[RC5] sounds like 640k

schoen at uclink4.Berkeley.EDU schoen at uclink4.Berkeley.EDU
Sun Jul 19 15:54:29 EDT 1998


Raymond Tant writes:

> PULLED FROM ARTICLE OFF MSNBC:
> 
> >       “I could easily see a situation where someone could do this as a
> science >project in five or six years,” said John Gilmore, co-founder of the
> EFF. But such an >attack with a single machine would not work on the much
> stronger Triple DES >algorithm, used in many banking networks, he added.
> 
> >120 BITS OF KEY
>  >      “At 90 bits of key, it begins to get tough to do this type of crack.
> At 120 bits of >key, it’s pretty much impossible,” Gilmore said.
> 
> Maybe this week......
> (Nothing is impossible)

You can predict what it might cost in the future, given that Moore's Law
holds.  Even with Moore's Law growth, computers with present architectures
will never crack strong 128-bit ciphers.

> Granted I  know thats one hellava keyspace, but all it takes is 4 of those
> machines now....

Nope, 4 of these machines can crack a 56-bit message in 1/4 the time, but
that's not at all the same as cracking a 224-bit message in the same time.
Time required scales exponentially with keylength, not linearly.

> lets see - 250,000 to break des-ii-2????    If someone uses 120 bits to
> encrypt their data and someone wants their data - it is worth 1,000,000 to
> build 4 of those machines....

So this won't work the way you think.  A 112-bit message is not just twice as
hard to crack as a 56-bit message; it is 2^56 times as hard.  Doubling the
keylength _squares_ the required time to recover the key.

It would take the EFF more money than the entire GDP of the United States
to break a 120-bit cipher using equivalent techniques, by their own optimistic
predictions.  And that's a dramatic understatement.

Exponential growth is an amazing thing.

-- 
   Seth David Schoen L&S '01 (undeclared) / schoen at uclink4.berkeley.edu
Magna dis immortalibus habenda est atque huic ipsi Iovi Statori, antiquissimo
custodi huius urbis, gratia, quod hanc tam taetram, tam horribilem tamque
infestam rei publicae pestem totiens iam effugimus.  -- Cicero, in Catilinam I
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