Is it possible to decrypt MD5 hashes?

Question

Someone told me that he has seen software systems that:

1. retrieve MD5 encrypted passwords from other systems;
2. decrypt the encrypted passwords and

Answer 1

There is no way of "reverting" a hash function in terms of finding the inverse function for it. As mentioned before, this is the whole point of having a hash function. It should not be reversible and it should allow for fast hash value calculation. So the only way to find an input string which yields a given hash value is to try out all possible combinations. This is called brute force attack for that reason.

Trying all possible combinations takes a lot of time and this is also the reason why hash values are used to store passwords in a relatively safe way. If an attacker is able to access your database with all the user passwords inside, you loose in any case. If you have hash values and (idealistically speaking) strong passwords, it will be a lot harder to get the passwords out of the hash values for the attacker.

Storing the hash values is also no performance problem because computing the hash value is relatively fast. So what most systems do is computing the hash value of the password the user keyed in (which is fast) and then compare it to the stored hash value in their user database.