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Simple, secure encryption and decryption for Python 2.7 and 3
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README.md

simple-crypt

Simple, secure encryption and decryption for Python 2.7 and 3.

Now on pypi: pip install simple-crypt (note that the pypi name includes a hyphen).

This provides two functions, which encrypt and decrypt data, delegating all the hard work to the pycrypto library (which must also be installed).

Examples

The API

The two calls:

from simplecrypt import encrypt, decrypt

ciphertext = encrypt(password, 'my secret message')
plaintext = decrypt(password, ciphertext)

Interactive Use

A simple Python 3 program:

from binascii import hexlify
from getpass import getpass
from sys import stdin

from simplecrypt import encrypt, decrypt

# read the password from the user (without displaying it)
password = getpass("password: ")

# read the (single line) plaintext we will encrypt
print("message: ")
message = stdin.readline()

# encrypt the plaintext.  we explicitly convert to bytes first (optional)
ciphertext = encrypt(password, message.encode('utf8'))

# the ciphertext plaintext is bytes, so we display it as a hex string
print("ciphertext: %s" % hexlify(ciphertext))

# now decrypt the plaintext (using the same salt and password)
plaintext = decrypt(password, ciphertext)

# the decrypted plaintext is bytes, but we can convert it back to a string
print("plaintext: %s" % plaintext)
print("plaintext as string: %s" % plaintext.decode('utf8'))

Which, when run, produces something like the following (the actual encrypted message will be different each time, as a random salt is used for each message):

password: ******

message:
hello world
ciphertext: b'73630001b1c39575390d5720f2a80e7a06fbddbf2c844d6b8eaf845d4a9e140d46a54c6729e74b0ddeb1cb82dee81691123faf8f41900c5a6c5b755ed8ae195ff2410290bcb8dc2ee3a2126c594b711d'
plaintext: b'hello world\n'
plaintext as string: hello world

Also, it's perhaps worth noting that the overhead (the extra length of the encrypted data, compared to the message) is constant. It looks a lot here, because the message is very small, but for most practical uses should not be an issue.

Using Files

When the following program is run, if the file "encrypted.txt" does not exist, then it is created with the contents "10 green bottles".

If the file does exist, it is read, and the number of green bottles is reduced. If there are no green bottles left, then the file is deleted, otherwise it is written with the new number.

from simplecrypt import encrypt, decrypt
from os.path import exists
from os import unlink

PASSWORD = "secret"
FILENAME = "encrypted.txt"

def main():
    # read or create the file
    if exists(FILENAME):
        print("reading...")
        data = read_encrypted(PASSWORD, FILENAME)
        print("read %s from %s" % (data, FILENAME))
        n_bottles = int(data.split(" ")[0]) - 1
    else:
        n_bottles = 10
    # write the file
    if n_bottles > 0:
        data = "%d green bottles" % n_bottles
        print("writing...")
        write_encrypted(PASSWORD, FILENAME, data)
        print("wrote %s to %s" % (data, FILENAME))
    else:
        unlink(FILENAME)
        print("deleted %s" % FILENAME)

def read_encrypted(password, filename, string=True):
    with open(filename, 'rb') as input:
        ciphertext = input.read()
        plaintext = decrypt(password, ciphertext)
        if string:
            return plaintext.decode('utf8')
        else:
            return plaintext

def write_encrypted(password, filename, plaintext):
    with open(filename, 'wb') as output:
        ciphertext = encrypt(password, plaintext)
        output.write(ciphertext)

if __name__ == '__main__':
    main()

This program is included in src/simplecrypt/example-file.py and we can run it as follows:

> python3 src/simplecrypt/example-file.py
writing...
wrote 10 green bottles to encrypted.txt
> python3 src/simplecrypt/example-file.py
reading...
read 10 green bottles from encrypted.txt
writing...
wrote 9 green bottles to encrypted.txt
> 
...
> python3 src/simplecrypt/example-file.py
reading...
read 1 green bottles from encrypted.txt
deleted encrypted.txt
>

Alternatives

This code is intended to be "easy to use" and "hard to use wrong". An alternative for more experienced users (who might, for example, want to use more rounds in the PBKDF, or an explicit key) is python-aead.

As far as I can tell, python-aead uses very similar algorithms to those found here.

Algorithms

The algorithms used follow the recommendations at http://www.daemonology.net/blog/2009-06-11-cryptographic-right-answers.html and http://www.daemonology.net/blog/2009-06-24-encrypt-then-mac.html, as far as I can tell:

  • The password is expanded to two 256 bit keys using PBKDF2 with a 256 bit random salt (increased from 128 bits in release 3.0.0), SHA256, and 100,000 iterations (increased from 10,000 in release 4.0.0).

  • AES256 CTR mode is used to encrypt the data with one key. The first 64 bits of the salt are used as a message nonce (of half the block size); the incremental part of the counter uses the remaining 64 bits (see section B.2 of http://csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf).

  • An encrypted messages starts with a 4 byte header ("sc" in ASCII followed by either two zero bytes (pre-3.0), or a zero and a one).

  • An SHA256 HMAC (of header, salt, and encrypted message) is calculated using the other key.

  • The final message consists of the header, salt, encrypted data, and HMAC, concatenated in that order.

  • On decryption, the header is checked and the HMAC validated before decryption.

The entire implementation is here.

Discussion and criticism of the design can be found on HN (also), codereview.stackexchange and crypto.stackexchange. Grateful thanks to all commentators (particularly marshray); mistakes remain mine.

Please note that the general design, based on the cryptographic right answers, is intended to give 128 bits of security - any attack would require around 2^128 guesses. This comes from birthday collisions on the 256 bit HMAC and random numbers (since release 3.0). AES256 is used because it provides additional security if, for example, some key bits are revealed through timing attacks (see link above or chapter 7 of Practical Cryptography).

Latest News

Release 4.1 obscures the output of the random number generator. This should not be necessary, but guards against a possible attack if the random number generator is compromised in some way. Functionality and interoperability are otherwise unchanged.

Release 4.0 increases the number of iterations used in the PBKDF (this will make encryption and decryption noticeably slower) and adds a reference to python-aead.

Release 3.0 increases the size of the salt used from 128 to 256 bits.

The header changes with each major version release (after 2.0), so data encrypted by previous releases can be detected and decrypted correctly. However, data encrypted by later major releases (after 2.0) cannot be decrypted by earlier releases (instead, an error with a helpful message is generated).

Release 2.0 was fully compatible with 1.0 on Python 3 (same API and identical results). However, thanks to d10n it also supported Python 2.7 (tested with Python 2.7.5, 3.0.1 and 3.3.2).

I (Andrew Cooke) am not sure Python 2.7 support is such a good idea. You should really use something like keyczar. But there seems to be a demand for this, so better the devil you know...

Warnings

  1. The whole idea of encrypting with a password is not so smart these days. If you think you need to do this, try reading about Google's keyczar which instead uses a keystore (unfortunately, at the time of writing, keyczar does not support Python 3, as far as I can tell, but that should change soon).

  2. When you call these routines the password is stored in memory as a Python string. This means that malicious code running on the same machine might be able to read the password (or even that the password could be written to swap space on disk). One way to reduce the risk is to have the crypto part of your code run as a separate process that exists for a limited time.

  3. All encrypted messages start with a 4 byte header (ASCII "sc", followed by the version number). So an adversary is able to recognize that the data are encrypted (and not simply random). You can avoid this by discarding the first 4 bytes of the encrypted data, but you must of course replace them before decrypting, and the code will not inter-operate between versions.

  4. I have considered extending the code to handle inputs larger than can be held in memory. While this is possible, the HMAC is not validated until decrypted data are returned. Which is asking for trouble - people are going to use the data as they are decrypted - and shows that the current design is inappropriate for such use. Someone needs to design a better solution (eg. with HMAC checks for each "block" - but even that allows data to be silently truncated at the end of a block).

(c) 2012-2015 Andrew Cooke, andrew@acooke.org; 2013 d10n, david@bitinvert.com. Released into the public domain for any use, but with absolutely no warranty.

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