app/gdata/Crypto/PublicKey/ElGamal.py

#
#   ElGamal.py : ElGamal encryption/decryption and signatures
#
#  Part of the Python Cryptography Toolkit
#
# Distribute and use freely; there are no restrictions on further
# dissemination and usage except those imposed by the laws of your
# country of residence.  This software is provided "as is" without
# warranty of fitness for use or suitability for any purpose, express
# or implied. Use at your own risk or not at all.
#

__revision__ = "$Id: ElGamal.py,v 1.9 2003/04/04 19:44:26 akuchling Exp $"

from Crypto.PublicKey.pubkey import *
from Crypto.Util import number

class error (Exception):
    pass

# Generate an ElGamal key with N bits
def generate(bits, randfunc, progress_func=None):
    """generate(bits:int, randfunc:callable, progress_func:callable)

    Generate an ElGamal key of length 'bits', using 'randfunc' to get
    random data and 'progress_func', if present, to display
    the progress of the key generation.
    """
    obj=ElGamalobj()
    # Generate prime p
    if progress_func:
        progress_func('p\n')
    obj.p=bignum(getPrime(bits, randfunc))
    # Generate random number g
    if progress_func:
        progress_func('g\n')
    size=bits-1-(ord(randfunc(1)) & 63) # g will be from 1--64 bits smaller than p
    if size<1:
        size=bits-1
    while (1):
        obj.g=bignum(getPrime(size, randfunc))
        if obj.g < obj.p:
            break
        size=(size+1) % bits
        if size==0:
            size=4
    # Generate random number x
    if progress_func:
        progress_func('x\n')
    while (1):
        size=bits-1-ord(randfunc(1)) # x will be from 1 to 256 bits smaller than p
        if size>2:
            break
    while (1):
        obj.x=bignum(getPrime(size, randfunc))
        if obj.x < obj.p:
            break
        size = (size+1) % bits
        if size==0:
            size=4
    if progress_func:
        progress_func('y\n')
    obj.y = pow(obj.g, obj.x, obj.p)
    return obj

def construct(tuple):
    """construct(tuple:(long,long,long,long)|(long,long,long,long,long)))
             : ElGamalobj
    Construct an ElGamal key from a 3- or 4-tuple of numbers.
    """

    obj=ElGamalobj()
    if len(tuple) not in [3,4]:
        raise error, 'argument for construct() wrong length'
    for i in range(len(tuple)):
        field = obj.keydata[i]
        setattr(obj, field, tuple[i])
    return obj

class ElGamalobj(pubkey):
    keydata=['p', 'g', 'y', 'x']

    def _encrypt(self, M, K):
        a=pow(self.g, K, self.p)
        b=( M*pow(self.y, K, self.p) ) % self.p
        return ( a,b )

    def _decrypt(self, M):
        if (not hasattr(self, 'x')):
            raise error, 'Private key not available in this object'
        ax=pow(M[0], self.x, self.p)
        plaintext=(M[1] * inverse(ax, self.p ) ) % self.p
        return plaintext

    def _sign(self, M, K):
        if (not hasattr(self, 'x')):
            raise error, 'Private key not available in this object'
        p1=self.p-1
        if (GCD(K, p1)!=1):
            raise error, 'Bad K value: GCD(K,p-1)!=1'
        a=pow(self.g, K, self.p)
        t=(M-self.x*a) % p1
        while t<0: t=t+p1
        b=(t*inverse(K, p1)) % p1
        return (a, b)

    def _verify(self, M, sig):
        v1=pow(self.y, sig[0], self.p)
        v1=(v1*pow(sig[0], sig[1], self.p)) % self.p
        v2=pow(self.g, M, self.p)
        if v1==v2:
            return 1
        return 0

    def size(self):
        "Return the maximum number of bits that can be handled by this key."
        return number.size(self.p) - 1

    def has_private(self):
        """Return a Boolean denoting whether the object contains
        private components."""
        if hasattr(self, 'x'):
            return 1
        else:
            return 0

    def publickey(self):
        """Return a new key object containing only the public information."""
        return construct((self.p, self.g, self.y))


object=ElGamalobj