Quite a while ago, I started playing with random numbers on 8 Bit machines. I don’t think anyone is doing “serious” work on these machines, but playing with Ciphers and Crypto got me at least curious about how a Commodore 64 generates random numbers.

We’re on ANOTHER random number generation technique today. Let’s work on the Lagged Fibonacci Sequence, a fun way to use the Fibonacci Sequence on itself to generate pseudo-random numbers.

Cryptonomicon Pontifex/Solitaire

We’re back to my favorite book for another round. We’ve done quite a bit with Cryptonomicon so far. One of my favorite moments is when Enoch Root introduces a cipher that two prisoners used to communicate using just a humble playing card deck.

A while ago, I explained in detail how to generate one time pad sheets and use them to encrypt and decrypt messages.

Hashes

We did an implementation of a once popular hashing function in Part 8 on RC4 without really going into why.

Recap

So far we’ve

• Reversed a string
• Shifted bits left and right (mostly right)
• Brute forced a known simple cipher
• Channeled our inner John Connor by cracking ATM machines
• Searched through a large number looking for matches
• Learned how to use XOR to recover data
• Created a Linear Feedback Shift Register to generate better random numbers
• Implemented a “real” but outdated cipher: RC4

Disclaimer: I’m sure I’ll get an angry email from a mathematician about this, but we’re going for understanding here. No one expects to be looking for large primes on a Commodore 64. Calm down.

Recap

So far in this 8 part series on doing weird things with Ciphers on old computers we’ve:

So far in this 7 part (shocked I’ve made it through 7!) series on doing weird things with Ciphers on old computers we’ve:

Review

So far we’ve:

• Reversed a string
• Shifted bits left and right (mostly right)
• Brute forced a known simple cipher
• Faked it, but then looking for a 4 digit known pin
• Searched through a large number looking for matches

We now have most of the basic skills in Assembly to start doing some real work. Let’s tackle what is arguably the most important of the bitwise operators for ciphers and encryption: The exclusive OR. (XOR/EOR)

A common task in our code is to determine the length of something. In this example, let’s check the length of a string like “are you keeping up?”

Let’s recap a little

So far we’ve:

• Reversed a string
• Shifted bits left and right (mostly right)
• Brute forced a known simple cipher
• Faked it, but then looking for a 4 digit known pin

What we need next in our toolbox is to be able to search through a large number of values to look for something. This will build on the previous skills and also reinforce working with tables and 24 bit math on an 8Bit Processor.

A few minutes into Terminator 2, the young John Connor takes his Atari Portfolio out of his bag and inserts a foil covered card into an ATM machine and proceeds to crack a pin number and steal some cash from an unsuspecting account.

Well here we are at part three and it’s a lot to pack into one page.

In Part 1 we did a simple reverse of a string. While simple, these software steps will help build the skills we need going forward in Assembly without feeling like I skipped right to a 300 line after doing a short one. One foot in front of the other and all that.

Sometime during my weeklong exploration of SHA-256 on a Commodore realized that there was a whole world of exploration that could be explored. Just because a Computer system is old, doesn’t mean it’s not interesting or useful.

Last week, at this bat-time on this bat-channel, I talked about checking off one of my 35 year old bucket list items by learning Commodore assembly language. (shut up, you’re old too.) Coincidentally, I picked this up at the time I was re-reading Cryptonomicon by Neal Stephenson for the ump-teenth time. There are a few books that I consider so good that they’re worth re-visiting, and this is one of them.