How can I decrypt message from user input in python using cryptography?

Question

Im able to decrypt the message running the script, but not when I use the user input method to get the encrypted message.

I have this simple script:

from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes
from cryptography.hazmat.backends import default_backend
from cryptography.hazmat.primitives import padding
padder = padding.PKCS7(128).padder()
backend = default_backend()

def operation_getmessage():
    messagesinput = input("Please, insert messages to decrypt: ")
    messagesinput = bytes(messagesinput, "utf-8")
    message = padder.update(messagesinput)
    message += padder.finalize()
    return message

keyinput = input("Please insert the key used for encrypt : ")
print(keyinput)
key = bytes(str(keyinput), 'ascii')
print("key value is :", key)
message = operation_getmessage()

cipher = Cipher(algorithms.TripleDES(key), modes.ECB(), backend=backend)
encryptor = cipher.encryptor()
message_encrypt = encryptor.update(message) + encryptor.finalize()
print("Value of message_encrypted:", message_encrypt)
test = operation_getmessage()
print("Value of test: ", test)
decryptor = cipher.decryptor()
message_decrypt = decryptor.update(test) + decryptor.finalize()
print("Value of message_decrypted is: ", message_decrypt)
unpadder = padding.PKCS7(128).unpadder()
data = unpadder.update(message_decrypt)
print("Value of message is: ", data + unpadder.finalize())

As I said it works when I run the script skipping the user input message test = operation_getmessage(), but if I tried to get the message from the input I receiving this error:

  Traceback (most recent call last):
    File "/Users/JMD/PycharmProjects/M-CM-Task1/Test.py", line 23, in <module>
      test = operation_getmessage()
    File "/Users/JMD/PycharmProjects/M-CM-Task1/Test.py", line 10, in   operation_getmessage
      message = padder.update(messagesinput)
    File ".../lib/python3.7/site-packages/cryptography/hazmat/primitives/padding.py", line 118, in update
      self._buffer, data, self.block_size)
    File ".../lib/python3.7/site packages/cryptography/hazmat/primitives/padding.py", line 41, in _byte_padding_update
      raise AlreadyFinalized("Context was already finalized.")
    cryptography.exceptions.AlreadyFinalized: Context was already finalised.

this is the whole script exit:

Please insert the key used for encrypt : 123456789012345678901234
123456789012345678901234
key value is : b'123456789012345678901234'
Please, insert messages to decrypt: a secure message
Value of message_encrypted: b'\xe0\xd9\xa7\x85\x07\xe4\x1d;\x19\xcb\xce\xa1*\xbbo\xefT\xae\x15|v\xce\xa2\x88T\xae\x15|v\xce\xa2\x88'
Please, insert messages to decrypt: b'\xe0\xd9\xa7\x85\x07\xe4\x1d;\x19\xcb\xce\xa1*\xbbo\xefT\xae\x15|v\xce\xa2\x88T\xae\x15|v\xce\xa2\x88'

Traceback (most recent call last):
  File "Test.py", line 23, in <module>
test = operation_getmessage()
  File "Test.py", line 10, in operation_getmessage
message = padder.update(messagesinput)
  File ".../lib/python3.7/site-packages/cryptography/hazmat/primitives/padding.py", line 118, in update
self._buffer, data, self.block_size)
  File ".../lib/python3.7/site- packages/cryptography/hazmat/primitives/padding.py", line 41, in _byte_padding_update
raise AlreadyFinalized("Context was already finalized.")
cryptography.exceptions.AlreadyFinalized: Context was already finalized.

Process finished with exit code 1

You should better encode your encrypted text (in Base64 for example) so that you have only printables (and inputables) characters. — Kevin FONTAINE
– Kevin FONTAINE, Commented Oct 24, 2018 at 9:34
you can also generate public/private keys Pairs at both end, Generate Diffi Hellman keys using one's public key and others private key and then use AES encryption for messeges. — GraphicalDot
– GraphicalDot, Commented Oct 24, 2018 at 9:38

Ralf · Accepted Answer · 2018-10-24 09:35:11Z

4

Looking at the docs:

After finalize() has been called this object can no longer be used; update() and finalize() will raise an AlreadyFinalized exception.

Try initializing the padder inside the function operation_getmessage(), so that it is a new padder every time the function gets called.

def operation_getmessage():
    padder = padding.PKCS7(128).padder()
    ...
    return message

answered Oct 24, 2018 at 9:35

Ralf

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Comments

cardamom · Accepted Answer · 2018-10-24 10:09:22Z

I have used the suggestions from @Ralf and @Kevin_Fontaine to fix your code so it works. Here is the corrected script, a few lines are changed and marked with comments:

import base64   # new line here
from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes
from cryptography.hazmat.backends import default_backend
from cryptography.hazmat.primitives import padding

backend = default_backend()

def operation_getmessage():
    padder = padding.PKCS7(128).padder()  # new line here
    messagesinput = input("Please, insert messages to decrypt: ")
    messagesinput = bytes(messagesinput, "utf-8")
    message = padder.update(messagesinput)
    message += padder.finalize()
    return message

keyinput = input("Please insert the key used for encrypt : ")
print(keyinput)
key = bytes(str(keyinput), 'ascii')
print("key value is :", key)
message = operation_getmessage()

cipher = Cipher(algorithms.TripleDES(key), modes.ECB(), backend=backend)
encryptor = cipher.encryptor()
message_encrypt = encryptor.update(message) + encryptor.finalize()
print("Value of message_encrypted:", base64.b64encode(message_encrypt))  # new line here
test = operation_getmessage()
test = base64.b64decode(test)  # new line here
print("Value of test: ", test)
decryptor = cipher.decryptor()
message_decrypt = decryptor.update(test) + decryptor.finalize()
print("Value of message_decrypted is: ", message_decrypt)
unpadder = padding.PKCS7(128).unpadder()
data = unpadder.update(message_decrypt)
print("Value of message is: ", data + unpadder.finalize())

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