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    <p>Both, I suspect. The benefit is fairly minimal.</p> <p>You have forgotten about the runtime cost of acquiring or generating cryptographic-quality random numbers. at one extreme, when a finite supply of randomness is available (/dev/random on some systems for instance), your code may have to wait a long time for more random bytes.</p> <p>At the other extreme, when you are getting your random bytes from a PRNG, you could expose yourself to problems if you're using the same random source to generate your keys. If you're sending encrypted data to multiple recipients one after another, you have given the previous recipient a whole bunch of information about the state of the PRNG which will be used to pick the key for your next comms session. If your PRNG algorithm is ever broken, which is IMO more likely than a good plaintext attack on full AES, you're much worse off than if you had used deliberately-deterministic padding.</p> <p>In either case, however you get the padding, it's more computationally intensive than PKCS#5 padding.</p> <p>As an aside, it is fairly standard to compress potentially-repetitive data with e.g. deflate before encrypting it; this reduces the redundancy in the data, which can make certain attacks more difficult to perform.</p> <p>One last recommendation: deriving the key with a mechanism in which only the username and password vary is very dangerous. If you are going to use it, make sure you use a Hash algorithm with no known flaws (not SHA-1, not MD-5). cf <a href="http://it.slashdot.org/story/11/01/13/2024237/Amazon-EC2-Enables-Cheap-Brute-Force-Attacks" rel="nofollow">this slashdot story</a></p> <p>Hope this helps.</p>
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    1. POIs there a benefit to padding with random data in AES encryption?
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    1. USPhil Willoughby
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    1. COExcellent points. I think I'll save my cryptographic-quality random numbers for the key and IV side. I'm looking at PBKDF2 for key derivation, but it uses SHA-1.
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    2. COPBKDF2 is pluggable by design; you can in principle use something stronger such as SHA-256 (or SHA-3 once we know what that is). Of course an individual PBKDF2 implementation may not allow you to specify the pseudorandom function to use.
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    3. COI ended up using the PBKDF2 package for Python (http://www.dlitz.net/software/python-pbkdf2/) that can use a different hashing function, and I'm using SHA-256. After some experimentation, I'm happy to use compression and PKCS#5-style padding, and save the strong random number generator for the key side of things.
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