From: "Hal E. Fulton" <hal9000@...>
Date: 2001-11-13T21:08:15+09:00
Subject: [ruby-talk:25004] Re: Mapping sequential strings to random strings: How would you do this?

----- Original Message -----
From: Niklas Frykholm <niklasf@cs.umu.se>
To: ruby-talk ML <ruby-talk@ruby-lang.org>
Sent: Tuesday, November 13, 2001 5:16 AM
Subject: [ruby-talk:25003] Re: Mapping sequential strings to random strings:
How would you do this?


> > Now, I wonder if there is a way to make the strings shorter? ;)
>
> Well that was not in the requirements ;)
>
> You can shorten the output from MD5 but it will increase the
> probability of collisions. For example if you have 1000 documents and
> use 8 characters from MD5 you have a collision probability of
> approximately 2/1000. Or in general, for m documents and n characters
>
> # Approximate, only valid when m*m << 2.0**(4*n)
>
> def coll(m,n)
> (m*m)/(2.0**(4*n))
> end
>
> But you can eliminate collisions entirely by prepending a document
> identifier (such as 001) to each name. Then the hash part will just
> make the name hard to guess. With four characters in that part, it will
> take on average 32000 attempts to guess it. Perhaps that is OK.
>
> name = id + MD5.md5(id + secret).hexdigest[0,4] + '.html'
>
> And if you want to make it even shorter you could recode the id and
> the hash in Base64 ;)

Thanks again!

Everything is intuitive once you understand it,
as someone said...

Hal