Part 6 – Addendum

This is an addendum to a five part series:

1. Introduction
2. Encryption and Hashes
3. Simple Hashes and Collisions
4. Reduction Functions
5. Rainbow Tables and Chains
6. Addendum

I’ll update this more later (or just replace it), but wanted to comment on the fact that there is an important difference between my sample rainbow tables and those in the Wikipedia article.  in my rainbow tables, the leftmost column is plaintext, and the rightmost is a hash.  In the wikipedia article, the rightmost column is also plaintext.  It is the same table, but they do one extra reduction (R3) to get the plaintext.  This means that you can’t look up your hashed value in the table – the first thing you MUST do is run your last reduction, and then look up the resultant plaintext.  Why would you do this?

Space.

In my example (and the wikipedia example), the plaintext and hashes are similar sizes (and small).  In reality, if you’re dealing with MD5 or SHA-1 hashes, your plaintext values are going to be MUCH shorter!  So, you can save a lot of space by doing one more reduction, and using the reduced values instead.  Of course, you increase the risk of collisions (and run extra calculations), but that is the price of admission.