Cannot decode encrypted rsa message python3

I’m encoding a message with rsa.encrypt but then I cannot convert the encrypted data to a str with .decode(). That’s a bit strange because the encrypted data is a bytes string and there shouldn’t be any problem converting that to a str.

data = [self.id, data, self.my_pubkey] # actually don't care about type of components, they are correct

My code:

import json
import rsa

def msg(query_type, data):
    if query_type == 'PubKey':
        try:
            query = {"Type": "PubKey",
                     "Message": {"PubKey": data[0],
                                 "Id": data[1]
                                 }
                     }
            to_send = json.dumps(query)
            to_send = to_send.encode()
            return to_send
        except Exception as ex:
            print("Error in creating message")
            print(ex)
    elif query_type == 'Message':
        try:
            encrypted_data = rsa.encrypt(data[1].encode('utf-8'), data[2])
            print(encrypted_data.decode('utf-8'))
            query = {"Type": "Message",
                     "Message": {"Id": data[0],
                                 "Data": str(encrypted_data)[2:-1]
                                 }
                     }
            pub = rsa.lo
            to_send = json.dumps(query)
            to_send = to_send.encode()
            return to_send
        except Exception as ex:
            print("Error in creating message")
            print(ex)
        except Exception as ex:
            to_send = str(ex).encode()
            return to_send

But, I’m getting this error:

Error in creating message
'utf-8' codec can't decode byte 0xfc in position 5: invalid start byte
Exception in thread Thread-2:
Traceback (most recent call last):
  File "C:UsersvladiAppDataLocalProgramsPythonPython37libthreading.py", line 926, in _bootstrap_inner
    self.run()
  File "C:UsersvladiAppDataLocalProgramsPythonPython37libthreading.py", line 870, in run
    self._target(*self._args, **self._kwargs)
  File "C:UsersvladiDocumentsProgrammingpythonServer_ClientClientclient.py", line 28, in send
    self.sock.send(str(len(data_to_send)).encode())
TypeError: object of type 'NoneType' has no len()```

Answer

Decoding a byte string as utf-8 only makes sense if you know the bytes represent valid utf-8 data. utf-8 will not decode arbitrary byte strings: there’s a specific format (when bytes are >= 0x80, they are interpreted as either “start” or “continuation” bytes and must follow certain patterns; see the Wikipedia page for more information).

On the other hand, encrypting data (using almost any encryption algorithm) will generate random-looking byte strings that almost certainly will not be valid utf-8.

The solution is to treat the output of the encryption process as a byte string – do not attempt to decode it to a string, as it will not make sense as a string. Python provides the bytes/str distinction precisely for this kind of case: bytes are for binary data (e.g. encrypted data), strings are for textual data.

For dumping binary data (as byte strings) into JSON, I suggest using an encoding like Base64 to encode the bytes into ASCII, rather than trying to use a string. This will be more efficient and much easier to debug.