dhke-aes!

cryptography/dhke-aes/DESCRIPTION.md

@@ -0,0 +1,3 @@
+Armed with your knowledge of DHKE, you will now build your first cryptosystem that resembles something real!
+You'll use DHKE to negotiate an AES key, and the challenge will use that key to encrypt the flag.
+Decrypt it, and win!

cryptography/dhke-aes/run

@@ -0,0 +1,50 @@
+#!/opt/pwn.college/python
+
+import sys
+from base64 import b64encode
+from Crypto.Cipher import AES
+from Crypto.Util.Padding import pad
+from Crypto.Random.random import getrandbits
+
+flag = open("/flag", "rb").read()
+assert len(flag) <= 256
+
+# 2048-bit MODP Group from RFC3526
+p = int.from_bytes(bytes.fromhex(
+    "FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1 "
+    "29024E08 8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD "
+    "EF9519B3 CD3A431B 302B0A6D F25F1437 4FE1356D 6D51C245 "
+    "E485B576 625E7EC6 F44C42E9 A637ED6B 0BFF5CB6 F406B7ED "
+    "EE386BFB 5A899FA5 AE9F2411 7C4B1FE6 49286651 ECE45B3D "
+    "C2007CB8 A163BF05 98DA4836 1C55D39A 69163FA8 FD24CF5F "
+    "83655D23 DCA3AD96 1C62F356 208552BB 9ED52907 7096966D "
+    "670C354E 4ABC9804 F1746C08 CA18217C 32905E46 2E36CE3B "
+    "E39E772C 180E8603 9B2783A2 EC07A28F B5C55DF0 6F4C52C9 "
+    "DE2BCBF6 95581718 3995497C EA956AE5 15D22618 98FA0510 "
+    "15728E5A 8AACAA68 FFFFFFFF FFFFFFFF"
+), "big")
+g = 2
+print(f"p = {p:#x}")
+print(f"g = {g:#x}")
+
+a = getrandbits(2048)
+A = pow(g, a, p)
+print(f"A = {A:#x}")
+
+try:
+    B = int(input("B? "), 16)
+except ValueError:
+    print("Invalid B value (not a hex number)", file=sys.stderr)
+    sys.exit(1)
+if B <= 2**1024:
+    print("Invalid B value (B <= 2**1024)", file=sys.stderr)
+    sys.exit(1)
+
+s = pow(B, a, p)
+key = s.to_bytes(256, "little")[:16]
+
+# friendship ended with DHKE, AES is my new best friend
+cipher = AES.new(key=key, mode=AES.MODE_CBC)
+flag = open("/flag", "rb").read()
+ciphertext = cipher.iv + cipher.encrypt(pad(flag, cipher.block_size))
+print(f"Flag Ciphertext (b64): {b64encode(ciphertext).decode()}")

cryptography/module.yml

@@ -43,6 +43,8 @@ challenges:
   name: "AES-CBC-POA-Encrypt"
 - id: level-6
   name: DHKE
+- id: dhke-aes
+  name: "DHKE-to-AES"
 - id: level-7
   name: RSA 1
 - id: level-8