Re: Git commit hash clash prevention

From: Jean-Luc Herren <jlh@gmx.ch>
Date: Oct 2, 2008, 11:08 UTC
Message-ID: <48E4ABC0.80100@gmx.ch>
In-Reply-To: <200810021118.15313.trast@student.ethz.ch>

Hello list!

Thomas Rast wrote:

Show 5 quoted lines

>   However, the expected number of objects needed to get a collision is
>   on the order of 2**80 (http://en.wikipedia.org/wiki/Birthday_attack),
>   and since there are (very roughly) 2**25 seconds in a year and 2**34
>   years in the age of the universe, that still leaves you with 2**21
>   ages of the universe to go.

In case it's interesting to someone, I once calculated (and wrote down) the math for the following scenario:

  - 10 billion humans are programming
  - They *each* produce 5000 git objects every day
  - They all push to the same huge repository
  - They keep this up for 50 years

With those highly exagerated assumptions, the probability of getting a hash collision in that huge git object database is 6e-13. Provided I got the math right.

So, mathematically speaking you have to say "yes, it *is* possible". But math aside it's perfectly correct to say "no, it won't happen, ever". (Speaking about the *accidental* case.)

jlh