This has the very desirable property of not being able to guess the name of the file even if you know the name of the "object".

In this case, extremely unlikely just means never unless you are being attacked by say Russians with secret technology and a room full of supercomputers. The world will almost certainly end before anyone guesses one of these numbers or before you generate two that are the same.

You can look at the probability table for the Birthday Attack in this Wikipedia article and decide how unlikely you would like this event to be: http://en.wikipedia.org/wiki/Birthday_problem

Also from the article: "In theory, MD5, 128 bits, should stay within that range until about 820 billion documents, even if its possible outputs are many more."

http://en.wikipedia.org/wiki/Md5#Vulnerability

Thanks!

dsc-450.jpg > user/acbdddc1ab1b73536fabbd3f4ffeea4e dsc-450.jpgsalt > f043ce111eb398cd280b67c80c6c8ca6 usersaltdsc-450.jpg > 272ed08a85fc1e47b97462fb053a7c29

You'd need at least a secret site salt, as well as a per-user salt. If you have only a site salt, I can figure out what file names would be by uploading my own. If you had only a per-user salt, I calculate it myself. The problem of your site salt getting out remains.

At the very least, you should randomly generate identifiers. This makes it less obvious which targets are worth looking at. What you really need, though, is something like per-folder access control.

I would also recommend adding a salt to stop an attacker from being able to guess URLs by hashing common filenames. Though using the time in the hash is likely to prevent this, it's low-cost and probably worth it.

So, I would recommend (in PHP):

  $url = md5( 'my_salt_string' . $time . $file_name );

Go read http://en.wikipedia.org/wiki/Salt_(cryptography) and come back once you understand what a salt is.