#! /usr/bin/env python
#
# Implementation of elliptic curves, for cryptographic applications.
#
# This module doesn't provide any way to choose a random elliptic
# curve, nor to verify that an elliptic curve was chosen randomly,
# because one can simply use NIST's standard curves.
#
# Notes from X9.62-1998 (draft):
#   Nomenclature:
#     - Q is a public key.
#     The "Elliptic Curve Domain Parameters" include:
#     - q is the "field size", which in our case equals p.
#     - p is a big prime.
#     - G is a point of prime order (5.1.1.1).
#     - n is the order of G (5.1.1.1).
#   Public-key validation (5.2.2):
#     - Verify that Q is not the point at infinity.
#     - Verify that X_Q and Y_Q are in [0,p-1].
#     - Verify that Q is on the curve.
#     - Verify that nQ is the point at infinity.
#   Signature generation (5.3):
#     - Pick random k from [1,n-1].
#   Signature checking (5.4.2):
#     - Verify that r and s are in [1,n-1].
#
# Version of 2008.11.25.
#
# Revision history:
#    2005.12.31 - Initial version.
#    2008.11.25 - Change CurveFp.is_on to contains_point.
#
# Written in 2005 by Peter Pearson and placed in the public domain.

import numbertheory

class CurveFp( object ):
  """Elliptic Curve over the field of integers modulo a prime."""
  def __init__( self, p, a, b ):
    """The curve of points satisfying y^2 = x^3 + a*x + b (mod p)."""
    self.__p = p
    self.__a = a
    self.__b = b

  def p( self ):
    return self.__p

  def a( self ):
    return self.__a

  def b( self ):
    return self.__b

  def contains_point( self, x, y ):
    """Is the point (x,y) on this curve?"""
    return ( y * y - ( x * x * x + self.__a * x + self.__b ) ) % self.__p == 0



class Point( object ):
  """A point on an elliptic curve. Altering x and y is forbidding,
     but they can be read by the x() and y() methods."""
  def __init__( self, curve, x, y, order = None ):
    """curve, x, y, order; order (optional) is the order of this point."""
    self.__curve = curve
    self.__x = x
    self.__y = y
    self.__order = order
    # self.curve is allowed to be None only for INFINITY:
    if self.__curve: assert self.__curve.contains_point( x, y )
    if order: assert self * order == INFINITY
 
  def __cmp__( self, other ):
    """Return 0 if the points are identical, 1 otherwise."""
    if self.__curve == other.__curve \
       and self.__x == other.__x \
       and self.__y == other.__y:
      return 0
    else:
      return 1

  def __add__( self, other ):
    """Add one point to another point."""
    
    # X9.62 B.3:

    if other == INFINITY: return self
    if self == INFINITY: return other
    assert self.__curve == other.__curve
    if self.__x == other.__x:
      if ( self.__y + other.__y ) % self.__curve.p() == 0:
        return INFINITY
      else:
        return self.double()

    p = self.__curve.p()

    l = ( ( other.__y - self.__y ) * \
          numbertheory.inverse_mod( other.__x - self.__x, p ) ) % p

    x3 = ( l * l - self.__x - other.__x ) % p
    y3 = ( l * ( self.__x - x3 ) - self.__y ) % p
    
    return Point( self.__curve, x3, y3 )

  def __mul__( self, other ):
    """Multiply a point by an integer."""

    def leftmost_bit( x ):
      assert x > 0
      result = 1L
      while result <= x: result = 2 * result
      return result / 2

    e = other
    if self.__order: e = e % self.__order
    if e == 0: return INFINITY
    if self == INFINITY: return INFINITY
    assert e > 0

    # From X9.62 D.3.2:

    e3 = 3 * e
    negative_self = Point( self.__curve, self.__x, -self.__y, self.__order )
    i = leftmost_bit( e3 ) / 2
    result = self
    # print "Multiplying %s by %d (e3 = %d):" % ( self, other, e3 )
    while i > 1:
      result = result.double()
      if ( e3 & i ) != 0 and ( e & i ) == 0: result = result + self
      if ( e3 & i ) == 0 and ( e & i ) != 0: result = result + negative_self
      # print ". . . i = %d, result = %s" % ( i, result )
      i = i / 2

    return result

  def __rmul__( self, other ):
    """Multiply a point by an integer."""
    
    return self * other

  def __str__( self ):
    if self == INFINITY: return "infinity"
    return "(%d,%d)" % ( self.__x, self.__y )

  def double( self ):
    """Return a new point that is twice the old."""

    if self == INFINITY:
      return INFINITY

    # X9.62 B.3:

    p = self.__curve.p()
    a = self.__curve.a()

    l = ( ( 3 * self.__x * self.__x + a ) * \
          numbertheory.inverse_mod( 2 * self.__y, p ) ) % p

    x3 = ( l * l - 2 * self.__x ) % p
    y3 = ( l * ( self.__x - x3 ) - self.__y ) % p
    
    return Point( self.__curve, x3, y3 )

  def x( self ):
    return self.__x

  def y( self ):
    return self.__y

  def curve( self ):
    return self.__curve
  
  def order( self ):
    return self.__order


# This one point is the Point At Infinity for all purposes:
INFINITY = Point( None, None, None )  

if __name__ == "__main__":

  def test_add( c, x1, y1, x2,  y2, x3, y3 ):
    """We expect that on curve c, (x1,y1) + (x2, y2 ) = (x3, y3)."""
    p1 = Point( c, x1, y1 )
    p2 = Point( c, x2, y2 )
    p3 = p1 + p2
    print "%s + %s = %s" % ( p1, p2, p3 ),
    if p3.x() != x3 or p3.y() != y3:
      print " Failure: should give (%d,%d)." % ( x3, y3 )
    else:
      print " Good."

  def test_double( c, x1, y1, x3, y3 ):
    """We expect that on curve c, 2*(x1,y1) = (x3, y3)."""
    p1 = Point( c, x1, y1 )
    p3 = p1.double()
    print "%s doubled = %s" % ( p1, p3 ),
    if p3.x() != x3 or p3.y() != y3:
      print " Failure: should give (%d,%d)." % ( x3, y3 )
    else:
      print " Good."

  def test_double_infinity( c ):
    """We expect that on curve c, 2*INFINITY = INFINITY."""
    p1 = INFINITY
    p3 = p1.double()
    print "%s doubled = %s" % ( p1, p3 ),
    if p3.x() != INFINITY.x() or p3.y() != INFINITY.y():
      print " Failure: should give (%d,%d)." % ( INFINITY.x(), INFINITY.y() )
    else:
      print " Good."

  def test_multiply( c, x1, y1, m, x3, y3 ):
    """We expect that on curve c, m*(x1,y1) = (x3,y3)."""
    p1 = Point( c, x1, y1 )
    p3 = p1 * m
    print "%s * %d = %s" % ( p1, m, p3 ),
    if p3.x() != x3 or p3.y() != y3:
      print " Failure: should give (%d,%d)." % ( x3, y3 )
    else:
      print " Good."


  # A few tests from X9.62 B.3:

  c = CurveFp( 23, 1, 1 )
  test_add( c, 3, 10, 9, 7, 17, 20 )
  test_double( c, 3, 10, 7, 12 )
  test_add( c, 3, 10, 3, 10, 7, 12 )	# (Should just invoke double.)
  test_multiply( c, 3, 10, 2, 7, 12 )

  test_double_infinity(c)

  # From X9.62 I.1 (p. 96):

  g = Point( c, 13, 7, 7 )

  check = INFINITY
  for i in range( 7 + 1 ):
    p = ( i % 7 ) * g
    print "%s * %d = %s, expected %s . . ." % ( g, i, p, check ),
    if p == check:
      print " Good."
    else:
      print " Bad."
    check = check + g

  # NIST Curve P-192:
  p = 6277101735386680763835789423207666416083908700390324961279L
  r = 6277101735386680763835789423176059013767194773182842284081L
  s = 0x3045ae6fc8422f64ed579528d38120eae12196d5L
  c = 0x3099d2bbbfcb2538542dcd5fb078b6ef5f3d6fe2c745de65L
  b = 0x64210519e59c80e70fa7e9ab72243049feb8deecc146b9b1L
  Gx = 0x188da80eb03090f67cbf20eb43a18800f4ff0afd82ff1012L
  Gy = 0x07192b95ffc8da78631011ed6b24cdd573f977a11e794811L

  c192 = CurveFp( p, -3, b )
  p192 = Point( c192, Gx, Gy, r )

  # Checking against some sample computations presented
  # in X9.62:

  d = 651056770906015076056810763456358567190100156695615665659L
  Q = d * p192
  if Q.x() != 0x62B12D60690CDCF330BABAB6E69763B471F994DD702D16A5L:
    print "p192 * d came out wrong."
  else:
    print "p192 * d came out right."

  k = 6140507067065001063065065565667405560006161556565665656654L
  R = k * p192
  if R.x() != 0x885052380FF147B734C330C43D39B2C4A89F29B0F749FEADL \
     or R.y() != 0x9CF9FA1CBEFEFB917747A3BB29C072B9289C2547884FD835L:
    print "k * p192 came out wrong."
  else:
    print "k * p192 came out right."

  u1 = 2563697409189434185194736134579731015366492496392189760599L
  u2 = 6266643813348617967186477710235785849136406323338782220568L
  temp = u1 * p192 + u2 * Q
  if temp.x() != 0x885052380FF147B734C330C43D39B2C4A89F29B0F749FEADL \
     or temp.y() != 0x9CF9FA1CBEFEFB917747A3BB29C072B9289C2547884FD835L:
    print "u1 * p192 + u2 * Q came out wrong."
  else:
    print "u1 * p192 + u2 * Q came out right."
