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#
# ElGamal.py : ElGamal encryption/decryption and signatures
#
# Part of the Python Cryptography Toolkit
#
# Originally written by: A.M. Kuchling
#
# ===================================================================
# The contents of this file are dedicated to the public domain. To
# the extent that dedication to the public domain is not available,
# everyone is granted a worldwide, perpetual, royalty-free,
# non-exclusive license to exercise all rights associated with the
# contents of this file for any purpose whatsoever.
# No rights are reserved.
#
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
# EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
# MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
# NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
# BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
# ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
# CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
# SOFTWARE.
# ===================================================================
__all__ = ['generate', 'construct', 'ElGamalKey']
from Crypto import Random
from Crypto.Math.Primality import ( generate_probable_safe_prime,
test_probable_prime, COMPOSITE )
from Crypto.Math.Numbers import Integer
# Generate an ElGamal key with N bits
def generate(bits, randfunc):
"""Randomly generate a fresh, new ElGamal key.
The key will be safe for use for both encryption and signature
(although it should be used for **only one** purpose).
Args:
bits (int):
Key length, or size (in bits) of the modulus *p*.
The recommended value is 2048.
randfunc (callable):
Random number generation function; it should accept
a single integer *N* and return a string of random
*N* random bytes.
Return:
an :class:`ElGamalKey` object
"""
obj=ElGamalKey()
# Generate a safe prime p
# See Algorithm 4.86 in Handbook of Applied Cryptography
obj.p = generate_probable_safe_prime(exact_bits=bits, randfunc=randfunc)
q = (obj.p - 1) >> 1
# Generate generator g
while 1:
# Choose a square residue; it will generate a cyclic group of order q.
obj.g = pow(Integer.random_range(min_inclusive=2,
max_exclusive=obj.p,
randfunc=randfunc), 2, obj.p)
# We must avoid g=2 because of Bleichenbacher's attack described
# in "Generating ElGamal signatures without knowning the secret key",
# 1996
if obj.g in (1, 2):
continue
# Discard g if it divides p-1 because of the attack described
# in Note 11.67 (iii) in HAC
if (obj.p - 1) % obj.g == 0:
continue
# g^{-1} must not divide p-1 because of Khadir's attack
# described in "Conditions of the generator for forging ElGamal
# signature", 2011
ginv = obj.g.inverse(obj.p)
if (obj.p - 1) % ginv == 0:
continue
# Found
break
# Generate private key x
obj.x = Integer.random_range(min_inclusive=2,
max_exclusive=obj.p-1,
randfunc=randfunc)
# Generate public key y
obj.y = pow(obj.g, obj.x, obj.p)
return obj
def construct(tup):
r"""Construct an ElGamal key from a tuple of valid ElGamal components.
The modulus *p* must be a prime.
The following conditions must apply:
.. math::
\begin{align}
&1 < g < p-1 \\
&g^{p-1} = 1 \text{ mod } 1 \\
&1 < x < p-1 \\
&g^x = y \text{ mod } p
\end{align}
Args:
tup (tuple):
A tuple with either 3 or 4 integers,
in the following order:
1. Modulus (*p*).
2. Generator (*g*).
3. Public key (*y*).
4. Private key (*x*). Optional.
Raises:
ValueError: when the key being imported fails the most basic ElGamal validity checks.
Returns:
an :class:`ElGamalKey` object
"""
obj=ElGamalKey()
if len(tup) not in [3,4]:
raise ValueError('argument for construct() wrong length')
for i in range(len(tup)):
field = obj._keydata[i]
setattr(obj, field, Integer(tup[i]))
fmt_error = test_probable_prime(obj.p) == COMPOSITE
fmt_error |= obj.g<=1 or obj.g>=obj.p
fmt_error |= pow(obj.g, obj.p-1, obj.p)!=1
fmt_error |= obj.y<1 or obj.y>=obj.p
if len(tup)==4:
fmt_error |= obj.x<=1 or obj.x>=obj.p
fmt_error |= pow(obj.g, obj.x, obj.p)!=obj.y
if fmt_error:
raise ValueError("Invalid ElGamal key components")
return obj
class ElGamalKey(object):
r"""Class defining an ElGamal key.
Do not instantiate directly.
Use :func:`generate` or :func:`construct` instead.
:ivar p: Modulus
:vartype d: integer
:ivar g: Generator
:vartype e: integer
:ivar y: Public key component
:vartype y: integer
:ivar x: Private key component
:vartype x: integer
"""
#: Dictionary of ElGamal parameters.
#:
#: A public key will only have the following entries:
#:
#: - **y**, the public key.
#: - **g**, the generator.
#: - **p**, the modulus.
#:
#: A private key will also have:
#:
#: - **x**, the private key.
_keydata=['p', 'g', 'y', 'x']
def __init__(self, randfunc=None):
if randfunc is None:
randfunc = Random.new().read
self._randfunc = randfunc
def _encrypt(self, M, K):
a=pow(self.g, K, self.p)
b=( pow(self.y, K, self.p)*M ) % self.p
return [int(a), int(b)]
def _decrypt(self, M):
if (not hasattr(self, 'x')):
raise TypeError('Private key not available in this object')
r = Integer.random_range(min_inclusive=2,
max_exclusive=self.p-1,
randfunc=self._randfunc)
a_blind = (pow(self.g, r, self.p) * M[0]) % self.p
ax=pow(a_blind, self.x, self.p)
plaintext_blind = (ax.inverse(self.p) * M[1] ) % self.p
plaintext = (plaintext_blind * pow(self.y, r, self.p)) % self.p
return int(plaintext)
def _sign(self, M, K):
if (not hasattr(self, 'x')):
raise TypeError('Private key not available in this object')
p1=self.p-1
K = Integer(K)
if (K.gcd(p1)!=1):
raise ValueError('Bad K value: GCD(K,p-1)!=1')
a=pow(self.g, K, self.p)
t=(Integer(M)-self.x*a) % p1
while t<0: t=t+p1
b=(t*K.inverse(p1)) % p1
return [int(a), int(b)]
def _verify(self, M, sig):
sig = [Integer(x) for x in sig]
if sig[0]<1 or sig[0]>self.p-1:
return 0
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 has_private(self):
"""Whether this is an ElGamal private key"""
if hasattr(self, 'x'):
return 1
else:
return 0
def can_encrypt(self):
return True
def can_sign(self):
return True
def publickey(self):
"""A matching ElGamal public key.
Returns:
a new :class:`ElGamalKey` object
"""
return construct((self.p, self.g, self.y))
def __eq__(self, other):
if bool(self.has_private()) != bool(other.has_private()):
return False
result = True
for comp in self._keydata:
result = result and (getattr(self.key, comp, None) ==
getattr(other.key, comp, None))
return result
def __ne__(self, other):
return not self.__eq__(other)
def __getstate__(self):
# ElGamal key is not pickable
from pickle import PicklingError
raise PicklingError
# Methods defined in PyCrypto that we don't support anymore
def sign(self, M, K):
raise NotImplementedError
def verify(self, M, signature):
raise NotImplementedError
def encrypt(self, plaintext, K):
raise NotImplementedError
def decrypt(self, ciphertext):
raise NotImplementedError
def blind(self, M, B):
raise NotImplementedError
def unblind(self, M, B):
raise NotImplementedError
def size(self):
raise NotImplementedError