Compressed data vulnerable to known-plaintext?

[Mark M.]

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On Sun, 2 Jun 1996, Anonymous wrote:

> Someone who claimed to be David F. Ogren said on Sat, 1 Jun 1996:
> 
> > > The brute force system decrypts the first, and second blocks (8
> > > bytes each) of the cyphertext, XORs them, and compares the result
> > > with "PKZIP2.1".  If the comparison is equal it has the key.
> 
> > I will concede that having a known header, such as a PKZIP header,
> > does weaken a crypto to certain degree, but I still believe that it is
> > not a significant problem.  Here's why:
> 
> Why not simply use two session keys, and encrypt the headers with one 
> while encrypting the actual data with the other? That seems to solve both 
> problems, except that more CPU cycles are required.

An easier solution would be to just strip of the headers.  If the header is
always the same, then it is redundant.  If it varies, then it cannot be used
as known-plaintext.

- -- Mark

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