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python/gdata/Crypto/PublicKey/ElGamal.py

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+#
+#   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