Custom Encryption

custom_encryption/custom_encryption.py

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+#!/home/maxime/.pyvenv/bin/python3
+from random import randint
+import sys
+
+
+def generator(g, x, p):
+    return pow(g, x) % p
+
+
+def encrypt(plaintext, key):
+    cipher = []
+    for char in plaintext:
+        cipher.append(((ord(char) * key*311)))
+    return cipher
+
+
+def is_prime(p):
+    v = 0
+    for i in range(2, p + 1):
+        if p % i == 0:
+            v = v + 1
+    if v > 1:
+        return False
+    else:
+        return True
+
+
+def dynamic_xor_encrypt(plaintext, text_key):
+    cipher_text = ""
+    key_length = len(text_key)
+    for i, char in enumerate(plaintext[::-1]):
+        key_char = text_key[i % key_length]
+        encrypted_char = chr(ord(char) ^ ord(key_char))
+        cipher_text += encrypted_char
+    return cipher_text
+
+
+def test(plain_text, text_key):
+    p = 97
+    g = 31
+    if not is_prime(p) and not is_prime(g):
+        print("Enter prime numbers")
+        return
+    a = randint(p-10, p)
+    b = randint(g-10, g)
+    print(f"a = {a}")
+    print(f"b = {b}")
+    u = generator(g, a, p)
+    v = generator(g, b, p)
+    key = generator(v, a, p)
+    b_key = generator(u, b, p)
+    shared_key = None
+    if key == b_key:
+        shared_key = key
+    else:
+        print("Invalid key")
+        return
+    semi_cipher = dynamic_xor_encrypt(plain_text, text_key)
+    cipher = encrypt(semi_cipher, shared_key)
+    print(f'cipher is: {cipher}')
+
+
+if __name__ == "__main__":
+    message = sys.argv[1]
+    test(message, "trudeau")

custom_encryption/enc_flag

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+a = 88
+b = 26
+cipher is: [97965, 185045, 740180, 946995, 1012305, 21770, 827260, 751065, 718410, 457170, 0, 903455, 228585, 54425, 740180, 0, 239470, 936110, 10885, 674870, 261240, 293895, 65310, 65310, 185045, 65310, 283010, 555135, 348320, 533365, 283010, 76195, 130620, 185045]

custom_encryption/sol.py

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+#!/home/maxime/.pyvenv/bin/python3
+from pwn import *
+
+with open("enc_flag", 'r') as f:
+    text = f.readlines()
+    a = int(text[0][4:].strip())
+    b = int(text[1][4:].strip())
+    cipher = text[2][11:].replace('[', '').replace(']', '').replace(' ', '').strip().split(',')
+    
+    p = 97
+    g = 31
+    text_key = "trudeau"
+
+    key = pow(g, a*b, p)
+    semi_cipher = ""
+    for i in cipher:
+        i = int(i.strip())
+        semi_cipher += chr(i // (311*key))
+    plaintext = ""
+    for i, char in enumerate(semi_cipher):
+        plaintext += chr(ord(char) ^ ord(text_key[i%len(text_key)]))
+    plaintext = plaintext[::-1]
+    print(plaintext)
+