[slver]

It's a challenge, but I'd probably look into any decoders that parallelize the decoding and see if I can abuse this.

[wizzwizz4]

You'd need to exploit an implementation bug, or have the user be streaming or something (so some squares appear before others as the output of your timing attack).

[tsegratis]

Just wait, its only a short while til JPEG contains the pdf spec and a javascript interpreter