On 06/05/2011 15:52, Le_Forgeron wrote:
> Le 06/05/2011 15:16, Invisible a écrit :
>>> DES was used as an illustration. Moreover, the 3*56 bits of 3DES have a
>>> lower entropy (so, 3*56 bits is *not* 168, at least in cryptography) due
>>> to issue in the model.
>>
>> How do you work that one out?
>
> IIRC, it's part of the discussion in the SSH RFCs about the number of
> secure bits.
> Or it was in a SSH book. 3DES should actually be considered insecure, if
> it was not for the administrations of USA to have it deployed a lot.
>
> The wiki about 3DES in french states that 3*56 is 112 due to m-i-t-m
> attack. The english version states the same, with added comment about
> NIST reducing the actual value to something around 80.

Right. So what you're saying is that due to the possibility of a meet in 
the middle attack, 3DES is less secure than a well-designed 168-bit 
block cipher.

This is different from saying "a 3DES key has less than 168 bits of 
entropy". Entropy is a specific property of a piece of data, and a 
properly chosen 3DES key would indeed have 168 bits of entropy. What 
you're saying is that not all of that entropy actually counts towards 
the difficulty of breaking the cipher, which is a different thing.

Also: Yes, DES is obsolete, and even 3DES shouldn't really be used any more.

Three guesses which cipher our VPN tunnel uses...

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