Re: MITM attack on 3TDES

[Alexander Klimov <alserkli () inbox ru>]

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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