SecurinetsQuals2k21 | MiTM

#securinetsquals2k21

from Crypto.Util.number import long_to_bytes
from Crypto.Util.Padding import pad
from Crypto.Cipher import AES
from secret import flag
import hashlib
import random
import os
import signal

class DHx():
    def __init__(self):
        self.g = 2
        self.p = 0xf18d09115c60ea0e71137b1b35810d0c774f98faae5abcfa98d2e2924715278da4f2738fc5e3d077546373484585288f0637796f52b7584f9158e0f86557b320fe71558251c852e0992eb42028b9117adffa461d25c8ce5b949957abd2a217a011e2986f93e1aadb8c31e8fa787d2710683676f8be5eca76b1badba33f601f45
        self.private = random.randint(1, self.p - 1)
        self.secret = None

    def getPublicKey(self):
        return pow(self.g, self.private, self.p)

    def share(self, x : int):
        assert x > 1 and x < self.p
        return pow(x, self.private, self.p)

    def getSharedSecret(self, x : int):
        assert x > 1 and x < self.p
        self.secret = pow(x, self.private, self.p)

    def getFingerprint(self):
        return hashlib.sha256(long_to_bytes(self.secret)).hexdigest()

    def checkFingerprint(self, h1 : str, h2 : str):
        return h1 == h2 == self.getFingerprint()

    def encryptFlag(self):
        iv = os.urandom(16)
        key = hashlib.sha1(long_to_bytes(self.secret)).digest()[:16]
        return iv.hex() + AES.new(key, AES.MODE_CBC, iv).encrypt(pad(flag, 16)).hex()

signal.alarm(60)

Alice = DHx()
Bob = DHx()
Carol = DHx()

A = Alice.getPublicKey()
print("Alice sends to Bob: {}".format(A))
A = int(input("Forward to Bob: "))
B = Bob.share(A)
print("Bob sends to Carol: {}".format(B))
B = int(input("Forward to Carol: "))
Carol.getSharedSecret(B)

B = Bob.getPublicKey()
print("Bob sends to Carol: {}".format(B))
B = int(input("Forward to Carol: "))
C = Carol.share(B)
print("Carol sends to Alice: {}".format(C))
C = int(input("Forward to Alice: "))
Alice.getSharedSecret(C)

C = Carol.getPublicKey()
print("Carol sends to Alice: {}".format(C))
C = int(input("Forward to Alice: "))
A = Alice.share(C)
print("Alice sends to Bob: {}".format(A))
A = int(input("Forward to Bob: "))
Bob.getSharedSecret(A)

print("Alice says: ")
if (Alice.checkFingerprint(Carol.getFingerprint(), Bob.getFingerprint())):
    print(Alice.encryptFlag())
else:
    print("ABORT MISSION! Walls have ears; Be careful what you say as people may be eavesdropping.")

Diffie-Hellmanのman in the middleをやる。それだけだと簡単すぎるので3人の鍵のfingerprintが一致していなければならない。

man in the middleでを渡してそれぞれの秘密鍵がいい感じにとなっていれば（すなわち、全部偶数なら）最終的に全員が共通の鍵としてを持つ

from ptrlib import Socket
from Crypto.Util.number import long_to_bytes
from Crypto.Cipher import AES
import hashlib
import time

while True:
    time.sleep(1)
    sock = Socket("crypto1.q21.ctfsecurinets.com", 1337)
    p = 0xf18d09115c60ea0e71137b1b35810d0c774f98faae5abcfa98d2e2924715278da4f2738fc5e3d077546373484585288f0637796f52b7584f9158e0f86557b320fe71558251c852e0992eb42028b9117adffa461d25c8ce5b949957abd2a217a011e2986f93e1aadb8c31e8fa787d2710683676f8be5eca76b1badba33f601f45

    sock.sendlineafter("Forward to Bob: ", str(p - 1))
    AB = int(sock.recvlineafter("Carol: "))
    if AB == 1:
        sock.close()
        continue
    sock.sendlineafter("Forward to Carol: ", str(AB))
    print("[+] progress 1")

    sock.sendlineafter("Forward to Carol: ", str(p - 1))
    BC = int(sock.recvlineafter("Alice: "))
    if BC == 1:
        sock.close()
        continue
    sock.sendlineafter("Forward to Alice: ", str(BC))
    print("[+] progress 2")

    sock.sendlineafter("Forward to Alice: ", str(p - 1))
    CA = int(sock.recvlineafter("Bob: "))
    if CA == 1:
        sock.close()
        continue
    sock.sendlineafter("Forward to Bob: ", str(CA))
    print("[+] progress 3")

    print(sock.recvline())
    line = sock.recvline().decode()
    if "ABORT" in line:
        sock.close()
        continue

    key = hashlib.sha1(long_to_bytes(p - 1)).digest()[:16]
    iv, cipher = bytes.fromhex(line[:32]), bytes.fromhex(line[32:])
    aes = AES.new(key, AES.MODE_CBC, iv)
    print(aes.decrypt(cipher))

    break