http://cr.yp.to/talks.html#2005.06.01 has slides that people might find
useful as an overview of what's going on. In particular, there's a list
of six obstacles to performing array lookups in constant time. People
who mention just one of the obstacles are oversimplifying the problem.

"Hal Finney" writes:
> The one extra piece of information it does return is the encryption of an
> all-zero packet.  So there is a small element of chosen plaintext involved.

Known plaintext, not chosen plaintext. I used timings to identify 105
key bits and then compared the remaining 2^23 keys against a known
plaintext-ciphertext pair, namely the encrypted zero that you mention.

One can carry out the final search with nothing more than known
ciphertext: try decrypting the ciphertext with each key and see which
result looks most plausible. It should even be possible to carry out a
timing attack with nothing more than known ciphertext, by focusing on
either the time variability in the last AES-encryption round or the time
variability in the first AES-decryption round.

Of course, most applications have some known plaintext, and some
applications allow chosen plaintext, so even a chosen-plaintext attack
is considered to be a fatal flaw in a cryptographic standard. The user
isn't supposed to have to worry that someone who influences part of the
plaintext will be able to read all the rest.

---D. J. Bernstein, Associate Professor, Department of Mathematics,
Statistics, and Computer Science, University of Illinois at Chicago

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