Python mutable error when comparing two very large numbers

Question

I am new to python and I am writing a secure ftp server / client to handle basic file upload / download (but encrypted).

So that the client has a secret key, I encrypt and send a randomized 32-byte number. The client must decrypt the number, add it, re-encrypt and return it to the server. The server checks if the response is a random number + 1, and then proceeds to the connection if it is equivalent. The problem is that this works in about 90% of cases, but sometimes it fails (even when the client sends a good response).

if int.from_bytes(challenge, "big") + 1 == int.from_bytes(response, "big"): print("Good\nExpected: {0}\nReceived: {0}".format( int.from_bytes(challenge, "big") + 1, int.from_bytes(response, "big"))) else: print("Bad\nExpected: {0}\nReceived: {0}".format( int.from_bytes(challenge, "big") + 1, int.from_bytes(response, "big"))) 

9 times out of 10, this enters the True state, but sometimes it fails. These are examples of when he failed:

 Bad Expected: 65159048323870645118410560973513118036375130115063959378348917255680432299875 Received: 65159048323870645118410560973513118036375130115063959378348917255680432299875 Bad Expected: 94602782648778784750235610259612519850690550920952731294858863927077528757933 Received: 94602782648778784750235610259612519850690550920952731294858863927077528757933 

Please note that each of the above expected and received is virtually identical.

Here are some examples of successful numbers:

 Good Expected: 91751260209520864629218443027060768890746721638897648279482154562044918570881 Received: 91751260209520864629218443027060768890746721638897648279482154562044918570881 Good Expected: 104504930179798203375748204555227260444250367405369759767776407892919812999121 Received: 104504930179798203375748204555227260444250367405369759767776407892919812999121 

Any idea what is going on? Thank you for your time. Edit: code snippet:

Server side:

 challenge = os.urandom(32) socket.send_msg(challenge, encrypt=True) response = socket.recv_msg(32, decrypt=True) if int.from_bytes(challenge, "big") + 1 != int.from_bytes(response, "big"): print("Expected: {0}\nReceived: {0}".format(int.from_bytes(challenge, "big") + 1, int.from_bytes(response, "big"))) 

client side:

 challenge = self._socket.recv_raw(32, decrypt=True) challenge = int.from_bytes(challenge, "big") + 1 self.sckt.send_msg(challenge.to_bytes(32, "big"), encrypt=True) 

Please note that I have carefully tested the encryption code and socket messaging protocol, this is not a problem.

Answer 1

The problem is with your format calls. You use {0} twice, which means that you get the first positional argument repeated twice in the output text. The integer response transformation is never printed.

To fix the report (no problem causes inconsistent data), you must either use {0} , or {1} once, or leave your numbers completely (Python will automatically use {0} for the first {} and {1} for second, etc.