[Huusker]

The MD5 hash has 340,282,366,920,938,463,463,374,607,431,768,211,456 possible signatures, which is greater than the number of atoms in the universe.

[ThePythonicCow]

Otherwise known as 2 ** 128. That doesn't mean that there isn't some practical way to manufacture a string to match a given number.

For example, let's say the md5 sum was computed by simply looking at the binary representation of the data as one big number, module 2**128. Then any data pattern ending in the same last 128 bits would have that same sum, and md5 would be worthless.

Or for a slightly more realistic example, consider the classic check sum -- gotten just by adding up each byte in the data. Any input data stream with the same bytes rearranged will have the same sum.

Which isn't to say that md5 is much of a risk for such. Just to say that it could be, and then the number of bits wouldn't necessarily help.

[FoxPro]

And that is Md5s potential weakness. It has a limited amount of available combinations, purporting to be able to uniquely identify any infinite combination of these “atoms”. There is a logical problem right there.

I didn’t post this to discuss MD5 or computer backup. I like the new hash based backup systems. I was just explaining how it works. I would backup my computer using this method, and sleep well at night.

I think MD5 is the coolest computer thing I have ever seen, and use it everyday for all sorts of things.