3 ms·

If you find this kind of stuff interesting, take Dan Boneh's Cryptography class on Coursera. It covers issues like this very well... https://www.coursera.org/c

• andrewcooke 14y ago
  as i said, i think i understand CTR mode. or thought i did. i was more curious about how you could seek to a particular counter value. and then was even more confused by the idea that neither nonce nor counter can be repeated, as i thought it was the combination that had to be unique. so maybe i do need to take that course. hmmm.
  • tptacek 14y ago
    There are systems that use CTR mode as a way to do "random access" bulk encryption, because Schneier suggested that in both his major crypto books. The specific exploitable condition is indeed the recurrence of a specific nonce/counter tuple; the point is, there are systems in which attackers can induce that condition, as opposed to simply having the system blunder into it (for instance, by using the same nonce every time).
    • andrewcooke 14y ago
      ah, ok, thanks. "random access" makes everything much clearer.
      • tptacek 14y ago
        You end up with similar problems when you try to use CTR with 64 bit block ciphers, like Blowfish and DES-EDE --- both of which are very common.