# -*- coding: utf-8 -*-
#
#  PubKey/RSA/_slowmath.py : Pure Python implementation of the RSA portions of _fastmath
#
# Written in 2008 by Dwayne C. Litzenberger <dlitz@dlitz.net>
#
# ===================================================================
# 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.
# ===================================================================

"""Pure Python implementation of the RSA-related portions of Crypto.PublicKey._fastmath."""

__revision__ = "$Id$"

__all__ = ['rsa_construct']

import sys

if sys.version_info[0] == 2 and sys.version_info[1] == 1:
    from Crypto.Util.py21compat import *
from Crypto.Util.number import size, inverse, GCD

class error(Exception):
    pass

class _RSAKey(object):
    def _blind(self, m, r):
        # compute r**e * m (mod n)
        return (m * pow(r, self.e, self.n)) % self.n

    def _unblind(self, m, r):
        # compute m / r (mod n)
        return inverse(r, self.n) * m % self.n

    def _decrypt(self, c):
        # compute c**d (mod n)
        if not self.has_private():
            raise TypeError("No private key")
        if (hasattr(self,'p') and hasattr(self,'q') and hasattr(self,'u')):
            m1 = pow(c, self.d % (self.p-1), self.p)
            m2 = pow(c, self.d % (self.q-1), self.q)
            h = m2 - m1
            if (h<0):
                h = h + self.q
            h = h*self.u % self.q
            return h*self.p+m1
        return pow(c, self.d, self.n)

    def _encrypt(self, m):
        # compute m**d (mod n)
        return pow(m, self.e, self.n)

    def _sign(self, m):   # alias for _decrypt
        if not self.has_private():
            raise TypeError("No private key")
        return self._decrypt(m)

    def _verify(self, m, sig):
        return self._encrypt(sig) == m

    def has_private(self):
        return hasattr(self, 'd')

    def size(self):
        """Return the maximum number of bits that can be encrypted"""
        return size(self.n) - 1

def rsa_construct(n, e, d=None, p=None, q=None, u=None):
    """Construct an RSAKey object"""
    assert isinstance(n, long)
    assert isinstance(e, long)
    assert isinstance(d, (long, type(None)))
    assert isinstance(p, (long, type(None)))
    assert isinstance(q, (long, type(None)))
    assert isinstance(u, (long, type(None)))
    obj = _RSAKey()
    obj.n = n
    obj.e = e
    if d is None:
        return obj
    obj.d = d
    if p is not None and q is not None:
        obj.p = p
        obj.q = q
    else:
        # Compute factors p and q from the private exponent d.
        # We assume that n has no more than two factors.
        # See 8.2.2(i) in Handbook of Applied Cryptography.
        ktot = d*e-1
        # The quantity d*e-1 is a multiple of phi(n), even,
        # and can be represented as t*2^s.
        t = ktot
        while t%2==0:
            t=divmod(t,2)[0]
        # Cycle through all multiplicative inverses in Zn.
        # The algorithm is non-deterministic, but there is a 50% chance
        # any candidate a leads to successful factoring.
        # See "Digitalized Signatures and Public Key Functions as Intractable
        # as Factorization", M. Rabin, 1979
        spotted = 0
        a = 2
        while not spotted and a<1000:
            k = t
            # Cycle through all values a^{t*2^i}=a^k
            while k<ktot:
                cand = pow(a,k,n)
                # Check if a^k is a non-trivial root of unity (mod n)
                if cand!=1 and cand!=(n-1) and pow(cand,2,n)==1:
                    # We have found a number such that (cand-1)(cand+1)=0 (mod n).
                    # Either of the terms divides n.
                    obj.p = GCD(cand+1,n)
                    spotted = 1
                    break
                k = k*2
            # This value was not any good... let's try another!
            a = a+2
        if not spotted:
            raise ValueError("Unable to compute factors p and q from exponent d.")
        # Found !
        assert ((n % obj.p)==0)
        obj.q = divmod(n,obj.p)[0]
    if u is not None:
        obj.u = u
    else:
        obj.u = inverse(obj.p, obj.q)
    return obj

class _DSAKey(object):
    def size(self):
        """Return the maximum number of bits that can be encrypted"""
        return size(self.p) - 1

    def has_private(self):
        return hasattr(self, 'x')

    def _sign(self, m, k, blind):   # alias for _decrypt
        # SECURITY TODO - We _should_ be computing SHA1(m), but we don't because that's the API.
        if not self.has_private():
            raise TypeError("No private key")
        if not (1L < k < self.q):
            raise ValueError("k is not between 2 and q-1")
        inv_blind_k = inverse(blind * k, self.q) # Compute (blind * k)**-1 mod q
        blind_x = self.x * blind
        r = pow(self.g, k, self.p) % self.q  # r = (g**k mod p) mod q
        s = (inv_blind_k * (m * blind + blind_x * r)) % self.q
        return (r, s)

    def _verify(self, m, r, s):
        # SECURITY TODO - We _should_ be computing SHA1(m), but we don't because that's the API.
        if not (0 < r < self.q) or not (0 < s < self.q):
            return False
        w = inverse(s, self.q)
        u1 = (m*w) % self.q
        u2 = (r*w) % self.q
        v = (pow(self.g, u1, self.p) * pow(self.y, u2, self.p) % self.p) % self.q
        return v == r

def dsa_construct(y, g, p, q, x=None):
    assert isinstance(y, long)
    assert isinstance(g, long)
    assert isinstance(p, long)
    assert isinstance(q, long)
    assert isinstance(x, (long, type(None)))
    obj = _DSAKey()
    obj.y = y
    obj.g = g
    obj.p = p
    obj.q = q
    if x is not None: obj.x = x
    return obj


# vim:set ts=4 sw=4 sts=4 expandtab: