This is a very basic pure Python implementation of the Paillier Homomorphic Cryptosystem.

The idea of homomorphic computation is to encrypt some numbers, perform algebraic operations like "add" and "multiply" on cyphertexts, then decrypt the result and find it to be exactly the same as if corresponding "+" and "*" operations were applied to the plaintexts.

In other words, a homomorphic cryptosystem enables cryptographically secure computations in an untrusted environment.

Paillier cryptosystem is a probabilistic asymmetric algorithm for public key cryptography. Paillier cryptosystem is partially homomorphic as it can only add encrypted numbers or multiply an encrypted number by an unencrypted multiplier.

This pure Python implementation exploits Python's long type with its arbitrary precision arithmetics. Public key is serializable, thus it can be pickled along with the encrypted numbers and sent to a remote server for computation.

The code is loosely based on Thep and a few ActiveState recipes.

Please note that this implementation's primary purpose is education; it is not suitable for production use as it is.

The paillier.py module has no external dependencies besides included primes.py. Simply run demo.py to see it in action.

To run unit tests please install Nose:

$ pip install -r requirements.txt
$ nosetests
...............
Ran 814 tests in 11.544s
OK

$ ipython
Python 2.7.1 (r271:86832, Jun 16 2011, 16:59:05)
Type "copyright", "credits" or "license" for more information.

In [1]: from paillier.paillier import *

In [2]: priv, pub = generate_keypair(128)

In [3]: x = encrypt(pub, 2)

In [4]: y = encrypt(pub, 3)

In [5]: x,y
Out[5]:
(72109737005643982735171545918..., 9615446835366886883470187...)

In [6]: z = e_add(pub, x, y)

In [7]: z
Out[7]: 71624230283745591274688669...

In [8]: decrypt(priv, pub, z)
Out[8]: 5L

LGPL v3, see LICENSE

(C) 2011 Mike Ivanov