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AES Details
Uses Substitution/Permutation Network vs. Feistel Network
No known attacks other than side-channels (timing, heat, electrical consumption)
Recently (2003) approved for classified work up to SECRET on 128 bit keys, and TOP SECRET on 192 and 256 bit keys (HW must pass normal NSA certification)
Notes:
Side channel attacks can be very effective, resulting in a break in as little as 65 ms. See http://www.wisdom.weizmann.ac.il/~tromer/papers/cache.pdf and http://cr.yp.to/papers.html#cachetiming
See http://www.cnss.gov/Assets/pdf/cnssp_15_fs.pdf for details of approval for use.