Cannot decode encrypted rsa message python3

Question

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:\Users\vladi\AppData\Local\Programs\Python\Python37\lib\threading.py", line 926, in _bootstrap_inner
    self.run()
  File "C:\Users\vladi\AppData\Local\Programs\Python\Python37\lib\threading.py", line 870, in run
    self._target(*self._args, **self._kwargs)
  File "C:\Users\vladi\Documents\Programming\python\Server_Client\Client\client.py", line 28, in send
    self.sock.send(str(len(data_to_send)).encode())
TypeError: object of type 'NoneType' has no len()```

actually, this is the main error, and the position of the wrong byte changing while I'm changing pubkey 'utf-8' codec can't decode byte 0xfc in position 5: invalid start byte — Vladik Ainshtein
– Vladik Ainshtein, Commented Dec 10, 2020 at 8:01
I wonder what kind of result you're expecting when you decode a random bytes to a utf8 string? Base64, as mentioned in nneonneos answer, is the right way. — jps
– jps, Commented Dec 10, 2020 at 9:19

Vladik Ainshtein · Accepted Answer · 2020-12-10 14:59:08Z

if someone will need it someday

import rsa
import base64
---------
pub, priv = rsa.newkeys(512)
---------
message_to_send = "Hello Vlad!"
b_message = message_to_send.encode()
encrypted = rsa.encrypt(b_message, pub)
encrypted_b64 = base64.b64encode(encrypted)
encrypted_b64_string = encrypted_b64.decode()
# -- sending via socket --
message_to_recieve = encrypted_b64_string
encrypted_b64 = message_to_recieve.encode()
encrypted = base64.b64decode(encrypted_b64)
b_final_message = rsa.decrypt(encrypted, priv)
final_message = b_final_message.decode()
-----------------------
print('message_to_send')
print(message_to_send)
print('b_message')
print(b_message)
print('encrypted')
print(encrypted)
print('encrypted_b64')
print(encrypted_b64)
print('encrypted_b64_string')
print(encrypted_b64_string)
print('message_to_recieve')
print(message_to_recieve)
print('encrypted_b64')
print(encrypted_b64)
print('encrypted')
print(encrypted)
print('b_final_message')
print(b_final_message)
print('final_message')
print(final_message)
---------------
message_to_send
Hello Vlad!
b_message
b'Hello Vlad!'
encrypted
b'5\x11\xeds\r\rF)&mt\x8bR\xc7\x9cf\x98/V#b[\x04\xe3\x90\x1f$R?=\xaa\xe0\x02\xba\xbeg\xec^F\xae+\x17\xab\xc1\xd1b[\xed\xce\xd8\x15\x18~99\x8a\xc3\xe0^2\x88Iy\xb5'
encrypted_b64
b'NRHtcw0NRikmbXSLUsecZpgvViNiWwTjkB8kUj89quACur5n7F5GrisXq8HRYlvtztgVGH45OYrD4F4yiEl5tQ=='
encrypted_b64_string
NRHtcw0NRikmbXSLUsecZpgvViNiWwTjkB8kUj89quACur5n7F5GrisXq8HRYlvtztgVGH45OYrD4F4yiEl5tQ==
message_to_recieve
NRHtcw0NRikmbXSLUsecZpgvViNiWwTjkB8kUj89quACur5n7F5GrisXq8HRYlvtztgVGH45OYrD4F4yiEl5tQ==
encrypted_b64
b'NRHtcw0NRikmbXSLUsecZpgvViNiWwTjkB8kUj89quACur5n7F5GrisXq8HRYlvtztgVGH45OYrD4F4yiEl5tQ=='
encrypted
b'5\x11\xeds\r\rF)&mt\x8bR\xc7\x9cf\x98/V#b[\x04\xe3\x90\x1f$R?=\xaa\xe0\x02\xba\xbeg\xec^F\xae+\x17\xab\xc1\xd1b[\xed\xce\xd8\x15\x18~99\x8a\xc3\xe0^2\x88Iy\xb5'
b_final_message
b'Hello Vlad!'
final_message
Hello Vlad!

Man, thank you!. I spend many hours on that and your answer helped me a lot. Thaaaaaaaaanks

nneonneo · Accepted Answer · 2020-12-10 08:25:40Z

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.

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