Re: MITM attack on 3TDES

["Saqib Ali" <docbook.xml () gmail com>]

shahin,
can you please elaborate on what you mean by that.

thanks


On 9/12/06, Shahin Ansari <zohal52 () yahoo com> wrote:
> I believe shorter effective key lenght is related to fact that all attacks
> are a combination of known text in addition to the effort to crack the key.
> Therefore the effective key lenght is shorter than the size of key space.
>
> Alexander Klimov <alserkli () inbox ru> wrote:  On Wed, 6 Sep 2006, Saqib Ali
> wrote:
>
> > Can anyone explain how the MITM works on 3TDES (three distinct keys)?
> > I am typically interested in finding out why 3TDES has effective
> > key-length of 112-bit
>
> Get a (plain text, cipher text) pair (m,c) encrypted with some unknown
> key (k1,k2,k3):
>
> s = E(k1,m)
> t = E(k2,s)
> c = E(k3,t)
>
> For all possible (k1,k2) pairs (2^112 possibilities) calculate
>
> t' = E(k2,E(k1,m))
>
> For all possible k3 (2^56 possibilities) calculate
>
> t'' = D(k3,c)
>
> Sort the sets of t' and t'' and find (k1,k2) and k3 such that t'=t''
>
> Check each such (k1,k2,k3) with several additional (plain text, cipher
> text) pairs to find the right key.
>
> --
> Regards,
> ASK
>
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Saqib Ali, CISSP, ISSAP
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EARN A MASTER OF SCIENCE IN INFORMATION ASSURANCE - ONLINE
The NSA has designated Norwich University a center of Academic Excellence 
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education and the case study affords you unmatched consulting experience. 
Using interactive e-Learning technology, you can earn this esteemed degree, 
without disrupting your career or home life.

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