How to generate a file from its MD5 sum

Question

The title says it all. From my understanding, every file has a unique MD5 checksum. Is it possible to reverse-engineer the file from its sum?

For example, let's just say a video's sum was 5. I know, but its just an example. Could you write a program where you enter 5 and it generates a video?

In other words, instead of generating a sum from a file, you get a file from a sum.

Answer 1

To expand on what Jim W said, any hash function is one-way, which means they're functions that don't need to be easily reversible — while some may have inverses, most do not.

MD5 is a cryptographic hash function, which means it's intentionally designed to be very difficult to reverse. MD5 in particular is relatively weak, there are vulnerabilities that make it easy to find collisions — two files with the same MD5 hash.

Since an MD5 hash is only 128 bits, there are 2^128 different possible MD5 hashes, and while that's a very large number, there are still many, many more files than that in the world (potentially an infinite number, in fact), so it is inevitable that some files will hash to the same value. This, as user2864740 pointed out in a comment, is known as the pigeonhole principle.

A strong cryptographic hash function — like SHA-256 — is one for which it's considered computationally infeasible to reliably find such collisions.

Answer 2

No, it's one way - otherwise be great method of compression!