RSA_Algorithm.py

import math
'''RSA is an encryption algorithm, used to
securely transmit messages over the internet.
It is based on the principle that it is easy to multiply large numbers,
but factoring large numbers is very difficult.
'''


def inverse_mod(p, m):
    p = p % m
    for i in range(1, m):
        if(p * i) % m == 1:
            return i
    return 1

if __name__ == '__main__':
    '''p and q  are prime numbers. pt is plain text.
    e is derived number which lies between 1 and (p-1)*(q-1)
    '''
    p, q, e, pt = [int(x) for x in input("Enter\
    p, q, e and plaintext: ").split()]
    n = p * q
    fn = (p-1) * (q-1)
    d = inverse_mod(e, fn)
    ct = pow(pt, e, n)
    print(f'Encryption for PlainText = {pt}, is CipherText = {ct}')
    new_pt = pow(ct, d, n)
    print(f'Decryption for CipherText = {ct}, is PlainText = {new_pt}')

'''Sample input and output
Enter p, q, e and plaintext: 7 13 5 10
Encryption for PlainText = 10, is CipherText = 82
Decryption for CipherText = 82, is PlainText = 10

Complexities
time: O(n)
space: O(1)
'''