 5D 8E EA 01 72 1C F4 FA 76 A9 64 00 00 00 00 00 00 00 00 00 00 00 00 00 00 10 00 -&gt; 94 36 15 B9 95 5D 8E EA 01 72 1C F4 FA 76 BA 55 00 00 00 00 00 00 00 00 00 00 00 00 00 00 11 00 -&gt; 94 36 15 B9 95 5D 8E EA 01 72 1C F4 FA 76 B9 54 00 00 00 00 00 00 00 00 00 00 00 00 00 01 01 00 -&gt; 94 36 15 B9 95 5D 8E EA 01 72 1C F4 FA 75 A8 64 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 02 -&gt; 73 36 15 B9 95 5D 8E EA 01 72 1C F4 FA 76 AA 63 </code></pre> <p>P.S. This is not a "hacking" in the common sense. I'm writing a 3D model viewer for an unfamous format (.bmd), but some of the models are encrypted, so I want to implement my own internal decrypter instead of using an external executable.</p>
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    1. COHave any examples for inputs with hex values other than 1 and 0? What does `00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 02` give you?
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    2. CO@Null Set, yes, sure. It gives me `73 36 15 B9 95 5D 8E EA 01 72 1C F4 FA 76 AA 63`, I'll add this to the examples list.
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    3. COAn excellent object lesson in not inventing your own ciphers, because someone (in this case @Angel and the SO community) will break them. ;)
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      1. USNick Johnson
 

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