/gdata/Crypto/PublicKey/ElGamal.py

  1#
  2#   ElGamal.py : ElGamal encryption/decryption and signatures
  3#
  4#  Part of the Python Cryptography Toolkit
  5#
  6# Distribute and use freely; there are no restrictions on further
  7# dissemination and usage except those imposed by the laws of your
  8# country of residence.  This software is provided "as is" without
  9# warranty of fitness for use or suitability for any purpose, express
 10# or implied. Use at your own risk or not at all.
 11#
 12
 13__revision__ = "$Id: ElGamal.py,v 1.9 2003/04/04 19:44:26 akuchling Exp $"
 14
 15from Crypto.PublicKey.pubkey import *
 16from Crypto.Util import number
 17
 18class error (Exception):
 19    pass
 20
 21# Generate an ElGamal key with N bits
 22def generate(bits, randfunc, progress_func=None):
 23    """generate(bits:int, randfunc:callable, progress_func:callable)
 24
 25    Generate an ElGamal key of length 'bits', using 'randfunc' to get
 26    random data and 'progress_func', if present, to display
 27    the progress of the key generation.
 28    """
 29    obj=ElGamalobj()
 30    # Generate prime p
 31    if progress_func:
 32        progress_func('p\n')
 33    obj.p=bignum(getPrime(bits, randfunc))
 34    # Generate random number g
 35    if progress_func:
 36        progress_func('g\n')
 37    size=bits-1-(ord(randfunc(1)) & 63) # g will be from 1--64 bits smaller than p
 38    if size<1:
 39        size=bits-1
 40    while (1):
 41        obj.g=bignum(getPrime(size, randfunc))
 42        if obj.g < obj.p:
 43            break
 44        size=(size+1) % bits
 45        if size==0:
 46            size=4
 47    # Generate random number x
 48    if progress_func:
 49        progress_func('x\n')
 50    while (1):
 51        size=bits-1-ord(randfunc(1)) # x will be from 1 to 256 bits smaller than p
 52        if size>2:
 53            break
 54    while (1):
 55        obj.x=bignum(getPrime(size, randfunc))
 56        if obj.x < obj.p:
 57            break
 58        size = (size+1) % bits
 59        if size==0:
 60            size=4
 61    if progress_func:
 62        progress_func('y\n')
 63    obj.y = pow(obj.g, obj.x, obj.p)
 64    return obj
 65
 66def construct(tuple):
 67    """construct(tuple:(long,long,long,long)|(long,long,long,long,long)))
 68             : ElGamalobj
 69    Construct an ElGamal key from a 3- or 4-tuple of numbers.
 70    """
 71
 72    obj=ElGamalobj()
 73    if len(tuple) not in [3,4]:
 74        raise error, 'argument for construct() wrong length'
 75    for i in range(len(tuple)):
 76        field = obj.keydata[i]
 77        setattr(obj, field, tuple[i])
 78    return obj
 79
 80class ElGamalobj(pubkey):
 81    keydata=['p', 'g', 'y', 'x']
 82
 83    def _encrypt(self, M, K):
 84        a=pow(self.g, K, self.p)
 85        b=( M*pow(self.y, K, self.p) ) % self.p
 86        return ( a,b )
 87
 88    def _decrypt(self, M):
 89        if (not hasattr(self, 'x')):
 90            raise error, 'Private key not available in this object'
 91        ax=pow(M[0], self.x, self.p)
 92        plaintext=(M[1] * inverse(ax, self.p ) ) % self.p
 93        return plaintext
 94
 95    def _sign(self, M, K):
 96        if (not hasattr(self, 'x')):
 97            raise error, 'Private key not available in this object'
 98        p1=self.p-1
 99        if (GCD(K, p1)!=1):
100            raise error, 'Bad K value: GCD(K,p-1)!=1'
101        a=pow(self.g, K, self.p)
102        t=(M-self.x*a) % p1
103        while t<0: t=t+p1
104        b=(t*inverse(K, p1)) % p1
105        return (a, b)
106
107    def _verify(self, M, sig):
108        v1=pow(self.y, sig[0], self.p)
109        v1=(v1*pow(sig[0], sig[1], self.p)) % self.p
110        v2=pow(self.g, M, self.p)
111        if v1==v2:
112            return 1
113        return 0
114
115    def size(self):
116        "Return the maximum number of bits that can be handled by this key."
117        return number.size(self.p) - 1
118
119    def has_private(self):
120        """Return a Boolean denoting whether the object contains
121        private components."""
122        if hasattr(self, 'x'):
123            return 1
124        else:
125            return 0
126
127    def publickey(self):
128        """Return a new key object containing only the public information."""
129        return construct((self.p, self.g, self.y))
130
131
132object=ElGamalobj