Re: Brute Force attack Question

["Mark M." <markm@voicenet.com>]

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On Wed, 24 Jul 1996, Igor Chudov wrote:

> Suppose Alice sends letters to BoB, and they always exchange plain 
> text ASCII data. Suppose also that they use DES for encryption.
> They are afraid that Perry intercepts their messages and tries to brute
> force their DES key.
> 
> Perry has 100,000 computers (and 20,000 couriers alone:) and his brute
> force attacks are as follows: he tries all keys in succession, looks at
> the decrypted texts, and *if* the decrypted text looks like a potential
> message (has only ASCII characters for example) he looks at that key closer
> as it is likely that he has found the right key.
> 
> What is Alice and Bob decide to obscure their letters and add random
> NON-ASCII  characters at random places? They may agree to just ignore
> all non-ASCII characters, so these characters would never change the
> meaning of their letters. If they do that, Perry does not have any easy
> way to tell whether he really recovered the right plaintext or not, because
> even correct key would still produce a lot of non-ASCII characters.
> 
> If percentage of ASCII characters in all 256 byte space is 40%, Alice
> and Bob may agree to put in junk characters to make up exactly 60% of
> the message. This way messages will look like random character data.
> 
> Is there any good method for attackers to circumvent this obscurity?
> What is the general method to make a judgment whether the recovered
> text really is a plain text if Alice and Bob noisify their letters?
> 
> I can think of this: we sift through all recovered plaintexts and remove
> all non-ASCII bytes, and then do some simple testing to see whether
> the remaining ASCII data resembles normal English texts. This kind
> of testing seems to be quite expensive though, compared to just testing
> for ASCII vs. non-ascii bytes. Anything else I am missig?

If the attacker uses a known-plaintext attack, then all this is a non-issue.
However, if the attacker is using a ciphertext-only attack, looking for the
MSB to be 0 is a good way to find a correct decryption.  Also, randomness tests
could be run on recovered plaintexts.  This is why compression before
encryption is a good idea.  If the plaintext is completely random, then there
is no way to crack the ciphertext.

- -- Mark

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