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5 views2 pages

Exp 5

Uploaded by

sejal nandkumar patil
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as DOCX, PDF, TXT or read online on Scribd
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def gcd(a, b):

while b:

a, b = b, a % b

return a

def modinv(a, m):

# Modular inverse using Extended Euclidean Algorithm

a=a%m

for x in range(1, m):

if (a * x) % m == 1:

return x

raise ValueError(f"No modular inverse for a={a} mod {m}")

def affine_encrypt(plaintext, a, b):

if gcd(a, 26) != 1:

raise ValueError("'a' must be coprime with 26.")

plaintext = plaintext.upper()

ciphertext = ""

for char in plaintext:

if char.isalpha():

x = ord(char) - ord('A')

y = (a * x + b) % 26

ciphertext += chr(y + ord('A'))

else:

ciphertext += char # Keep non-alpha as is

return ciphertext

def affine_decrypt(ciphertext, a, b):


if gcd(a, 26) != 1:

raise ValueError("'a' must be coprime with 26.")

a_inv = modinv(a, 26)

plaintext = ""

for char in ciphertext:

if char.isalpha():

y = ord(char) - ord('A')

x = (a_inv * (y - b)) % 26

plaintext += chr(x + ord('A'))

else:

plaintext += char # Keep non-alpha as is

return plaintext

# === Example usage ===

a=5

b=8

plaintext = "AFFINE CIPHER"

encrypted = affine_encrypt(plaintext, a, b)

decrypted = affine_decrypt(encrypted, a, b)

print(f"Plaintext: {plaintext}")

print(f"Encrypted: {encrypted}")

print(f"Decrypted: {decrypted}")

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