[RC5] 40 bit encryption

Jeff Gilchrist jeffg at cips.ca
Fri Mar 15 10:39:11 EST 2002

Quoting "Brad T." <bradtod at rocketmail.com>:

> how long would it take to crack 40bit encryption? Assuming a Athlon TB
> 1.4ghz computer was used. Some web sites still use this level of
> security so how safe are they?

Do you have this Athlon machine?  By 40bit encryption, I'm guessing you are 
referring specifically to RC5 40bit?  Run the d.net client and see how many 
keys/sec your machine will do.

According to the D.Net benchmark pages, a TBird 1.4GHz will do about 5 million 
keys/second.  A 40 bit key has 2^40 keys = 1,099,511,627,776 keys.  It would 
take that machine 2^40 keys divided by 4,986,524 keys/sec = 220,496 seconds = 
61.25 hours.

To guarentee a find it would take 2.5 days but statistically you only need to 
check about half the keys so 1.25 days which is pretty darn fast.  Just add a 
few machines and that is down to hours (ie: there is little security at all).

A new Athlon XP 1800 (I'm guessing that is the 1800+ which is not actually 1800 
MHz) will do 5,669,816 keys/sec would only require 2.25 days total to crack 


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