I wrote a comment about this a year and a half ago, and I'm just go to re-paste it verbatim here:

You want sick? Here's sick.

An AES block is 16 bytes long. You want to encrypt the lyrics to "Yellow Submarine". You can't just repeat AES for each 16 byte block, because then you can tell where the words "yellow submarine" --- 1 AES block --- occur: they'll encrypt to the same block. So you use CBC mode, XOR'ing each plaintext block to the previous ciphertext block, creating a unique ciphertext every time you encrypt those lyrics. But "Yellow Submarine" is 902 characters long; the last block is 6 bytes short. So you pad the message which a count of the pad characters --- 06h 06h 06h 06h 06h 06h.

With me so far?

Decrypt the message. Check the padding: it should repeat as many times as the value of the last byte. If not, you decrypted badly. Send an error to that effect.

Hey guess what! I can decrypt parts of your message!

If I stick a random AES block in your message, it will scramble the padded block. Every once in awhile, that scrambled message will end in 01h. 01h is valid padding. The message will be gibberish, but you at least won't tell me it had bad padding.

You wouldn't tell me if you decrypted to bad padding if I generated 02h 02h either. But that's much less likely than 01h.

Now I know that your plaintext XOR my random block produces a last byte of 01h. Solve.

Now my point: this is actually way old news --- back to Bleichenbacher and Vaudenay and probably before that (I'm not the resident expert) --- but attacks like this have broken peer-reviewed versions of TLS, the most peer-reviewed crypto protocol ever designed.

Don't build crypto.

Some quick back-story

Vaudenay described this attack almost 10 years ago, in this paper:

http://lasecwww.epfl.ch/php_code/publications/search.php?ref...

It's very readable for a crypto paper.

We blogged about padding oracles in our AES post 2 years ago.

Then Thai Duong and Juliano Rizzo independently re-discovered them and went on a tear looking for software vulnerable to the flaw. Just a few months ago, they busted up Java Server Faces with the exact same vulnerability.

The nature of the attack is this: in almost all modern web frameworks, session storage can be stored "clientside" in fat cookies. These cookies are encrypted so that users can't modify their contents. Because of this vulnerability, attackers can take legitimate fat cookies, decrypt them, and (often) modify their contents. Those attackers can then (say) issue fraudulent payments, change their user ID, alter their account balance, or inject SQL or Javascript into code that never expected to be exposed to attack because it was guarded with AES.

This vulnerability is also a galactic pain in the ass to address:

* It exploits the common-case error handling behavior of most web stacks (generate exception, do something that visibly indicates to the user that an exception was generated)

* It's not enough just to stifle the errors; you have to make observable behavior in the bad-padding case identical to the good-padding case.

* If you're MAC'ing packets as well as encrypting them, you also have to make sure the timing is the same in the bad-padding and good-padding case, which means you have to "pretend" to MAC the bad-padding packets, which is extremely counterintuitive (you have to catch an exception then pass "something" to a MAC verification routine).

Some food for thought

It's 2010, vulnerability research is no longer a cottage industry (it's hundreds of millions annually), and we're just now hearing about exploits of one of the simplest crypto flaws there is (there is almost no math to this one).

Software security people simply don't read crypto papers.

Crypto researchers, at least until recently, don't break software.

God only knows how much other scary stuff is buried in those papers waiting for an enterprising bug hunter to try them out.