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| 1 | +def is_prime(n): |
| 2 | + if n < 2: |
| 3 | + return False |
| 4 | + for i in range(2, int(n**0.5) + 1): |
| 5 | + if n % i == 0: |
| 6 | + return False |
| 7 | + return True |
| 8 | + |
| 9 | +def gcd(a, b): |
| 10 | + while b != 0: |
| 11 | + a, b = b, a % b |
| 12 | + return a |
| 13 | + |
| 14 | +def multiplicative_inverse(a, b): |
| 15 | + s1, s2, m = 1, 0, b |
| 16 | + while b != 0: |
| 17 | + q, r = divmod(a, b) |
| 18 | + a, b = b, r |
| 19 | + s1, s2 = s2, s1 - q * s2 |
| 20 | + return s1 % m |
| 21 | + |
| 22 | +def powermod(x, y, p): |
| 23 | + res = 1 |
| 24 | + x = x % p |
| 25 | + while y > 0: |
| 26 | + if y % 2 == 1: |
| 27 | + res = (res * x) % p |
| 28 | + y //= 2 |
| 29 | + x = (x * x) % p |
| 30 | + return res |
| 31 | + |
| 32 | +def main(): |
| 33 | + while True: |
| 34 | + res = input('Do you want to enter prime numbers (y) or let the algorithm do it for you (n) or exit (e)? (y/n/e): ') |
| 35 | + if res == 'y': |
| 36 | + while True: |
| 37 | + p = int(input('Enter a prime number: ')) |
| 38 | + if is_prime(p): |
| 39 | + break |
| 40 | + else: |
| 41 | + print(p, 'is not a prime number') |
| 42 | + |
| 43 | + while True: |
| 44 | + q = int(input('Enter a different prime number: ')) |
| 45 | + if is_prime(q) and p * q > 26: |
| 46 | + break |
| 47 | + else: |
| 48 | + print('Both prime numbers are the same or the product is less than 26!') |
| 49 | + |
| 50 | + n = p * q |
| 51 | + phi_n = (p - 1) * (q - 1) |
| 52 | + |
| 53 | + while True: |
| 54 | + a = int(input(f'Enter a number such that Greatest Common Divisor with {phi_n} is 1: ')) |
| 55 | + if gcd(a, phi_n) != 1: |
| 56 | + continue |
| 57 | + else: |
| 58 | + break |
| 59 | + |
| 60 | + b = multiplicative_inverse(a, phi_n) |
| 61 | + message = input('Enter the message to be encrypted (lower case): ').lower() |
| 62 | + |
| 63 | + encrypted_string = "" |
| 64 | + encrypted_num = [] |
| 65 | + |
| 66 | + for ch in message: |
| 67 | + if ch != ' ': |
| 68 | + m = ord(ch) - 97 |
| 69 | + e = powermod(m, a, n) |
| 70 | + encrypted_num.append(e) |
| 71 | + encrypted_string += chr(e % 26 + 97) |
| 72 | + else: |
| 73 | + encrypted_string += ' ' |
| 74 | + |
| 75 | + print('Encrypted message is:', encrypted_string) |
| 76 | + res = input("Do you want to decrypt it too? (y/n): ") |
| 77 | + if res == 'y': |
| 78 | + decrypted = '' |
| 79 | + j = 0 |
| 80 | + for ch in encrypted_string: |
| 81 | + if ch != ' ': |
| 82 | + e = encrypted_num[j] |
| 83 | + m = powermod(e, b, n) |
| 84 | + ch = chr(m + 97) |
| 85 | + decrypted += ch |
| 86 | + j += 1 |
| 87 | + else: |
| 88 | + decrypted += ' ' |
| 89 | + |
| 90 | + print("Decrypted message is:", decrypted) |
| 91 | + else: |
| 92 | + ans = input("Do you want to continue? (y/n): ") |
| 93 | + if ans == 'y': |
| 94 | + continue |
| 95 | + else: |
| 96 | + break |
| 97 | + elif res == 'n': |
| 98 | + p, q, a, b = 13, 17, 5, 77 |
| 99 | + n = p * q |
| 100 | + message = input('Enter the message to be encrypted (lower case): ').lower() |
| 101 | + |
| 102 | + encrypted_string = "" |
| 103 | + encrypted_num = [] |
| 104 | + |
| 105 | + for ch in message: |
| 106 | + if ch != ' ': |
| 107 | + m = ord(ch) - 97 |
| 108 | + e = powermod(m, a, n) |
| 109 | + encrypted_num.append(e) |
| 110 | + encrypted_string += chr(e % 26 + 97) |
| 111 | + else: |
| 112 | + encrypted_string += ' ' |
| 113 | + |
| 114 | + print('Encrypted message is:', encrypted_string) |
| 115 | + res = input("Do you want to decrypt it too? (y/n): ") |
| 116 | + if res == 'y': |
| 117 | + decrypted = '' |
| 118 | + j = 0 |
| 119 | + for ch in encrypted_string: |
| 120 | + if ch != ' ': |
| 121 | + e = encrypted_num[j] |
| 122 | + m = powermod(e, b, n) |
| 123 | + ch = chr(m + 97) |
| 124 | + decrypted += ch |
| 125 | + j += 1 |
| 126 | + else: |
| 127 | + decrypted += ' ' |
| 128 | + |
| 129 | + print("Decrypted message is:", decrypted) |
| 130 | + else: |
| 131 | + ans = input("Do you want to continue? (y/n): ") |
| 132 | + if ans == 'y': |
| 133 | + continue |
| 134 | + else: |
| 135 | + break |
| 136 | + elif res == 'e': |
| 137 | + break |
| 138 | + else: |
| 139 | + print('Invalid command!') |
| 140 | + continue |
| 141 | + |
| 142 | +if __name__ == '__main__': |
| 143 | + main() |
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