cscg ist super

2026/cscg/crypto/intro1/main.py

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+#!/usr/bin/env pypy3
+
+import os
+from pydoc import plain
+from sys import byteorder
+from Crypto.Cipher import AES
+from Crypto.Util import Counter
+import hashlib
+
+# Create a secret.py file with a variable `FLAG` for local testing :)
+from secret import FLAG
+
+secret_key = os.urandom(16)
+
+def encrypt(plaintext, counter):
+    m = hashlib.sha256()
+    m.update(counter.to_bytes(8, byteorder="big"))
+
+    alg = AES.new(secret_key, AES.MODE_CTR, nonce=m.digest()[0:8])
+    ciphertext = alg.encrypt(plaintext)
+
+    return ciphertext.hex()
+
+
+def main():
+    print("DES is broken, long live the secure AES encryption!")
+    print("Give me a plaintext and I'll encrypt it a few times for you. For more security of course!")
+
+    try:
+        plaintext = bytes.fromhex(input("Enter some plaintext (hex): "))
+    except ValueError:
+        print("Please enter a hex string next time.")
+        exit(0)
+    
+    for i in range(256):
+        print(f"Ciphertext {i:03d}: {encrypt(plaintext, i)}")
+    
+    print("Flag:", encrypt(FLAG.encode("ascii"), int.from_bytes(os.urandom(1), byteorder="big")))
+
+if __name__ == "__main__":
+    main()

2026/cscg/crypto/intro1/secret.py

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+FLAG = "CSCG{i_love_my_mum}"

2026/cscg/crypto/intro1/solve.py

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+from pwn import context, args, process, remote, success, xor
+
+context.update(arch='amd64', os='linux', log_level='info')
+
+URL = 'iaet6burymdllg3rflxg4ym7vk-1024-intro-crypto-1.challenge.cscg.live'
+PORT = 443
+
+def start():
+    if args.REMOTE:
+        return remote(URL, PORT, ssl=True)
+    else:
+        return process(['python3', './main.py'])
+
+io = start()
+
+payload_len = 512
+io.sendlineafter(b"Enter some plaintext (hex): ", (b"00" * payload_len))
+
+keystreams = []
+for i in range(256):
+    line = io.recvline()
+    ks = bytes.fromhex(line.strip().split(b" ")[-1].decode())
+    keystreams.append(ks)
+
+io.recvuntil(b"Flag: ")
+flag_enc = bytes.fromhex(io.recvline().strip().decode())
+
+io.close()
+
+for ks in keystreams:
+    candidate = xor(flag_enc, ks[:len(flag_enc)])
+    if b"CSCG{" in candidate or b"flag{" in candidate:
+        success(f"Found Flag: {candidate.decode()}")
+        break