# [RC5] 40 bit encryption

Zorba the Hutt zorbathut at uswest.net
Fri Mar 15 16:05:04 EST 2002

```On the other hand, if you know some of the protocol of the encrypted data,
you might be able to search for the magic numbers :)

-Zorba

----- Original Message -----
From: "Paul Goyette" <paul at whooppee.com>
To: <rc5 at lists.distributed.net>
Sent: Friday, March 15, 2002 12:31 PM
Subject: Re: [RC5] 40 bit encryption

> Comparing the dnetc client's key rate isn't really useful here.
>
> 1. The client does not completely decode the message, it just tries
>    each key to see if the first few bytes of the result matches the
>    known text.
>
> 2. Aha!  dnetc knows what the answer is, so all we have to do is
>    find a key that gives us the right answer.  In your situation,
>    you are asking how long it would take to decode an _unknown_
>    message, and that has to take a lot longer, especially since you
>    probably won't know that you've succeeded!
>
> On Fri, 15 Mar 2002, Jeff Gilchrist wrote:
>
> > 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
> > 40bit.
> >
> > Regards,
> > Jeff.
> >
> >
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