from cryptography.fernet import Fernet
key = Fernet.generate_key()
f = Fernet(key)
l = f.encrypt(bytes(input(), encoding="UTF-8"))   #ввожу строку, зразу перетворюю в байти і шифрую
file = open("C:/Users/xxxl/Desktop/eff1c.txt", 'wb')  #!!!
file.write(l)
file.close()
file = open("C:/Users/xxxl/Desktop/eff1c.txt", 'rb')
print(f.decrypt(file.read()).decode("UTF-8"))     #вивожу на екран все що в документі
file.close()                                      #при цьому спочатку розшифровую і тоді з байтів перевожу в стрічку

Traceback (most recent call last):
  File "C:\Users\xxxl\AppData\Local\Programs\Python\Python37\lib\site-packages\cryptography\fernet.py", line 106, in _verify_signature
    h.verify(data[-32:])
  File "C:\Users\xxxl\AppData\Local\Programs\Python\Python37\lib\site-packages\cryptography\hazmat\primitives\hmac.py", line 69, in verify
    ctx.verify(signature)
  File "C:\Users\xxxl\AppData\Local\Programs\Python\Python37\lib\site-packages\cryptography\hazmat\backends\openssl\hmac.py", line 73, in verify
    raise InvalidSignature("Signature did not match digest.")
cryptography.exceptions.InvalidSignature: Signature did not match digest.

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "D:/d/illya/practice/practice/practic_x_1.py", line 22, in <module>
    print(f.decrypt(file.read()).decode("UTF-8"))     #вивожу на екран все що в документі
  File "C:\Users\xxxl\AppData\Local\Programs\Python\Python37\lib\site-packages\cryptography\fernet.py", line 75, in decrypt
    return self._decrypt_data(data, timestamp, ttl)
  File "C:\Users\xxxl\AppData\Local\Programs\Python\Python37\lib\site-packages\cryptography\fernet.py", line 119, in _decrypt_data
    self._verify_signature(data)
  File "C:\Users\xxxl\AppData\Local\Programs\Python\Python37\lib\site-packages\cryptography\fernet.py", line 108, in _verify_signature
    raise InvalidToken
cryptography.fernet.InvalidToken

Traceback (most recent call last):
  File "D:/d/illya/practice/practice/practic_x_1.py", line 20, in <module>
    print(f.decrypt(file.read()).decode("UTF-8"))
  File "C:\Users\xxxl\AppData\Local\Programs\Python\Python37\lib\site-packages\cryptography\fernet.py", line 74, in decrypt
    timestamp, data = Fernet._get_unverified_token_data(token)
  File "C:\Users\xxxl\AppData\Local\Programs\Python\Python37\lib\site-packages\cryptography\fernet.py", line 94, in _get_unverified_token_data
    raise InvalidToken
cryptography.fernet.InvalidToken

from cryptography.fernet import Fernet
key = Fernet.generate_key()
f = Fernet(key)
l = f.encrypt(bytes(input(), encoding="UTF-8"))   
file = open("C:/Users/xxxl/Desktop/eff1c.txt", 'wb+')  #міняється тільки значення 'wb+' на 'ab+'
file.write(l)
print(f.decrypt(file.read()).decode("UTF-8"))
file.close()