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PORandom access encryption with AES In Counter mode using Fortuna PRNG:
 

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    <p>I'm building file-encryption based on AES that have to be able to work in random-access mode (accesing any part of the file). AES in Counter for example can be used, but it is well known that we need an unique sequence never used twice. Is it ok to use a simplified Fortuna PRNG in this case (encrypting a counter with a randomly chosen unique key specific to the particular file)? Are there weak points in this approach? </p> <p>So encryption/decryption can look like this</p> <p>Encryption of a block at Offset:</p> <pre><code>rndsubseq = AESEnc(Offset, FileUniqueKey) xoredplaintext = plaintext xor rndsubseq ciphertext = AESEnc(xoredplaintext, PasswordBasedKey) </code></pre> <p>Decryption of a block at Offset:</p> <pre><code>rndsubseq = AESEnc(Offset, FileUniqueKey) xoredplaintext = AESDec(ciphertext, PasswordBasedKey) plaintext = xoredplaintext xor rndsubseq </code></pre> <p>One observation. I came to the idea used in Fortuna by myself and surely discovered later that it is already invented. But as I read everywhere the key point about it is security, but there's another good point: it is a great random-access pseudo random numbers generator so to speak (in simplified form). So the PRNG that not only produces very good sequence (I tested it with Ent and Die Hard) but also allow to access any sub-sequence if you know the step number. So is it generally ok to use Fortuna as a "Random-access" PRNG in security applications?</p> <p>EDIT:</p> <p>In other words, what I suggest is to use Fortuna PRNG as a tweak to form a tweakable AES Cipher with random-access ability. I read the work of Liskov, Rivest and Wagner, but could not understand what was the main difference between a cipher in a mode of operation and a tweakable cipher. They said they suggested to bring this approach from high level inside the cipher itself, but for example in my case xoring the plain text with the tweak, is this a tweak or not?</p>
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