On 1 déc. 08, at 13:52, Martin Jambon wrote:

> If I understand your problem correctly, this is the so-called birthday
> problem with 2^128 days in a year. The Wikipedia article gives useful
> approximations:
>
> http://en.wikipedia.org/wiki/Birthday_problem

Thank you for the link, this was very informative. And there is even a  
table linking desired probability of collision to number of outputs in  
this page:

http://en.wikipedia.org/wiki/Birthday_attack

Alan