🚀 go-pugleaf

In article <1992Nov5.000740.1755@qiclab.scn.rain.com>, leonard@qiclab.scn.rain.com (Leonard Erickson) writes:
> gilchr@ee.ualberta.ca (Andrew Gilchrist) writes:
> 
> 
> >What are theoretic ways to deter a plaintext attack?
> 
> >i.e. The opponent is given two ciphertext messages, the plaintext of
> >the first message, and the encryption method.
> 
> 
> >Now, obviously the longest key in the world will not help.
> 
> >How does one protect the key, so that it can be reused.
> 
> You *can't*.
> 
> Reusing keys *will* compromise the key if the enmey has the known
> plaintext. 

That's not true at all.  Rather, it's not true for most types of
cryptosystems.  Consider DES -- as far as the non-classified world
knows, it's immune to known-plaintext attacks.  Even if you run DES
in output feedback mode, generating an XOR stream similar to what
the original poster asked about, the *key* is the 56 bits you fed
into the DES engine, not the output from DES acting as a cryptographically
strong random number generator.