🚀 go-pugleaf

In article <2611@ecsvax.UUCP>, hes@ecsvax.UUCP (Henry Schaffer) writes:
> In article <682@rpics.RPI.EDU>, yerazuws@rpics.RPI.EDU (Crah) writes:
> > In article <434@ethz.UUCP>, wyle@ethz.UUCP (Mitchell Wyle) writes:
> >  ...  	
> > Your big problem is going to be a known-plaintext spoofing attack.  
> > If you can find out (or guess, because you've seen the plaintext of
> > several other transmissions and KNOW they always start with the
> > date, time, city of origin and sender's full name and title, in standard
> > format)...
>  
>   I think that this is even harder than this indicates.  A "good guess" is
> not sufficient, because *any* guess will give a "key" for that block, and
> there is no way to tell if the guess/key is correct.  So you have to have
> the *exact* plaintext.
> 

What I mean by a good guess is one that gives you a high probability of 
being exactly correct- hence your spoof message is decrypted correctly 
and believed authentic.

> > spoofing, and known-plaintext attacks.  The probability of a known-plaintext
> > (or chosen-plaintext ) spoof succeeding (because of the way the interior 
> > checksum was generated) was one chance in 2^32.
>   This only works if the interior checksum is generated by a method which the
> interceptor can't duplicate.


Exactly.  For example, use a pair of 32-bit words from the keytape.  One word
specifies the initial condition of the CRC register, and the second is 
XORed into the CRC output just before concatenation of that register 
onto the text to be transmitted.  Given either of the 32-bit words, 
I can calculate what the other should have been to give the observed 
cyphertext stream- but this still only cuts me down to an ensemble of 
2^32 pairs of possible words, only one of which will give the "right" 
cyphertext for all plaintext messages, the other 2^32 - 1 will work 
for the specific known plaintext but will (with high probability, given
a good CRC) fail on other plaintexts.  So, the algorithm can be public
and only the key tape needs security.  Knowing two block's wordpairs 
doesn't help, because they were interior-checked by two different pairs
of words.

> >      .........................  Read a text on information theory
> > (such as Pierce's _Symbols, Signals, and Noise_ and you'll understand))
>  ...
>   Agreed.
 
Definitely.  A good book is something truly worthwhile.

> > 	
> > 	-Bill Yerazunis
> 
> --henry schaffer  n c state univ

	-Bill Yerazunis
 
	"NSA food?  Here?  Why bother?"