LFSR -Linear Feedback Shift Register

Documentation Python

Installation

with pip

``````pip install pylfsr
``````

Build from the source

Download the repository or clone it with git, after cd in directory build it from source with

``````python setup.py install
``````

Examples

Example 1: 5-bit LFSR with feedback polynomial x^5 + x^2 + 1

``````# import LFSR
import numpy as np
from pylfsr import LFSR

L = LFSR()

# print the info
L.info()

5 bit LFSR with feedback polynomial  x^5 + x^2 + 1
Expected Period (if polynomial is primitive) =  31
Current :
State        :  [1 1 1 1 1]
Count        :  0
Output bit   : -1
feedback bit : -1
``````
``````L.next()
L.runKCycle(10)
L.runFullCycle()
L.info()
``````

Example 2**: 5-bit LFSR with custom state and feedback polynomial

``````state = [0,0,0,1,0]
fpoly = [5,4,3,2]
L = LFSR(fpoly=fpoly,initstate =state, verbose=True)
L.info()
tempseq = L.runKCycle(10)
L.set(fpoly=[5,3])
``````

Example 3**: 23-bit LFSR with custom state and feedback polynomial

``````L = LFSR(fpoly=[23,18],initstate ='random',verbose=True)
L.info()
L.runKCycle(10)
L.info()
seq = L.seq
``````

Example 4**: Get the feedback polynomial or list

Reference : http://www.partow.net/programming/polynomials/index.html

``````L = LFSR()
# list of 5-bit feedback polynomials
fpoly = L.get_fpolyList(m=5)

# list of all feedback polynomials as a dictionary
fpolyDict = L.get_fpolyList()
``````

Changing feedback polynomial in between

``````L.changeFpoly(newfpoly =[23,14],reset=False)
seq1 = L.runKCycle(20)

# Change after 20 clocks
L.changeFpoly(newfpoly =[23,9],reset=False)
seq2 = L.runKCycle(20)
``````

For A5/1 GSM Stream cipher generator

Reference Article: Enhancement of A5/1: https://doi.org/10.1109/ETNCC.2011.5958486

``````# Three LFSRs initialzed with 'ones' though they are intialized with encription key
R1 = LFSR(fpoly = [19,18,17,14])
R2 = LFSR(fpoly = [23,22,21,8])
R3 = LFSR(fpoly = [22,21])

# clocking bits
b1 = R1.state
b2 = R1.state
b3 = R1.state

``````

MATLAB

Folder : https://github.com/Nikeshbajaj/Linear_Feedback_Shift_Register/tree/master/matlabfiles

Description Genrate randon binary sequence using LFSR for any given feedback taps (polynomial), This will also check Three fundamental Property of LFSR

1. Balance Property
2. Runlength Property
3. Autocorrelation Property

This MATLAB Code work for any length of LFSR with given taps (feedback polynomial) -Universal, There are three files LFSRv1.m an LFSRv2.m, LFSRv3.m

LFSRv1

This function will return all the states of LFSR and will check Three fundamental Property of LFSR (1) Balance Property (2) Runlength Property (3) Autocorrelation Property

EXAMPLE

``````s=[1 1 0 0 1]
t=[5 2]
[seq c] =LFSRv1(s,t)
``````

LFSRv2

This function will return only generated sequence will all the states of LFSR, no verification of properties are done here. Use this function to avoid verification each time you execute the program.

EXAMPLE

``````s=[1 1 0 0 1]
t=[5 2]
[seq c] =LFSRv2(s,t)
``````

LFSRv3 (faster)

seq = LFSRv3(s,t,N) this function generates N bit sequence only. This is faster then other two functions, as this does not gives each state of LFSR

EXAMPLE

``````s=[1 1 0 0 1]
t=[5 2]
seq =LFSRv3(s,t,50)
``````

Tips

• If you want to use this function in middle of any program, use LFSRv2 or LFSRv1 with verification =0.
• If you want to make it fast for long length of LFSR,use LFSRv3.m