Example Rainbow Table Generation

Question

I have read up on rainbow tables and think I understand the idea behind them. However, I find that it would be better for me to actually attempt to implement a (very basic) rainbow table generator in order to get a proper feel to using rainbow tables.

However, I face the problem of there not being descriptions on how to actually generate one. More specifically, I have found no descriptions on what reduction functions to use. Nor have I found out how many hashes a chain contains. Everything I have found has all been "hash then reduce" or "use OphCrack/RainbowCrack".

Is there information that takes me through the actual steps to generate chains and search through them? I want to learn how to make the tables, not how to use other peoples' programs.

Please do not link me to these websites:

http://kestas.kuliukas.com/RainbowTables/
http://project-rainbowcrack.com/tutorial.htm
http://www.ethicalhacker.net/content/view/94/24/
http://www.thetazzone.com/tutorial-rainbow-tables/
http://en.wikipedia.org/wiki/Rainbow_table

score 10 · Accepted Answer · edited Jul 25 '14 at 16:06

Wikipedia shows you how to create a rainbow table.

The information you are looking for is under "Precomputed hash chains"

The reduction function tries to create a new input, the non hashed part which is the password in rainbow tables, from a hash. We want to choose a reduction function that will try to equally reduce the hash into new inputs such that it tries not to create the same input so many times.

Consider the following example. We want to create a rainbow table for all 5 character number combinations for the md5 function.

Lets start out with 12345 as the input. The md5 hash is 827ccb0eea8a706c4c34a16891f84e7b. The reduction function can be as simple as taking the first 5 numbers and using that as the next input. This method will produce reasonably distributed new inputs.

So the hash

"827 ccb 0 eea 8 a706c4c4a16891f84e7b" would create "82708"

The next hash would be 71b8e22700e63c2a0c1bad6506549d3b then would be reduced to 71822 accordingly.

So our current chain will consist of:

12345 -> 827ccb0eea8a706c4c34a16891f84e7b ->

82708 -> 71b8e22700e63c2a0c1bad6506549d3b ->

71822

The chain can go as long as you want, until it his a previous input, after which it enters a cycle - i.e. when it hits that point, it will just repeat itself and it will be useless.

Having a longer chain is going to reduce the space need to store these values but increase the time it needs to recalculate the entire chain.

All that would need to be stored would be the starting and ending point of the chain.

Given a hash, you use your same reduction function on it to get an input and repeats itself until that matches the end of a chain.

If it matches the end of a chain, you can just recalculate those values to determine which input it was.

NOTE: You want to make sure that you have every value from 00000 to 99999 in these chains, otherwise it will be incomplete.

If you need characters a to z instead of 0 - 9, you could just convert the hash to base 26.

Example Rainbow Table Generation

1 Answers1