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I want to write AES from scratch without using the built in libraries of java. I know there are hundreds of AES programs (and websites) out there but I can't seem to find a site that gives me the details I think I would/should need to write my own Java program.

I know that AES has S-boxes like DES has. Were would I look for them? Also, I would like more info on the AES "counter mode".

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  • $\begingroup$ Just a note: For productive use, you often want to use existing implementations instead of creating your own, since they probably are better shielded against side-channel attacks, and might also be more efficient (e.g. might use AES-specific instructions on modern processors). For learning about the internal workings it is a good idea to implement it yourself, though. $\endgroup$
    – Paŭlo Ebermann
    Commented Sep 29, 2011 at 23:50
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    $\begingroup$ For reference, a Java implementation of FIPS AES can be found in the Bouncy Castle open source project: bouncycastle.org/specifications.html $\endgroup$
    – Gary
    Commented Sep 30, 2011 at 15:24
  • $\begingroup$ My python implementation of AES is supposed to be very easy to read, and it's got a good few comments. For what it's worth, it might help you or someone else implementing aes: code.google.com/p/zs-crypto/source/browse/zs/cipher/aes.py. A tip: read this code together with the wikipedia articles on AES. Most of the maths turns out to be really straightforward. $\endgroup$
    – Stefano Palazzo
    Commented Nov 9, 2011 at 9:17

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The AES standard is reasonably clear, and (theoretically) sufficient to implement the AES itself. For block cipher modes of operation (including counter mode), see NIST Special Publication 800-38A.

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One thing I would like to add to Thomas's answer: when implementing a cipher, it's useful to have a test vector with the cipher intermediate states (if you get it right the first time, you don't need it; if you get a detail wrong, it makes finding that detail a lot easier).

The document http://csrc.nist.gov/publications/fips/fips197/fips-197.pdf has all those intermediate states (both for the key expansion and the actual block encrypt operations; the decrypt is just a reversal of the encrypt operations, so they don't need to spell it out explicitly).

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  • $\begingroup$ While I do thanks you for the help. Thomas Pornin first link is the same as the one you posted. $\endgroup$
    – Dennis Hayden
    Commented Sep 29, 2011 at 15:42
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    $\begingroup$ Oops, I missed that his 'The AES standard' was a link... $\endgroup$
    – poncho
    Commented Sep 29, 2011 at 20:51
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In the Rijndael proposal document a clear explanation of the possible implementations is given. It was later completed by D. J. Bernstein (https://cr.yp.to/aes-speed.html) with further optimizations. However as said before, be careful of the possible side channel attacks which may applied to your implementation.

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