linear feedback shift register explained

pop().insert(pos, item) inserts some item at the index pos.pop() just consumes the last element off the right and returns it to you. We need a way to feed things in from the left and consume them off the right. Looking at our duplo shift register, we have a list. Every time that happens, every spot in the shift register bumps over to the right once. Tick 4: A -> 1 0 1 -> B # The rightmost 1 has made it to point B, all that's left in the register is _1 0 1_ 1īasically, in a shift register, new data comes in from the left and out the right side one digit at a time.

Tick 2: A -> 1 1 0 -> B # continuing, the numbers in the register are the rightmost bits of 1 0 _1 1_ Tick 1: A -> 1 0 0 -> B # the number is consumed from the right, A is sending the rightmost bit of 1 0 1 _1_ Tick 0: A -> 0 0 0 -> B # our register is the thing between the arrows, and it starts empty, with 0 0 0 Each line represents the system after a single clock tick. An example: Sending the numbers 1 0 1 1 from point A to point B with a 3 bit shift register. We can imagine it as a sort of a queue with a fixed number of spots, and every time a clock tick passes, all the numbers in our shift register hop over to the right one spot. One use of a digital circuit shift register is to get a series of numbers from point A to point B with a fixed delay. I'm going to ignore the linear feedback aspect for a moment and briefly describe a traditional shift register. lfsr = # We'll come back to this later, I SWEAR. For example, let's take the number 1337, and put it in our baby's-first-shift-register, which we'll model using a list. A shift register is just a container that holds a single number, but keeps the digits separate. Let's make a linear feedback shift register.įirst, we have to understand the behavior of a shift register, which we'll look at using a sort of blown up model of one. I can't seem to find this written in a clear way.Īny comments or suggestions (if say you want to suggest some starting code to help me write it myself.) would be very helpful. I've looked around the web, but some scripts are too involved (I don't want to try to understand an OO LFSR at the moment), and others are for variations on the regular LFSR (like Fibonacci, or Galois). If anyone can suggest one, I'd be grateful.

I thought if there was a 10-15 line python script I could review, it might make more sense. I've read about it countless times, but I am having trouble putting it together. I know the data stream is coming in from the left and certain places along the 0 to 31 bits, there are 'taps' (as defined by a primitive polynomial or a 32bit hex number), which XOR the data that flows into that bit, which is then fed back into the data stream.

I am trying to study weaknesses in basic PRNGs, and while I understand the generators I've learned thus far (namely a Linear Congruential Generator), I cannot seem to understand what's going on in an LFSR.

Linear feedback shift register explained