#
# 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
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