[G04T] Diffie-Hellman

from Crypto.Util.number import getPrime
from Crypto.Util.Padding import pad, unpad
from Crypto.Cipher import AES
import hashlib
import random

class Person(object):
    def __init__(self, p):
        self.p = p
        self.g = 2
        self.x = random.randint(2, self.p - 1)
    
    def calc_key(self):
        self.k = pow(self.g, self.x, self.p)
        return self.k

    def set_shared_key(self, k):
        self.sk = pow(k, self.x, self.p)
        print("sk : ",self.sk)
        aes_key = hashlib.md5(str(self.sk).encode()).digest()
        self.cipher = AES.new(aes_key, AES.MODE_ECB)

    def encrypt(self, pt):
        return self.cipher.encrypt(pad(pt, 16)).hex()

    def decrypt(self, ct):
        return unpad(self.cipher.decrypt(bytes.fromhex(ct)), 16)

def is_prime(n):
    if n <= 1:
        return False
    for i in range(2, int(n**0.5)+1):
        if n % i == 0:
            return False
    return True

def primitive_root(p):
    if p == 2:
        return 1
    p1 = 2
    p2 = (p-1) // p1

    while(p2 % p1 == 0):
        p1 *= 2
        p2 = (p-1) // p1

    g = pow(2, p2, p)

    while (pow(g, (p-1)//2, p) == 1):
        g = pow(g, 2, p)

    return g

flag = open("flag.txt", "r").read().encode()
prime = 16683502042145070731
print(f'prime : {prime}')

#exit()

alice = Person(prime)
bob = Person(prime)

alice_k = alice.calc_key()
print(f"Alice sends her key to Bob. Key: {alice_k}")

if alice_k == alice.g:
    exit("Malicious key !!")
bob.set_shared_key(alice_k)

bob_k = bob.calc_key()
print(f"Bob sends his key to Alice. Key: {bob_k}")

if bob_k == bob.g:
    exit("Malicious key !!")
alice.set_shared_key(bob_k)

print("They are sharing the part of flag")
print(f"Alice: {alice.encrypt(flag[:len(flag) // 2])}")
print(f"Bob: {bob.encrypt(flag[len(flag) // 2:])}")

 

공유키를 알려주고 있어서 해당 키 값으로 디크립트하면 됩니다.

 

$ nc goatwar.xyz 11000
prime : 16683502042145070731
Alice sends her key to Bob. Key: 4015290253982335710
sk :  9165507793789697032
Bob sends his key to Alice. Key: 4929127157915240762
sk :  9165507793789697032
They are sharing the part of flag
Alice: e479a9dcb605f253c39332a04ce5f726
Bob: 2e38b3ee0ec957aa62bdabd9e6b4ea42

 

from Crypto.Util.number import getPrime
from Crypto.Util.Padding import pad, unpad
from Crypto.Cipher import AES
import hashlib
import random

class Person(object):
    def __init__(self, p):
        self.p = p
        self.g = 2
        self.x = random.randint(2, self.p - 1)
    
    def calc_key(self):
        self.k = pow(self.g, self.x, self.p)
        return self.k

    def set_shared_key(self):
        self.sk = 9165507793789697032
        aes_key = hashlib.md5(str(self.sk).encode()).digest()
        self.cipher = AES.new(aes_key, AES.MODE_ECB)

    def encrypt(self, pt):
        return self.cipher.encrypt(pad(pt, 16)).hex()

    def decrypt(self, ct):
        return unpad(self.cipher.decrypt(bytes.fromhex(ct)), 16)

prime = 16683502042145070731
alice = Person(prime)
bob = Person(prime)
bob.set_shared_key()
alice.set_shared_key()

print(f"Alice: {alice.decrypt('e479a9dcb605f253c39332a04ce5f726')}")
print(f"Bob: {bob.decrypt('2e38b3ee0ec957aa62bdabd9e6b4ea42')}")

Alice: b'G04T{d1ff13_h3'
Bob: b'11m4n_15_10v3}'