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

Not directly. Because of the pigeonhole principle, there is (likely) more than one value that hashes to any given MD5 output. As such, you can't reverse it with certainty. Moreover, MD5 is made to make it difficult to find any such reversed hash (however there have been attacks that produce collisions - that is, produce two values that hash to the same result, but you can't control what the resulting MD5 value will be).

However, if you restrict the search space to, for example, common passwords with length under N, you might no longer have the irreversibility property (because the number of MD5 outputs is much greater than the number of strings in the domain of interest). Then you can use a rainbow table or similar to reverse hashes.