A shrinking algorithm for SHA256 rainbow tables
I am doing a sha256 hack project using rainbow tables. I am trying to attack 8 digit alphanumeric sequences. I understand exactly how rainbow tables work and how chains should be created and stored. However, I don't understand how to get the cut function to form the chains. I searched for it and thought about it for hours with no results. So what is a good chain shortening function and how can it prove that it covers all 8-digit alphanumeric sequences.
source to share
There are 10 ^ 9 different 8-digit sequences. There are 1,073,741,824 possible values for the first 30 bits of the SHA256 hash value. So one sane approach is to extract those 30 bits and use that modulo 10 ^ 9 number as your reduction function:
R(hash) = hash[0:30] % 10^9
It is unlikely that this actually covers all 8-digit sequences, but in practice it should be good enough due to the supposed "random" properties of SHA256. There is a slight bias towards numbers <= 2 ^ 30 - 10 ^ 9 though due to modulus.
source to share