How does the md5 hashing algorithm compress data to a fixed length?

Question

I know that MD5 produces a 128-bit digest. My question is, how does it produce this fixed length output from a message of 128bits+?

EDIT:

I have now a greater understanding of hashing functions now. After reading this article I have realized that hash functions are one-way, meaning that you can't convert the hash back to plaintext. I was under the misimpression that you could due to all the online services converting them back to strings, but I have realised that thats just rainbow tables (collections of string's mapped to pre-computed hashes).

Answer 1

When you generate an MD5 hash, you're not compressing the input data. Compression implies that you'll be able to uncompress it back to it's original state. MD5, on the other hand, is a one-way process. This is why it's used for password storage; you ideally have to know the original input string to be able to generate the same MD5 result again.

This page provides a nice graphic-equipped explanation of MD5 and similar hash functions, and how they're used: An Illustrated Guide to Cryptographic Hashes

Answer 2

Consider something like starting with a 128-bit value, and taking input 128 bits at a time, and XORing each of those input blocks with the existing value.

MD5 is considerably more complex than that, but the general idea is the same: input is processed 128 bits at a time. Each input block can change the value of the result, but has no effect on the length.