ALL the CTFS of Crypto2025 finally

crypto-simmetric/forge-a-cookie/chall.py

@@ -0,0 +1,80 @@
+from Cryptodome.Cipher import ChaCha20
+from Cryptodome.Random import get_random_bytes
+#from secret import flag
+import json
+import base64
+
+key = get_random_bytes(32)
+
+
+def make_cipher():
+    nonce = get_random_bytes(12)
+    cipher = ChaCha20.new(key=key, nonce=nonce)
+    return nonce, cipher
+
+
+def get_user_token(name):
+    nonce, cipher = make_cipher()
+    token = json.dumps({
+        "username": name
+    })
+    #token = json.dumps({
+    #    "admin": True
+    #})
+    print(f"{token.encode()}")
+
+    enc_token = cipher.encrypt(token.encode())
+    print(f"ENC_TOKEN:{enc_token}, NONCE:{nonce}")
+    return f"{base64.b64encode(nonce).decode()}.{base64.b64encode(enc_token).decode()}"
+
+
+def check_user_token(token):
+    nonce, token = token.split(".")
+    #It uses the nonce from the token
+    nonce = base64.b64decode(nonce)
+    print(f"CHECK NONCE {nonce} TOKEN {base64.b64decode(token)}")
+    #Uses the same KEY used at the beginning
+    #cipher = ChaCha20.new(key=key, nonce=nonce)
+    cipher = ChaCha20.new(key=key, nonce=nonce)
+    #Obtain from the token and the nonce the key used
+    #at the beginning forge new cookie enjoy
+    dec_token = cipher.decrypt(base64.b64decode(token))
+
+    user = json.loads(dec_token)
+    print(user.get("admin",False))
+    if user.get("admin", False) == True:
+        return True
+    else:
+        return False
+
+
+def get_flag(token):
+    #token = input("What is your token?\n> ").strip()
+    if check_user_token(token):
+        print("You are admin!")
+        print(f"This is your flag!\nPUPPA")
+    else:
+        print("HEY! WHAT ARE YOU DOING!?")
+        exit(1)
+
+
+if __name__ == "__main__":
+    name = input("Hi, please tell me your name!\n> ").strip()
+    token = get_user_token(name)
+    print("This is your token: " + token)
+    menu = \
+        "What do you want to do?\n" + \
+        "quit - quit the program\n" + \
+        "help - show this menu again\n" + \
+        "flag - get the flag\n" + \
+        "> "
+    while True:
+        cmd = input(menu).strip()
+
+        if cmd == "quit":
+            break
+        elif cmd == "help":
+            continue
+        elif cmd == "flag":
+            #check_user_token(token)
+            get_flag(token)