possible collision hashing uuid cakephp

问题:

Is it possible to have collisions if to use Security::hash on uuid() string ? I know that uuid() generates truly unique string, but I need them to be hashed, and I am worried if there is a possibility that the hashed string can be repeated.

Thanks

回答1:

Firstly, contrary to the name, a uuid does not create a truly unique string. It generates a string that is unique with very high probability(high enough that it can for pretty much all purposes be treated as unique).

As for your chances of getting a collision, that really depends on which hashing algorithm you are using. Assuming a well built hashing algorithm which distributes uniformly over it's output space, your odds of a collision with any two hashes is 1 / 2^n where n is the hash length in bits. The odds of any two hashes colliding in a birthday attack scenario can be approximated using the formula p(h) = h^2 / 2 m where h is the number of hashes you expect to generate and m is the output space (2^256 in the case of SHA256 for example).

So, the sum it all up, you will always have a chance of getting a hash collision regardless of what hashing algorithm you're using. However, in the case of pretty much anything equal to or greater than SHA256, the chance is so vanishingly small that is is not worth worrying about. Your time is better spent worrying about the chances of a bus running over your server in the next second.

回答2:

uuid can generate duplicates but the chance is very very very small.

Security::hash of cakePHP looks like the hash function of PHP.

If you use it with sha512 it should be pretty good.