Composing large block cipher from smaller ones
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From: J.D. on 6 Mar 2010 15:49 On Mar 6, 3:38 pm, Mok-Kong Shen <mok-kong.s...(a)t-online.de> wrote: > Am 06.03.2010 21:21, schrieb J.D.: > > >> As layman... > > > You know, there's a cure for your condition. And it doesn't involve > > laying in bed, asking us to spoon-feed you for the rest of your > > life... > > Should I behave like a few of others in this group in always attempting > to (undeservedly) present oneself as an 'expert'? > > If you have concrete 'objective' scientific arguments, then please > argue. But to do personal attacks unnecessarily wastes the bandwidth of > the group and is a very 'mean' conduct in my personal view. > > M. K. Shen > > ------------------------------------------------------------------------ > [A second time:] > > My favourite citation for scientific discussions: > > Was sich ueberhaupt sagen laesst, laesst sich klar sagen; > > und wovon man nicht sprechen kann, darueber muss man schweigen. > > L. Wittgenstein You seem to be under the impression that I have called you an idiot. I'm not calling you an idiot. I am calling you ignorant -- something that you admit that you are every time you call yourself a "layman". Ignorance is curable. Stupidity isn't. If you have a problem with people calling you ignorant I would suggest there is a very simple and elegant solution...
From: J.D. on 6 Mar 2010 15:59 On Mar 6, 3:41 pm, Mok-Kong Shen <mok-kong.s...(a)t-online.de> wrote: > J.D. wrote: > >>> Quite obvious: DES has only 56 bit key, and you're going to us it in > >>> CTR more. So where is the problem, just try all the keys. > > >> The point argued was whether to crack the "composite" ciphe, consiting > >> of u DES, needs "only slightly longer" than cracking one single DES. > >> As I said, that needs at least u times at much work. (In the now > >> revised version, one uses u DESs, that from u keys generate the keys > >> for the corresponding component DESs in the diverse rounds.) > > That's exactly the version he was talking about. Does this really > > need to be spelled out for you even more clearly? > > So where is the foundation of "your" 'only slightly longer'?? We were > talking about the entire composite cipher! > > M. K. Shen If you use DES --initialized with a Master Key and operating in Counter Mode-- to generate the keys for each round, then to break this scheme all you have to do is guess the Master Key (and for each guess, use it in counter mode to find the potential round-keys, and then test the potential round keys to see if they properly encrypt/decrypt the block). This is slightly harder than to break DES alone because for each guess of the Master Key you have to spend the time to generate all the round keys and then use them in the longer/larger cipher. That might get you an extra few bits of strength (as a form of key- strengthening). But it doesn't substantially increase the difficulty of a brute force attack.
From: Mok-Kong Shen on 6 Mar 2010 15:58 J.D. wrote: > You seem to be under the impression that I have called you an idiot. > I'm not calling you an idiot. I am calling you ignorant -- something > that you admit that you are every time you call yourself a "layman". > Ignorance is curable. Stupidity isn't. If you have a problem with > people calling you ignorant I would suggest there is a very simple and > elegant solution... You don't need to (and shouldn't) analyse at all other people's personality in discussions of this group. This is a sci-group. If you consider the stuff in a post you see to be scientifically at a level that you appreciate, then join in. Otherwise, simply ignore it. Optimize your precious time!! (I hope that you on later occasions would remember this.) M. K. Shen
From: Mok-Kong Shen on 6 Mar 2010 16:07 J.D. wrote: > If you use DES --initialized with a Master Key and operating in > Counter Mode-- to generate the keys for each round, then to break this > scheme all you have to do is guess the Master Key (and for each guess, > use it in counter mode to find the potential round-keys, and then test > the potential round keys to see if they properly encrypt/decrypt the > block). This is slightly harder than to break DES alone because for > each guess of the Master Key you have to spend the time to generate > all the round keys and then use them in the longer/larger cipher. > That might get you an extra few bits of strength (as a form of key- > strengthening). But it doesn't substantially increase the difficulty > of a brute force attack. The compsite cipher has u component DESs. The keys for each component DES in the different rounds is generated by one DES with a master key (different from the others). So the total effort to crack the composite cipher is at least u times (if not much more) of that required to crack one (simple, i.e. not composite) DES. u times is u times, it's certianly not 'only slightly longer'. M. K. Shen
From: J.D. on 6 Mar 2010 16:33
> You don't need to (and shouldn't) analyse at all other people's > personality in discussions of this group. Ignorance is not a personality trait. At least I hope it's not... >This is a sci-group. > If you consider the stuff in a post you see to be scientifically at > a level that you appreciate, then join in. Otherwise, simply ignore it. > Optimize your precious time!! (I hope that you on later occasions > would remember this.) > > M. K. Shen I am inviting you to cure your ignorance. Learning about cryptography is a tremendously rewarding journey, one that I enjoy every day and that I hope to enjoy for many years to come. It is certainly much more enjoyable than being dismissively lectured and mocked by faceless people on a newsgroup (and more rewarding than trolling, if --as some here believe-- that is what you are up to). So stop being a layman (and/or a troll)! Go forth, and educate thyself! And do not worry. I shall do as you wish and ignore your posts from now on... |