Overflow Exception when encrypting message with python-rsa

Question

I'm using Python 3 and Python-rsa (https://stuvel.eu/rsa) and I keep getting the same error when encrypting a message:

> Traceback (most recent call last):
``File "client.py", line 65, in <module>
msgg = encrypt_text(pubkey, msg)
File "client.py", line 54, in encrypt_text
return rsa.encrypt(msg.encode(), pubkey)
File "C:\Users\N0t_an_admin\AppData\Local\Programs\Python\Python36-32\lib\s
-packages\rsa\pkcs1.py", line 170, in encrypt
padded = _pad_for_encryption(message, keylength)
File "C:\Users\N0t_an_admin\AppData\Local\Programs\Python\Python36-32\lib\s
-packages\rsa\pkcs1.py", line 87, in _pad_for_encryption
' space for %i' % (msglength, max_msglength))
OverflowError: 1 bytes needed for message, but there is only space for -10

Code:

def get_server_pub():
    pubkey = listener("serverpub").decode("utf-8")
    pubkey = pubkey.strip(";")
    xd = rsa.PublicKey(n= int(pubkey[0]), e= int(pubkey[1]))
    return xd

def encrypt_text(pubkey, msg):
    return rsa.encrypt(msg.encode(), pubkey)


if __name__ == '__main__':
    print("Hello")
    if os.path.isfile('ckeys.json') == False:
        keys = createkeys()
        write_to_json_file('ckeys.json', keys)
    pubkey = get_server_pub()
    while True:
        msg = input("Message: ")
        msgg = encrypt_text(pubkey, msg)
        recived = listener(msgg)
        if not recived:
            continue
        print(recived)

The documentation says:

OverflowError – when the message is too large to fit in the padded block.

What do you use as Message?

Alfabravo
– Alfabravo

2018-04-05 16:52:38 +00:00
Commented Apr 5, 2018 at 16:52 — Alfabravo
– Alfabravo, Commented Apr 5, 2018 at 16:52
@Alfabravo a string. From msg = input("Message: ")

Luís Duarte
– Luís Duarte

2018-04-05 16:55:36 +00:00
Commented Apr 5, 2018 at 16:55 — Luís Duarte
– Luís Duarte, Commented Apr 5, 2018 at 16:55

Alfabravo · Accepted Answer · 2018-04-05 17:00:27Z

1

Checking the code for that library, the section to blame is this one

def _pad_for_encryption(message, target_length):
    r"""Pads the message for encryption, returning the padded message.
    :return: 00 02 RANDOM_DATA 00 MESSAGE
    >>> block = _pad_for_encryption(b'hello', 16)
    >>> len(block)
    16
    >>> block[0:2]
    b'\x00\x02'
    >>> block[-6:]
    b'\x00hello'
    """

    max_msglength = target_length - 11
    msglength = len(message)

    if msglength > max_msglength:
        raise OverflowError('%i bytes needed for message, but there is only'
' space for %i' % (msglength, max_msglength))

And that method is called here

keylength = common.byte_size(pub_key.n)
padded = _pad_for_encryption(message, keylength)

It seems that the key length is being calculated incorrectly, so the maxlength is -10. Check that you are getting the public key.

answered Apr 5, 2018 at 17:00

Alfabravo

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1 Comment

Luís Duarte Over a year ago

Yeah you're right, I wasn't getting the publickey in the correct way because I was using pubkey = pubkey.strip(";") instead of pubkey = pubkey.split(";")

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