[rc5] key security question

Henry W Miller mill0440 at gold.tc.umn.edu
Thu Jun 5 13:34:17 EDT 1997



On Thu, 5 Jun 1997, Benedikt Eric Heinen wrote:

> methods (neither do I have the mathematical knowledge to understand them),
> but I am curious - what happens when I encrypt some plaintext using 56bit
> RC5 and then encrypt the result with another RC5 56bit key.

They do this with DES all the time.  TDES (tripple DES) is considered the 
strongest encryption avaibal.  I think they use 3 different keys for 
encryption

> How secure will the resulting file be? How do those keys work? 
> 
>   - Is there still a single 56bit key that can decrypt the file (i.e.
>     the second encryption doesn't give any additional security)?

not unlesss there is a secerity hole in RC5 that would make it 
unnessicary to search the entire keyspace to find a key.  This is 
possibal, and RSA admits that RC5 is new and needs more experts looking 
at it before they can trust it fully, though so far it has held up to 
experts.

>   - Do the bits just add up (i.e. the resulting ciphertext will be 
>     like it was encrypted using a single 112 byte key)?

I think so, but I've not done any accual math to be sure.  there might be 
an additionaly effect that someone who knows you are using RC5 might not 
realise you are doing the double encryption at fisrt and may start off on 
teh wrong track, but this is not something to count on.

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