I was reading "serious Cryptography" by Jean-Philippe Aumasson,and when coming to Feedback Shift Registers,I didn't understand what role Polynomials play in the feedback function, does it have to do with polynomial addition,which is simuar to XOR? Also i made a simple FSR,just to understand it better,so I wanted to ask the question:
is this an example for a FSR?:
def feedback(list):
return(((list[-1]+list[-1])^list[-2]|list[-7]&list[0])^(list[-5]|list[-6]&list[-4]^list[-10]^list[-11])&(list[-3]|list[-7]&(~list[-9])^list[-11])|(list[-13]^list[-15]^~list[-4])^list[0]>>1) # the feedback function combines specific bytes from the key stream to generate a new byte
def encrypt_and_decrypt(nums,key):
key = list(key)
if len(key)>=len(nums):#if the key is larger than the plaintext
for i in range(len(key)):#just XOR the key to the plaintext and return the ciphertext
nums[i]^=key[i]
return nums
else:
for i in range(len(key)):
nums[i]^=key[i]
for i in range(len(key),len(nums)):
key.append(feedback(key))# generate new bits from the existing key stream
key = key[1:] # delete the first entry to save memory
nums[i]^=key[-1]# XOR the newly generated key byte to the plaintext byte
return nums
Another question I have is: if this really is an example for a LFSR,what prevents a known plaintext attacker,who knows the first bytes from the plaintext from just XORing that part to the ciphertext,getting the initial key and generating the key stream all over again?
I would thank everyone who answers me,as I am really new to this aspect,ans I'm just beginning to understand it,so thank you for your answers!
