@SureshPatil/

# Affine Cipher

## No description

Files
• main.py
main.py
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```
```from math import gcd
from collections import Counter
# import sympy

def c2i(c, alphabet):
return alphabet.index(c)
#"""Returns the index of c in the string alphabet, starting at 0"""
# Copy your method from subcipher.py here

def i2c(i, alphabet):
return alphabet[i]
#"""Returns the character at index i in alphabet"""
# Copy your method from subcipher.py here

def prepare_string(s, alphabet):
newS = ""
for char in s:
if char in alphabet:
newS += char
return newS
#"""removes characters from s not in alphabet, returns new string"""
# Copy your method from subcipher.py here

def mod_inverse(a, m):
for x in range(1,m):
if((a*x)%m == 1):
return x
print("No modular inverse found!")
return -1

def isNotCoprime(a, lenAlpha):
if lenAlpha % a == 0:
return True
for x in range(2, a):
if lenAlpha % x == 0 and a % x == 0:
return True
return False

def numCoprime(x):
num = x
for i in range(1, x):
if isNotCoprime(i, x):
num -= 1
return num

def coprimes(x):
num = x
coprimes = []
for i in range(1, x):
if isNotCoprime(i, x):
num -= 1
else:
coprimes.append(i)
return coprimes

def affine_encode(plaintext, alphabet, a, b):
if(isNotCoprime(a, len(alphabet))):
print("Invalid a-value!")
return -1
else:
ciphertext = ''
for letter in plaintext:
x = c2i(letter, alphabet)
encX = (a*x + b) % len(alphabet)
ciphertext += i2c(encX, alphabet)
return ciphertext

def affine_decode(ciphertext, alphabet, a, b):
if(isNotCoprime(a, len(alphabet))):
print("Invalid a-value!")
return -1
else:
plaintext = ''
for letter in ciphertext:
encX = c2i(letter, alphabet)
aInv = mod_inverse(a, len(alphabet))
x = (aInv*(encX - b) + len(alphabet)) % len(alphabet)
plaintext += i2c(x, alphabet)
return plaintext

# def phi(n):
#     y = n
#     for i in range(2,n+1):
#         if sympy.isprime(i) and n % i == 0:
#             y *= 1 - 1.0/i
#     return int(y)

alpha = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
# newAlpha = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789!"

# # Ex. 1
# plaintext_1 = prepare_string("THISISATEST", alpha)
# ciphertext_1 = affine_encode(plaintext_1, alpha, 17, 4)
# print(ciphertext_1)

# # Ex. 2
# ciphertext_2 = prepare_string("WIGZEYYPTUPUYP", alpha)
# plaintext_2 = affine_decode(ciphertext_2, alpha, 17, 4)
# print(plaintext_2)

# print("New Alpha beyond this point! ------------------------------")
# #Ex. 3
# plaintext_3 = prepare_string("C01DB100D3DC0D3R5!", newAlpha)
# ciphertext_3 = affine_encode(plaintext_3, newAlpha, 26, 5)
# print(ciphertext_3)
# plaintext_3 = affine_decode(ciphertext_3, newAlpha, 26, 5)
# print(plaintext_3)

# print("Table beyond this point! ------------------------------")

# for x in range(20, 61):
#   numA = numCoprime(x)
#   numB = x
#   numCombs = numA*numB - 1
#   print(x, numA, numB, numCombs)

# print(numCoprime(99999))

def d2i(d, alphabet):
return c2i(d[0], alphabet)*len(alphabet) + c2i(d[1], alphabet)

def i2d(i, alphabet):
d = ''
d += i2c(i//len(alphabet), alphabet)
d += i2c(i - (i//len(alphabet))*len(alphabet), alphabet)
return d

def affine_encode_digraphs(plaintext, alphabet, a, b):
if(len(plaintext) % 2  != 0):
plaintext += 'X'
ciphertext = ''
for index in range(0, len(plaintext), 2):
x = d2i(plaintext[index : index + 2], alphabet)
encX = (a*x + b) % (len(alphabet) * len(alphabet))
ciphertext += i2d(encX, alphabet)
return ciphertext

def affine_decode_digraphs(ciphertext, alphabet, a, b):
plaintext = ''
for index in range(0, len(ciphertext), 2):
encX = d2i(ciphertext[index : index + 2], alphabet)
aInv = mod_inverse(a, len(alphabet)*len(alphabet))
x = (aInv*(encX - b)) % (len(alphabet) * len(alphabet))
plaintext += i2d(x, alphabet)
return plaintext

# # Ex. 4
# plaintext_4 = prepare_string("THISISANOTHERTEST", alpha)
# ciphertext_4 = affine_encode_digraphs(plaintext_4, alpha, 81, 119)
# print(ciphertext_4)

# # Ex. 5
# ciphertext_5 = prepare_string("QFHIIRTBUUYNNUURJPXDYWFG", alpha)
# plaintext_5 = affine_decode_digraphs(ciphertext_5, alpha, 81, 119)
# print(plaintext_5)
# print(phi(26*26*26))
# print(26*26)
# print(phi(26*26) * 26*26 - 1)

# Ex. 2
# ciphertext_2 = prepare_string("XeiackzmpowbytywyvofTyterywxabkpauyrgtpoubargkqm", alpha)
# possibilities = []
# crib = "FAMOUS"
# for i in coprimes(26):
#   for j in range(26):
#     plaintext_2 = affine_decode(ciphertext_2, alpha, i, j)
#     possibilities.append(plaintext_2)

# for poss in possibilities:
#   if crib in poss and poss.index(crib) == 0:
#     print(poss)