adding validated services? patching forcad_local.py

ctfcup2025-school-final/sploits/sonobank/crypto_sploit.py

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+from sage.all import *
+from socket import socket, AF_INET, SOCK_DGRAM
+import uuid
+from Crypto.Cipher import AES
+from Crypto.Util.Padding import unpad
+import hashlib
+
+IP = "127.0.0.1"
+flag_id = "b0e29b3a-f560-449c-ad59-1b760c4aab47"
+
+PR = PolynomialRing(GF(2), "x")
+
+def bits(n):
+    return [int(c) for c in bin(n)[2:]][::-1]
+
+def unbits(bs):
+    return sum(int(x) << i for i, x in enumerate(list(bs)))
+
+get_crypto_params_payload = bytes((
+    0xF0,
+    0x7D,
+    0x53,
+    0x58,
+    0x05,
+    0xF7
+))
+
+sock = socket(AF_INET, SOCK_DGRAM)
+sock.bind(("0.0.0.0", 0))
+sock.connect((IP, 5004))
+
+sock.sendall(get_crypto_params_payload)
+msg = sock.recv(1024)
+
+modulus = PR(bits(int.from_bytes(msg[5:5 + 64], "big")))
+base = PR(bits(int.from_bytes(msg[5 + 64:5 + 64 + 64], "big")))
+
+
+get_crypto_params_payload = bytes((
+    0xF0,
+    0x7D,
+    0x53,
+    0x58,
+    0x03,
+    *uuid.UUID(flag_id).bytes,
+    0xF7
+))
+
+sock.sendall(get_crypto_params_payload)
+msg = sock.recv(1024)
+h = PR(bits(int.from_bytes(msg[5:5 + 64], "big")))
+c1 = PR(bits(int.from_bytes(msg[5 + 64:5 + 64*2], "big")))
+iv = msg[5 + 64*2:5 + 64*2 + 16]
+ct = msg[5 + 64*2 + 16:-1]
+
+fact = factor(modulus)
+assert len(fact) == 1
+assert fact[0][1] == 5
+D = fact[0][0].degree()
+
+# part modulo p
+R = GF(2 ** D, 'a', modulus=fact[0][0])
+print(R)
+r = R(c1).log(base)
+
+# remaining part
+R = PR.quotient(modulus)
+newbase = R(base) ** (2 ** D - 1)
+newres = R(c1) ** (2 ** D - 1)
+for x in range(1, 8):
+    if newbase ** x == newres:
+        break
+else:
+    print("failed to find part")
+r = crt([r, x], [2 ** D - 1, 8])
+assert R(base) ** r == R(c1)
+
+shared_key = int(unbits(list(R(h) ** r))).to_bytes(64, "big")
+key = hashlib.sha256(shared_key).digest()[:16]
+cipher = AES.new(key, AES.MODE_CBC, iv)
+pt = unpad(cipher.decrypt(ct), 16)
+print(pt)