1. from Crypto.Util.number import long_to_bytes, bytes_to_long, isPrime, inverse
2. import random, time
3.  
4. class RSA:
5.     def __init__(self):
6.         random.seed(int(time.time()))
7.         p = self.getPrime(random.getrandbits(512))
8.         q = self.getPrime(random.getrandbits(512))
9.         assert isPrime(p) == True and isPrime(q) == True
10.         self.n = p * q
11.         self.phi = (p-1)*(q-1)
12.         self.e = 65537
13.         self.d = inverse(self.e, self.phi)
14.         self.flag = "ISPCTF{}"
15.         self.enc_flag = hex(pow(bytes_to_long(self.flag.encode('utf-8')), self.e, self.n))
16.    
17.     def getPrime(self, num):
18.         if num % 2 == 0:
19.             num += 1
20.         while not isPrime(num):
21.             num += 2
22.         return num
23.     def title(self):
24.         return '''
25.        ----------------------------------------------------------------
26.        |     WELCOME TO MY RSA SERVICE ENCRYPT AND DECRYPT FOR FREE   |
27.        ----------------------------------------------------------------                                                
28.        |        1. Give me a message and then receive a encryption    |
29.        |        2. Give me a decryption and then receive a message    |
30.        |        3. Receive encrypted the flag as a gift               |
31.        |        4. Give me the key (d) to get the flag as rewards     |
32.        |--------------------------------------------------------------|
33.        '''
34.        
35.     def encrypt(self, mess):
36.         return hex(pow(bytes_to_long(mess.encode('utf-8')), self.e,self.p*self.q))
37.  
38.     def decrypt(self, isGetFlag, enc, key):
39.         try:
40.             if isGetFlag:
41.                 key = int(key, 16)
42.             else:
43.                 key = self.d
44.             mess = long_to_bytes(pow(int(enc, 16), key, self.n)).decode('utf-8')
45.             return mess
46.         except Exception as e:
47.             return "Invalid Key"
48.        
49.     def process(self):
50.         print(self.title())
51.         while True:
52.             try:
53.                 choice = int(input("Your choice: "))
54.                 if choice == 1:
55.                     mess = input("What would you like to encrypt? Type here: ")
56.                     enc = self.encrypt(mess)
57.                     print(f"Here your cipher: {enc}")
58.                 elif choice == 2:
59.                     enc = input("What would you like to decrypt? Type here: ")
60.                     data = self.decrypt(isGetFlag=False, enc=enc, key=None)
61.                     print(f"Here your data: {data}")
62.                 elif choice == 3:
63.                     print(f"Here your encryption of the flag: {self.enc_flag}")
64.                 elif choice == 4:
65.                     key = input("What is your key? Type here: ")
66.                     data = self.decrypt(isGetFlag=True, enc=self.enc_flag, key=key)
67.                     if data == self.flag:
68.                         print(f"Congratulation !! Please submit your flag and receive your reward !! {self.flag}")
69.                     else:
70.                         print(data)
71.                 else:
72.                     print(self.title())
73.             except Exception as e:
74.                 print("Something went wrong")
75.                
76.                
77. app = RSA()
78. app.process()