md5的一部分有多独特？

Question

I'm having a question regarding the uniqueness of md5 function.

I know that md5 (with microtime value) are not unique, however, they are pretty unique :)

How can I calculate the probability of a collision between two portions of an md5 hashes?

For example: The following in php that generates a 8 chars string from md5 result:

substr(md5(microtime()), 0, 8);

A second scenario - What if the index of it is unique (so it gets a different portion of the hash each time)?

substr(md5(microtime()), rand(0, 32), 8);

Answer 1

As you are asking about uniqueness of your string, it's actually a probability. Means as much string character you will use and as much the length of random string you make will get less chances of getting similar random string.

So, to get unique string you need to store string in your DB and compare with random string, if you found similar then again go for new fresh string , until you get unique string.