Finding encrytion algorithm

[Mike Rosing]

On Thu, 11 Jul 2002, gfgs pedo wrote:

> suppose a cryptanalysis only has encrypted data-how is
> going 2 know which is the encrytion algorithm used 2
> encrypt the data ,so that he can effeciently
> cryptanalyse if
>
> 1:>he has large amount of cipher text only
> 2:>has large amount of plain text and corresponding
> cipher text.
>
> There r so many encryption algorithms,how does he know
> which algorithm was used?

Depends on how they got the source.  They may know it's one of 5
possible choices because of the person who sent (or received) it.
If it's just found on a disk in a garbage dump with no connections
to anyone, it's a bit tougher.  But every algorithm has some statistical
signature and if you've got enough cipher text you can compare that
signature with known algorithms to home in on fewer choices.

I'm not sure having the plaintext helps much more, but you could
use random keys to create several ciphertexts with known algorithms and
compare the statistics just to see if they compare better.

It's definitly challenging :-)

Patience, persistence, truth,
Dr. mike