added default.nix and various ctf examples

ctf-solutions/symmetry.py

@@ -0,0 +1,39 @@
+'''
+Solution to https://cryptohack.org/courses/symmetric/symmetry/ 
+'''
+
+import os
+import requests
+
+from imp.math.util import xor_bytes, xor_str
+
+URL = 'https://aes.cryptohack.org/symmetry'
+
+def get_encrypted_flag() -> tuple[bytes, bytes]:
+    resp = requests.get(f'{URL}/encrypt_flag/')
+    ciphertext = resp.json()['ciphertext']
+    iv = bytes.fromhex(ciphertext[:32])
+    flag = bytes.fromhex(ciphertext[32:])
+    return iv, flag
+
+def encrypt(plaintext: bytes, iv: bytes) -> bytes:
+    plaintext, iv = plaintext.hex(), iv.hex()
+    resp = requests.get(f'{URL}/encrypt/{plaintext}/{iv}')
+    return bytes.fromhex(resp.json()['ciphertext'])
+
+def main() -> None:
+    iv, flag = get_encrypted_flag()
+    # generate a random plaintext of equal length to the flag
+    random_plaintext = os.urandom(len(flag))
+    random_ciphertext = encrypt(random_plaintext, iv)
+    # XOR random plaintext with corresponding ciphertext to
+    # get the set generated by IV under the keyed AES permutation
+    aes_orbit = xor_bytes(random_plaintext, random_ciphertext)
+    # XOR this orbit with the flag to get the hexed flag plaintext
+    flag_plaintext = xor_bytes(flag, aes_orbit)
+    print('Flag:', flag_plaintext.decode())
+    
+    print('IV:', iv.hex())
+
+if __name__ == '__main__':
+    main()