Python Help! Decrypt Function using a Multiplicative inverse function. Here is t

Question

Python Help! Decrypt Function using a Multiplicative inverse function.

Here is the code that I have so far, I have a multiplicative inverse function that use an Extended Euclidean Algorithm function. I need to use these the multinv function to create a decrypt function. "Write a function called decrypt that accepts three numbers (c, m, and k) and returns the corresponding plaintext (p) value as a number. You can assume the modulus (n) is 256. You will need to compute the multiplicative inverse of m mod 256 to decipher c."

# Problem 1: Extended Euclidean Algorithm
def egcd(a, b):
   if b == 0:
       return (1, 0)
   else:
       # Calculate q
       q = a // b
       # Calculate r
       r = a % b
       # Calculate (s, t) by calling egcd(b, r)
       (s,t) = egcd(b, r)
       return (t, s-q*t)

# Problem 2: Multiplicative Inverse
def multinv(a, n):
g = egcd(a, n)
return g[0] % n

# Problem 3: Decrypt a Single Value

def decrypt(c, m, k):

for testing purpose, running decrypt(80, 5, 11) should return 65

Explanation / Answer

def gcd(x, y):

while(y):
x, y = y , x % y
return x

def lcm(a, b):
  
return abs(a*b) / gcd(a,b)

def egcd(a, b):
  
if b == 0:
return (1, 0)

else:
q = a // b
r = a % b

(s, t) = egcd(b, r)

return (t, s - q * t)

def multinv(m, n):
  
(x, y) = egcd(m, n)

return x % n
  
def decrypt(m, e, k):

return (e-k) * multinv(m, 256) % 256

def linearcipher(bstr, m, k):

return bytes( [ (m*b+k) % 256 for b in bstr] )

def lineardecipher(lstr, m, k):
  
return bytes ( (decrypt (m, e, k) ) for e in lstr)

import unittest

class MyTest(unittest.TestCase):

def test_gcd(self):
self.assertEqual( gcd(42, 35) , 7 )
self.assertEqual( gcd(37, 16) , 1 )
self.assertEqual( gcd(52, 81), 1 )
self.assertEqual( gcd(10, 15), 5 )
self.assertEqual( gcd(5,5), 5)

def test_lcm(self):
self.assertEqual( lcm(3,1) , 3 )
self.assertEqual( lcm(5,0) , 0 )
self.assertEqual( lcm(11, 3), 33 )

def test_egcd(self):
self.assertEqual( egcd(6, 4) , (1, -1) )
self.assertEqual( egcd(6, 3) , (0, 1) )
self.assertEqual( egcd(9,6), (1,-1) )

def test_multinv(self):
self.assertEqual( multinv(15, 26) , 7 )
self.assertEqual( multinv(7,26), 15 )

def test_linearcipher(self):
self.assertEqual(linearcipher(b'Hello Sierra', 5, 11), b"s''6«ªEEð")
self.assertEqual(linearcipher(b'Discrete Structures', 11, 83), b'?ÖD”9ªOª³äO9Z”OZ9ªD')
self.assertEqual(linearcipher(b'Sierra', 11, 83), b'äÖª99~')

def test_lineardecipher(self):
self.assertEqual(lineardecipher(b"s''6«ªEEð", 5, 11), b'Hello Sierra')
  

def test_decrypt(self):
self.assertEqual( decrypt(5, 61, 11), 10)
self.assertEqual( decrypt(5, 80, 11), 65)
self.assertEqual( decrypt(5, 149, 11), 130)

unittest.main()

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