File Encryptor

This is a library used by MetaDisk to convergently encrypt and decrypt files. It contains helper methods to encrypt and decrypt files inline (without using extra space) and to stream decryption.

Installation

You can easily install file-encryptor using pip:

pip install file_encryptor

Usage

Here’s an example to encrypt a file inline using convergent encryption:

import file_encryptor.convergence

key = convergence.encrypt_inline_file("/path/to/file", None)

You can also specify a passphrase:

import file_encryptor.convergence

key = convergence.encrypt_inline_file("/path/to/file", "rainbow dinosaur secret")

To decrypt a file inline, you need the key that was returned by the encrypt method:

import file_encryptor.convergence

key = convergence.encrypt_inline_file("/path/to/file", "rainbow dinosaur secret")

convergence.decrypt_inline_file("/path/to/file", key)

The reason why you cannot use the passphrase directly is because the key is derived from both the passphrase and the SHA-256 of the original file.

For streaming applications, you can decrypt a file with a generator:

for chunk in convergence.decrypt_generator("/path/to/file", key):
    do_something_with_chunk(chunk)

Cryptoconcerns

The key generation mechanism is the following:

key = HMAC-SHA256(passphrase, hex(SHA256(file-contents)))

If no passphrase is given, a default is used.

The file itself is encrypted using AES128-CTR, from pycrypto. We’re not specifying any IV, thinking that for convergent encryption that is the right thing to do.

Testing

To run tests, execute the following command in the project root:

python setup.py test -a "--doctest-modules --pep8 -v tests/"

To run tests with detailed coverage output, execute:

coverage run setup.py test -a "--doctest-modules --pep8 -v tests/"
coverage report -m --include="file_encryptor/*"