0
$\begingroup$

An attacker is equipped with a computer that performs 10 trillion (1013) DES decryptions per second, what is the average time required, in hours, for a brute force attacker to break DES? Key size is 56 bits.

I tried 256/1013 divided by 3600 but the answer is incorrect.

$\endgroup$
  • 5
    $\begingroup$ $2^{56}/10^{13}$ is incorrect for three reasons: a) it does not account for the "average" word of the problem statement. b) it does not account for the "hours" word of the problem statement. c) nitpick: it does not account for the "time" word of the problem statement, for lack of unit: if it not asked how many hours are required... Second rule of sucess in exams: in a good problem, every word counts (first rule is: attend). $\endgroup$ – fgrieu Apr 30 '20 at 13:28
  • $\begingroup$ Is 'Bruce Force' done by comedic Australians? $\endgroup$ – dave_thompson_085 May 2 '20 at 1:56
0
$\begingroup$

You got $2^{56}$ different keys and the attacker can test $10^{13}$ keys per second.

In the worst case, the last tested key is the solution. For that you need $2^{56} / 10^{13} \approx 7205$ sec. -> around $2$ hours.

In the best case, the first tested key is the solution. This takes about 1 second.

On Average you need to test half of the keys. That means: $0.5 * (2^{56} / 10^{13}) \approx 1$ hour.

$\endgroup$
  • 2
    $\begingroup$ Redo your calculations, $2^{56} / 10^{13} \not\approx 8796093022208$ $\endgroup$ – poncho May 1 '20 at 14:03
  • $\begingroup$ also if he can test $10^{13}$ keys/second how come it takes a full second when the first key tested is the correct key? $\endgroup$ – kodlu May 1 '20 at 23:49
  • $\begingroup$ That's why i wrote "about 1 sec." You can lower that time to nearly 0 sec. But for calculating the average case, it makes no different. $\endgroup$ – Titanlord May 2 '20 at 14:18

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.