[rc5] key security question

Henry W Miller mill0440 at gold.tc.umn.edu
Fri Jun 6 08:55:56 EDT 1997



On Thu, 5 Jun 1997, Sean Reifschneider wrote:

> Let's talk some more reasonable numbers here.  What if we were saying
> RC5-8+RC5-8?  Saying that it's 16-bit encryption is VERY incorrect.
> If you encrypt twice with RC5-8, worst case you would have to BREAK
> the code twice.  Breaking it once would take searching 256 keys.
> Breaking it twice would take (worst case) 256+256 key searches (512).
> This is a *LONG* way off 65K key searches that would be required with
> RC5-16.

Yes you only have two codes to break.  BUT part of the definition of a 
good encryption algrythm is the output CANNOT be told from random 
garbage.  In particular the output should not be compressable.  So what 
happens is you try a key, get some random garbage, and you have to try 
all possibal keys on the restulr of the first key before discarding it.  
This you try a second key, and then all possibal keys on the result of 
the second key.  When you try the 234th key (which happens to be the key 
for this discussion) you have no way of knowing that you have the right 
first key until you try all possibal keys on it and hit on a solution.

> 3DES does not give 3x the protection of DES -- from what I recall it's
> something more like 1.3x the protection from what I recall.

I've read some books labeled "not for export" that claim TDES is better 
then IDEA, and the most secure encryption avaibal.  I'm not going to go 
into more details due to the international nature of this list.  (When I 
break the law I'm going to at least spread something useful, this 
argument isn't useful)

> 256-bits is enough to uniquely identify every atom which makes up
> the earth...  We're talking REAL big numbers here.

Someone told me the entire universe.  I don't feel like getting out my 
physics books and doing the calculations.  
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