CTFs and solutions

forge-a-cookie/attack.py

@@ -0,0 +1,58 @@
+#!/usr/bin/env python3
+from Cryptodome.Cipher import ChaCha20
+from Cryptodome.Random import get_random_bytes
+import json
+import base64
+def foo(name,nonce):
+
+    #token = json.dumps({
+    #    "username" : name
+    #})
+    token = json.dumps({
+        "admin" : True
+    })
+    if json.loads(token).get("admin",False) == True:
+        print("ADMIN achieved")
+    else:
+        print("erorr")
+    #ENCRYPTED token.encode()
+    tc = token.encode()
+    etc = base64.b64encode(tc).decode()
+    dec_token = base64.b64decode(etc)
+    user = json.loads(dec_token)
+    print(user)
+    print(user.get("admin",False))
+    print(f"Token encoded: {tc}")
+    print(f"Token:{token}")
+foo("admin",1)
+name = input("Give me name!\n").strip()
+token = json.dumps({
+    "username" : name
+})
+print(token.encode())
+
+def attack():
+    enc_token = input("Give me the token:")
+    nonce, token = enc_token.split(".")
+    print(f"{nonce}, {token}")
+    payload = json.dumps({
+        "username" : "aaaa"
+    }).encode()
+    adminPayload = json.dumps({
+        "admin" : True
+    }).encode()
+    adminTok = b''
+    tok = base64.b64decode(token)
+    keystream = b''
+    print(f"Len payload:{len(payload)} Len tok:{len(tok)}")
+    keystream = bytes([p ^ t for p, t in zip(payload, tok)])
+    print(len(keystream))
+    adminTok = bytes([aP ^ k for aP, k in zip(adminPayload, keystream)])
+    print(f"admin Token:{adminTok} len admin Payload:{len(adminPayload)} lenTok:{len(adminTok)}")
+    print(f"{nonce}.{base64.b64encode(adminTok).decode()}")
+    #for i,j in zip(tok,payload):
+
+    #    print(bytes(i^j))
+    #print(keystream)
+#attack("j5l1MgGWqU06x2GvgXGEnXkoFPs=")
+attack()

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)