For the most up to date version and information about this library,
visit

  http://acsc.csl.sri.com/libpaillier

Paillier [1] is a public key cryptosystem which offers an additive
homomorphism, making it very useful for privacy preserving
applications. This is a simple C library which implements Paillier key
generation, encryption, decryption, and also makes it easy to use the
homomorphism.

The excellent GNU Multiple Precision Arithmetic Library (GMP) is used
for the underlying number theoretic operations, so you will need to
have that installed before building libpaillier.

Generally, the library is written in the most straightforward manner
that you may imagine given that it is written in C and uses GMP. The
Paillier cryptosystem is not very complicated, and neither is
libpaillier.

Right now, the only documentation available is the series of comments
in the header file (paillier.h). That is probably all that is
necessary, however, given the simplicity of the library.

There are couple security considerations to keep in mind if you intend
to try to actually do something secure with libpaillier. Which you
absolutely shouldn't do anyway, since there are probably other issues
I'm overlooking.

When the paillier_keygen function selects a modulus n = p q, it does
not bother ensuring that p and q are strong primes [3]. While this was
at one point considered important (e.g., it is required by ANSI
X9.31), modern factoring algorithms make any advantage of strong
primes doubtful [4]. RSA Laboratories no longer recommends the
practice.

More importantly, at no point is any special effort made to securely
"shred" sensitive memory. This means that it is important that
functions dealing with private keys and plaintexts (e.g.,
paillier_keygen and paillier_enc) only be run on trusted machines. The
resulting ciphertexts and public keys, however, may of course be
handled in an untrusted manner.

The following paper introduced the basic algorithms that libpaillier
implements.

  Public-Key Cryptosystems Based on Composite Degree Residuosity
  Classes. Pascal Paillier. EUROCRYPT 1999.

This next paper is a collection of a number of improvements by Jurik
and Damgard. We don't do any of this stuff, so implementing some of
these things would be a great way to contribute.

  Extensions to the Paillier Cryptosystem with Applications to
  Cryptological Protocols. Mads Jurik. BRICS Dissertation Series,
  August 2003.

Please report any bugs, questions, comments, and contributions to
bethenco@cs.berkeley.edu.

Cheers,

John Bethencourt <bethenco@cs.berkeley.edu>
http://www.cs.berkeley.edu/~bethenco


[1] http://en.wikipedia.org/wiki/Paillier_cryptosystem

[2] http://www.swox.com/gmp

[3] http://en.wikipedia.org/wiki/Strong_prime

[4] http://eprint.iacr.org/2001/007

[5] http://www.rsasecurity.com/rsalabs/node.asp?id=2217