[cryptography] The Unbreakable Cipher

Collin RM Stocks collin at sibilance.org
Tue Oct 1 09:54:55 EDT 2013

On 09/25/2013 03:51 PM, Jonathan Katz wrote:
> On Wed, Sep 25, 2013 at 1:30 PM, Greg Rose <ggr at seer-grog.net
> <mailto:ggr at seer-grog.net>> wrote:
>     On Sep 25, 2013, at 9:40 , Jonathan Katz <jkatz at cs.umd.edu
>     <mailto:jkatz at cs.umd.edu>> wrote:
>      > "Every cipher is breakable, given enough traffic": in principle,
>     yes, as long as the traffic (formally, the entropy of the traffic)
>     is larger than the key length.
>     You misstated this. It's breakable if the *redundancy* of the
>     traffic is larger than the key length.
> Not so; this is most easily seen by taking the uniform distribution over
> n-bit messages, in which case the entropy is n and the redundancy is 0.
>     regards,
>     Greg.
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If the message is chosen from a uniform distribution over n bits (and 
assuming that the message is not used for something else after it is 
deciphered), the adversary will not be able to distinguish a correctly 
deciphered message from an incorrectly deciphered message, no matter how 
short the key is in comparison to the data.

Now, you could easily argue that there is absolutely no reason to send a 
message with those properties, but that isn't really the point.


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