Is it possible to create a file with a given size and MD5 hash?

Question

Of course I know is possible to create a file with a certain size and how to do it. But is it possible to create a file with a certain size and also a certain MD5 fingerprint?

Answer 1

Not being able to find a file with a certain hash is one of the defining properties of a cryptographic hash function such as md5.

While md5 has been broken, those breaks only allow you to create two different files (images) with the same hash, but not with a predetermined hash.

What you want is called a pre-image attack, and since md5 is still unbroken concerning that attack, you need to bruteforce.

This requires 2⁶⁴ (2^128/2) hashing operations on average, which is far beyond anything we can get with current computers.

Wikipedia has an article on pre-image attacks too:
http://en.wikipedia.org/wiki/Preimage_attack

Answer 2

Just creating a file with a given MD5 hash is difficult enough without adding the size requirement.

You could try a rainbow table, but that requires a long time to create and a huge amount of memory to store.

Answer 3

Theoretically, yes. Practically, it will most likely require computation of all permutations of all n bytes in a file -- baiscally, brute-forcing. Which means, this is hardly doable in any reasonable timeframe.