kartmaan / filecrypt

Python script to encrypt and decrypt files

Filecrypt

This Python script intended to be executed on the command line, allows to encrypt and decrypt files present in the current folder of the script. The program is based on the cryptography module and in particular its Fernet class allowing symmetric encryption using the 128-bit AES algorithm.

encrypt funtion

Allows to encrypt a file found in the script's current folder. Before encrypting the file, the program generates a filekey.key file containing the randomly generated secret key from which the file will be encrypted. This file is to be kept preciously in order to be able to decrypt the file later and prevent the file from being read by others.

The function must take two arguments:

• The name of the file to encrypt (with its extension)
• The choice of procedure:
  • ow : the file will be overwritten, in the case of an image file for example, it will therefore become unusable (corrupted) in its encrypted form
  • c : the file will be copied to the current folder before being overwritten

Case examples for psw.txt file encryption :

Overwrite mode :

python filecrypt.py encrypt psw.txt ow

Copy mode :

python filecrypt.py encrypt psw.txt c

The encryption operation will leave a filekey.key file in the current folder which must be kept safe

decrypt function

Used to decrypt a file found in the current folder using a key file also present in the folder and in which the secret key is located. The function must take 2 parameters :

• The name of the file to decrypt (with its extension)
• The name of the key file present in the current folder (with its extension)

Case examples for psw.txt file decryption :

python filecrypt.py decrypt psw.txt filekey.key

Some technical details

The Fernet class of the cryptography module used in this program is based on the AES (Advanced Encryption Standard) algorithm using a 128bit key. The algorithm is in CBC mode (Cipher Block Chaining) which means that the data is processed in 128-bit blocks each of which is independently encrypted. The data authentication functions are provided by the SHA-265 hashing algorithm.