note 52500 deleted from function.md5 by joey

Note Submitter: tommiboy ---- Regarding those many posts about MD5 and this-or-that hash function being "broken" or insecure because it has collisions, please note the following: 1. Every hash function has collisions, that is what hash functions are made for. MD5, as an example, turns N bits of input into 128 bits of output. Obviously, whenever N > 128 bits, then there MUST be collisions. This does not mean the function is broken, it means that the function does EXACTLY what you want it to do - it makes a secret unrecoverable. The important thing about "secure" hash functions is that it is hard to calculate an input that will produce a certain output (e.g. the same output as another user's password). It is impossible to reconstruct a password from a hash if the password has more than 128 bits since several passwords necessarily map to the same hash. No matter which supercomputers you use, you have a set of equations with several unknowns. It is possible to find SOME password that produces a valid hash, though. For every reasonable scenario, however, MD5 will do just fine. If you are concerned, store the password length as well. 2. Chaining the hash function means 128 bits of input producing 128 bits of output, this does not make sense, really. In fact, you greatly increase the likelihood of finding SOME password that produces the same double-hash. 3. Chaining MD5 with SHA makes little sense, too, as you feed 128 bits into a function that returns 256 bits. So the information that you keep around is 50% redundant. Security is in no way enhanced. 4. You can add "salt", i.e. a constant or variable string (for example calculated from the user id) that is concatenated to the input of the hash function, but that does not really make things a lot better. It does make a dictionary attack against a stolen password database harder, if nothing else. 5. Concerns about this-or-that hash being not good enough are rather silly since there are a lot of other ways which are by several orders of magnitude cheaper and easier to break into your system. It is not likely that any sane person will attempt to find collisions of a hash function to break into one single account. Users are only too happy to give out their password to "the administator who must verify that their password is correct". 6. What happens if two users accidentially choose passwords that have a hash collision? First, this will probably never happen, and second, if it does happen, then there is not much harm. Two users can have the same password and none of them will ever notice.