[RC5] RC5 question

Décio Luiz Gazzoni Filho decio at revistapcs.com.br
Fri Oct 3 19:10:33 EDT 2003


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> Hi,
>  I'm fairly new to Distributed.net; I started this summer after having done
> SETI for a few years, so I don't know if this was ever brought up before. I
> was just wondering if RC5 will ever go up to the 128 bit secret key
> contest? This could be more useful, since most sites use 128 bit security
> today. I don't know if there are encryption laws anymore; I thought that
> they got rid of them a few years ago, but even if there were, you could
> restrict the contest to American users only, couldn't you?

The problem is not export restrictions or anything (they've been lifted, as 
far as I know); it's just the sheer amount of work that has to be done. A 
book of mine states that since the inception of the computer through the year 
2000, there have been approximately a mole (6e23) of computer operations 
performed on all computers on Earth. In comparison, there are 3.4e38 RC5-128 
keys to check. Even taking into account that only 50% of keyspace is expected 
to be checked on average, that's 1.7e38 keys, or 1.7e40 cycles. Dividing out 
1.7e38 by 6e23, you get a number which is on the order of magnitude of a 
million times the age of the universe.

It's hopeless, possibly short of a breakthrough in quantum computing, or a 
researcher discovering an attack to RC5 significantly more efficient than 
brute force. Personally, I'm not holding my breath.

Décio
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