File Encryption

I recently wondered how difficult it is to encrypt a file. In this article I will show you two ways to do it. I use a text file content.txt with the content

This is a top secret message!
Don't show it to others!

Applications

GNU Privacy Guard (GnuPG or GPG) is a free-software replacement for Symantec's PGP cryptographic software suite, and is compliant with RFC 4880, the IETF standards-track specification of OpenPGP.

Check with gpg --list-secret-keys if you already have keys. If not, run

gpg –-gen-key

Encrypt file:

gpg -e -r [email protected] content.txt

Decrypt file:

gpg -d -o out.decrypted.txt content.txt.gpg

Python

In order to generate an encrypted file with Python, I use the fernet module of cryptography. It's not part of the standard library (source), but it is super wide spread.

It uses AES in CBC mode with a 128-bit key for encryption; using PKCS7 padding. Initialization vectors are generated using os.urandom().

from cryptography.fernet import Fernet

# Generate a key
key = Fernet.generate_key()
with open("keyfile", "wb") as fp:
    fp.write(key)

# Get bytes to encrypt
message = "This is secret!"
encoded = message.encode()

# Encrypt the bytes
fernet = Fernet(key)
encrypted = fernet.encrypt(encoded)

# Write encrypted file
with open("encrypted-file", "wb") as fp:
    fp.write(encrypted)

# Decrypt the bytes
fernet = Fernet(key)
decrypted = fernet.decrypt(encrypted)
print(decrypted.decode())

You can also generate the key from a password like this:

import base64
from cryptography.hazmat.backends import default_backend
from cryptography.hazmat.primitives import hashes
from cryptography.hazmat.primitives.kdf.pbkdf2 import PBKDF2HMAC

password = "something I can remember"

# Set this in the application as a constant
# Generate one for yourself with
# >>> import os; os.urandom(16)
salt = b"\xd8\x85\xd3`|\xa2\x82w\x11!\xcc\x8d\xa4\x8a:\xb4"

kdf = PBKDF2HMAC(
    algorithm=hashes.SHA256(),
    length=32,
    salt=salt,
    iterations=100_000,
    backend=default_backend(),
)
key = base64.urlsafe_b64encode(kdf.derive(password.encode()))

Attack Scenarios

For many attack scenarios there are better solutions than file encryption. But file encryption might provide additional security in case there is a breach elsewhere.

Lost Device

Alice was sitting in a taxi, using her laptop and her phone at the same time. She just quickly put her laptop next to her and had a look at her phone. Thinking about the flight she needed to catch, she forgot the laptop. The taxidriver sold the laptop on ebay and Bob bought it. He wants to see if there is valuable information on it and tries to get access.

Assessment: To protect agains this, Alice should use full disk encryption (FDE). If Alice doesn't have FDE, encrypting a single file might help for that single file. But as it only protects one file, it's certainly worse than FDE.

Root Access

Alice catched a virus! No, not COVID-19, but one on her computer. Bob now has remote access to her machine.

Assessment: In this case, I would say all hope is lost. The attacker can install a keylogger and will get the result he wants. Or a crypto trojan which prevents Alice from using her computer. Encrypting a single file might help if Alice doesn't access that file while the attacker has control. FDE would not help here at all.

User Access

Bob has access to the computer of Alice, but just normal user permissions. He cannot install new software. He can just run installed software under the account of Alice.

Assessment: Encrypting a single file helps as long as Alice doesn't access the file.

Man in the Middle: E-Mail

Alice sends data to Charlie via e-mail. Bob catches the message while it's transmitted.

Bob is here the Man in the Middle and makes a Man in the Middle (MitM) attack.

Assessment: The encryption of the e-mail would have helped. Using TLS would be preferable, though.

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