Subject: Whales falling on houses - was: Hash collision count

From: Jon Seymour <[email-masked]>
Date: Mon, 25 Apr 2005 08:25:32 +1000

.> 
> 1. Take your favorite text file, at least 160 characters long.
> 2. Choose 160 positions in this file.
> 3. For each position choose your favorite mispelling of that character.
> 4. Produce all 2^160 text files, all of the same length, choosing for
> each position either the original or the alternate character
> 5. Add an arbitrary file of the same length, different from the above
> 
> Two of these files have the same sha1 hash. Or, for that matter, for
> any 160 bit  hash the same is true.

If you were to create those files at 10^9 files per second, it would
take you 10^38 years before you were in position to take step 5. I am
about to turn 38 this week. Would that I could live to 10^38.

It's absolute rubbish to say that the best solution from an
<double-quote>engineering</double-quote> point of view is to eliminate
the infinitessimal possibility of a collision. Engineering is all
about assessing risk and making suitable trade-offs. Every day of the
week, "real" engineers accept life-threatening risks that put
thousands of peoples lives in danger. They do it because we live in a
world where risk cannot be eliminated, merely reduced to an acceptable
level.

I can't understand that you are a prepared to drive a car or fly in a
Boeing or Airbus that has a demonstrated risk of killing you, yet you
want to insist on eliminating a risk that at most might create an
interesting Slashdot headline: "Jolt-crazed programmer finds SHA1
collision - but later dies when whale falls on house".

jon.
-- 
homepage: http://www.zeta.org.au/~jon/
blog: http://orwelliantremors.blogspot.com/