numbertheory.py

#! /usr/bin/env python
#
# Provide some simple capabilities from number theory.
#
# Version of 2008.11.14.
#
# Written in 2005 and 2006 by Peter Pearson and placed in the public domain.
# Revision history:
#   2008.11.14: Use pow(base, exponent, modulus) for modular_exp.
#               Make gcd and lcm accept arbitrarly many arguments.

from __future__ import division

from six import integer_types, PY3
from six.moves import reduce
try:
    xrange
except NameError:
    xrange = range

import math
import warnings


class Error(Exception):
  """Base class for exceptions in this module."""
  pass


class SquareRootError(Error):
  pass


class NegativeExponentError(Error):
  pass


def modular_exp(base, exponent, modulus):
  "Raise base to exponent, reducing by modulus"
  if exponent < 0:
    raise NegativeExponentError("Negative exponents (%d) not allowed" \
                                % exponent)
  return pow(base, exponent, modulus)
#   result = 1L
#   x = exponent
#   b = base + 0L
#   while x > 0:
#     if x % 2 > 0: result = (result * b) % modulus
#     x = x // 2
#     b = (b * b) % modulus
#   return result


def polynomial_reduce_mod(poly, polymod, p):
  """Reduce poly by polymod, integer arithmetic modulo p.

  Polynomials are represented as lists of coefficients
  of increasing powers of x."""

  # This module has been tested only by extensive use
  # in calculating modular square roots.

  # Just to make this easy, require a monic polynomial:
  assert polymod[-1] == 1

  assert len(polymod) > 1

  while len(poly) >= len(polymod):
    if poly[-1] != 0:
      for i in xrange(2, len(polymod) + 1):
        poly[-i] = (poly[-i] - poly[-1] * polymod[-i]) % p
    poly = poly[0:-1]

  return poly


def polynomial_multiply_mod(m1, m2, polymod, p):
  """Polynomial multiplication modulo a polynomial over ints mod p.

  Polynomials are represented as lists of coefficients
  of increasing powers of x."""

  # This is just a seat-of-the-pants implementation.

  # This module has been tested only by extensive use
  # in calculating modular square roots.

  # Initialize the product to zero:

  prod = (len(m1) + len(m2) - 1) * [0]

  # Add together all the cross-terms:

  for i in xrange(len(m1)):
    for j in xrange(len(m2)):
      prod[i + j] = (prod[i + j] + m1[i] * m2[j]) % p

  return polynomial_reduce_mod(prod, polymod, p)


def polynomial_exp_mod(base, exponent, polymod, p):
  """Polynomial exponentiation modulo a polynomial over ints mod p.

  Polynomials are represented as lists of coefficients
  of increasing powers of x."""

  # Based on the Handbook of Applied Cryptography, algorithm 2.227.

  # This module has been tested only by extensive use
  # in calculating modular square roots.

  assert exponent < p

  if exponent == 0:
    return [1]

  G = base
  k = exponent
  if k % 2 == 1:
    s = G
  else:
    s = [1]

  while k > 1:
    k = k // 2
    G = polynomial_multiply_mod(G, G, polymod, p)
    if k % 2 == 1:
      s = polynomial_multiply_mod(G, s, polymod, p)

  return s


def jacobi(a, n):
  """Jacobi symbol"""

  # Based on the Handbook of Applied Cryptography (HAC), algorithm 2.149.

  # This function has been tested by comparison with a small
  # table printed in HAC, and by extensive use in calculating
  # modular square roots.

  assert n >= 3
  assert n % 2 == 1
  a = a % n
  if a == 0:
    return 0
  if a == 1:
    return 1
  a1, e = a, 0
  while a1 % 2 == 0:
    a1, e = a1 // 2, e + 1
  if e % 2 == 0 or n % 8 == 1 or n % 8 == 7:
    s = 1
  else:
    s = -1
  if a1 == 1:
    return s
  if n % 4 == 3 and a1 % 4 == 3:
    s = -s
  return s * jacobi(n % a1, a1)


def square_root_mod_prime(a, p):
  """Modular square root of a, mod p, p prime."""

  # Based on the Handbook of Applied Cryptography, algorithms 3.34 to 3.39.

  # This module has been tested for all values in [0,p-1] for
  # every prime p from 3 to 1229.

  assert 0 <= a < p
  assert 1 < p

  if a == 0:
    return 0
  if p == 2:
    return a

  jac = jacobi(a, p)
  if jac == -1:
    raise SquareRootError("%d has no square root modulo %d" \
                          % (a, p))

  if p % 4 == 3:
    return modular_exp(a, (p + 1) // 4, p)

  if p % 8 == 5:
    d = modular_exp(a, (p - 1) // 4, p)
    if d == 1:
      return modular_exp(a, (p + 3) // 8, p)
    if d == p - 1:
      return (2 * a * modular_exp(4 * a, (p - 5) // 8, p)) % p
    raise RuntimeError("Shouldn't get here.")

  if PY3:
    range_top = p
  else:
    # xrange on python2 can take integers representable as C long only
    range_top = min(0x7fffffff, p)
  for b in xrange(2, range_top):
    if jacobi(b * b - 4 * a, p) == -1:
      f = (a, -b, 1)
      ff = polynomial_exp_mod((0, 1), (p + 1) // 2, f, p)
      assert ff[1] == 0
      return ff[0]
  raise RuntimeError("No b found.")


def inverse_mod(a, m):
  """Inverse of a mod m."""

  if a < 0 or m <= a:
    a = a % m

  # From Ferguson and Schneier, roughly:

  c, d = a, m
  uc, vc, ud, vd = 1, 0, 0, 1
  while c != 0:
    q, c, d = divmod(d, c) + (c,)
    uc, vc, ud, vd = ud - q * uc, vd - q * vc, uc, vc

  # At this point, d is the GCD, and ud*a+vd*m = d.
  # If d == 1, this means that ud is a inverse.

  assert d == 1
  if ud > 0:
    return ud
  else:
    return ud + m


def gcd2(a, b):
  """Greatest common divisor using Euclid's algorithm."""
  while a:
    a, b = b % a, a
  return b


def gcd(*a):
  """Greatest common divisor.

  Usage: gcd([ 2, 4, 6 ])
  or:    gcd(2, 4, 6)
  """

  if len(a) > 1:
    return reduce(gcd2, a)
  if hasattr(a[0], "__iter__"):
    return reduce(gcd2, a[0])
  return a[0]


def lcm2(a, b):
  """Least common multiple of two integers."""

  return (a * b) // gcd(a, b)


def lcm(*a):
  """Least common multiple.

  Usage: lcm([ 3, 4, 5 ])
  or:    lcm(3, 4, 5)
  """

  if len(a) > 1:
    return reduce(lcm2, a)
  if hasattr(a[0], "__iter__"):
    return reduce(lcm2, a[0])
  return a[0]


def factorization(n):
  """Decompose n into a list of (prime,exponent) pairs."""

  assert isinstance(n, integer_types)

  if n < 2:
    return []

  result = []
  d = 2

  # Test the small primes:

  for d in smallprimes:
    if d > n:
      break
    q, r = divmod(n, d)
    if r == 0:
      count = 1
      while d <= n:
        n = q
        q, r = divmod(n, d)
        if r != 0:
          break
        count = count + 1
      result.append((d, count))

  # If n is still greater than the last of our small primes,
  # it may require further work:

  if n > smallprimes[-1]:
    if is_prime(n):   # If what's left is prime, it's easy:
      result.append((n, 1))
    else:               # Ugh. Search stupidly for a divisor:
      d = smallprimes[-1]
      while 1:
        d = d + 2               # Try the next divisor.
        q, r = divmod(n, d)
        if q < d:               # n < d*d means we're done, n = 1 or prime.
          break
        if r == 0:              # d divides n. How many times?
          count = 1
          n = q
          while d <= n:               # As long as d might still divide n,
            q, r = divmod(n, d)       # see if it does.
            if r != 0:
              break
            n = q                     # It does. Reduce n, increase count.
            count = count + 1
          result.append((d, count))
      if n > 1:
        result.append((n, 1))

  return result


def phi(n):  # pragma: no cover
  """Return the Euler totient function of n."""
  # deprecated in 0.14
  warnings.warn("Function is unused by library code. If you use this code, "
                "please open an issue in "
                "https://github.com/warner/python-ecdsa",
                DeprecationWarning)

  assert isinstance(n, integer_types)

  if n < 3:
    return 1

  result = 1
  ff = factorization(n)
  for f in ff:
    e = f[1]
    if e > 1:
      result = result * f[0] ** (e - 1) * (f[0] - 1)
    else:
      result = result * (f[0] - 1)
  return result


def carmichael(n):  # pragma: no cover
  """Return Carmichael function of n.

  Carmichael(n) is the smallest integer x such that
  m**x = 1 mod n for all m relatively prime to n.
  """
  # deprecated in 0.14
  warnings.warn("Function is unused by library code. If you use this code, "
                "please open an issue in "
                "https://github.com/warner/python-ecdsa",
                DeprecationWarning)

  return carmichael_of_factorized(factorization(n))


def carmichael_of_factorized(f_list):  # pragma: no cover
  """Return the Carmichael function of a number that is
  represented as a list of (prime,exponent) pairs.
  """
  # deprecated in 0.14
  warnings.warn("Function is unused by library code. If you use this code, "
                "please open an issue in "
                "https://github.com/warner/python-ecdsa",
                DeprecationWarning)

  if len(f_list) < 1:
    return 1

  result = carmichael_of_ppower(f_list[0])
  for i in xrange(1, len(f_list)):
    result = lcm(result, carmichael_of_ppower(f_list[i]))

  return result


def carmichael_of_ppower(pp):  # pragma: no cover
  """Carmichael function of the given power of the given prime.
  """
  # deprecated in 0.14
  warnings.warn("Function is unused by library code. If you use this code, "
                "please open an issue in "
                "https://github.com/warner/python-ecdsa",
                DeprecationWarning)

  p, a = pp
  if p == 2 and a > 2:
    return 2**(a - 2)
  else:
    return (p - 1) * p**(a - 1)


def order_mod(x, m):  # pragma: no cover
  """Return the order of x in the multiplicative group mod m.
  """
  # deprecated in 0.14
  warnings.warn("Function is unused by library code. If you use this code, "
                "please open an issue in "
                "https://github.com/warner/python-ecdsa",
                DeprecationWarning)

  # Warning: this implementation is not very clever, and will
  # take a long time if m is very large.

  if m <= 1:
    return 0

  assert gcd(x, m) == 1

  z = x
  result = 1
  while z != 1:
    z = (z * x) % m
    result = result + 1
  return result


def largest_factor_relatively_prime(a, b):  # pragma: no cover
  """Return the largest factor of a relatively prime to b.
  """
  # deprecated in 0.14
  warnings.warn("Function is unused by library code. If you use this code, "
                "please open an issue in "
                "https://github.com/warner/python-ecdsa",
                DeprecationWarning)

  while 1:
    d = gcd(a, b)
    if d <= 1:
      break
    b = d
    while 1:
      q, r = divmod(a, d)
      if r > 0:
        break
      a = q
  return a


def kinda_order_mod(x, m):  # pragma: no cover
  """Return the order of x in the multiplicative group mod m',
  where m' is the largest factor of m relatively prime to x.
  """
  # deprecated in 0.14
  warnings.warn("Function is unused by library code. If you use this code, "
                "please open an issue in "
                "https://github.com/warner/python-ecdsa",
                DeprecationWarning)

  return order_mod(x, largest_factor_relatively_prime(m, x))


def is_prime(n):
  """Return True if x is prime, False otherwise.

  We use the Miller-Rabin test, as given in Menezes et al. p. 138.
  This test is not exact: there are composite values n for which
  it returns True.

  In testing the odd numbers from 10000001 to 19999999,
  about 66 composites got past the first test,
  5 got past the second test, and none got past the third.
  Since factors of 2, 3, 5, 7, and 11 were detected during
  preliminary screening, the number of numbers tested by
  Miller-Rabin was (19999999 - 10000001)*(2/3)*(4/5)*(6/7)
  = 4.57 million.
  """

  # (This is used to study the risk of false positives:)
  global miller_rabin_test_count

  miller_rabin_test_count = 0

  if n <= smallprimes[-1]:
    if n in smallprimes:
      return True
    else:
      return False

  if gcd(n, 2 * 3 * 5 * 7 * 11) != 1:
    return False

  # Choose a number of iterations sufficient to reduce the
  # probability of accepting a composite below 2**-80
  # (from Menezes et al. Table 4.4):

  t = 40
  n_bits = 1 + int(math.log(n, 2))
  for k, tt in ((100, 27),
                (150, 18),
                (200, 15),
                (250, 12),
                (300, 9),
                (350, 8),
                (400, 7),
                (450, 6),
                (550, 5),
                (650, 4),
                (850, 3),
                (1300, 2),
                ):
    if n_bits < k:
      break
    t = tt

  # Run the test t times:

  s = 0
  r = n - 1
  while (r % 2) == 0:
    s = s + 1
    r = r // 2
  for i in xrange(t):
    a = smallprimes[i]
    y = modular_exp(a, r, n)
    if y != 1 and y != n - 1:
      j = 1
      while j <= s - 1 and y != n - 1:
        y = modular_exp(y, 2, n)
        if y == 1:
          miller_rabin_test_count = i + 1
          return False
        j = j + 1
      if y != n - 1:
        miller_rabin_test_count = i + 1
        return False
  return True


def next_prime(starting_value):
  "Return the smallest prime larger than the starting value."

  if starting_value < 2:
    return 2
  result = (starting_value + 1) | 1
  while not is_prime(result):
    result = result + 2
  return result


smallprimes = [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41,
               43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97,
               101, 103, 107, 109, 113, 127, 131, 137, 139, 149,
               151, 157, 163, 167, 173, 179, 181, 191, 193, 197,
               199, 211, 223, 227, 229, 233, 239, 241, 251, 257,
               263, 269, 271, 277, 281, 283, 293, 307, 311, 313,
               317, 331, 337, 347, 349, 353, 359, 367, 373, 379,
               383, 389, 397, 401, 409, 419, 421, 431, 433, 439,
               443, 449, 457, 461, 463, 467, 479, 487, 491, 499,
               503, 509, 521, 523, 541, 547, 557, 563, 569, 571,
               577, 587, 593, 599, 601, 607, 613, 617, 619, 631,
               641, 643, 647, 653, 659, 661, 673, 677, 683, 691,
               701, 709, 719, 727, 733, 739, 743, 751, 757, 761,
               769, 773, 787, 797, 809, 811, 821, 823, 827, 829,
               839, 853, 857, 859, 863, 877, 881, 883, 887, 907,
               911, 919, 929, 937, 941, 947, 953, 967, 971, 977,
               983, 991, 997, 1009, 1013, 1019, 1021, 1031, 1033,
               1039, 1049, 1051, 1061, 1063, 1069, 1087, 1091, 1093,
               1097, 1103, 1109, 1117, 1123, 1129, 1151, 1153, 1163,
               1171, 1181, 1187, 1193, 1201, 1213, 1217, 1223, 1229]

miller_rabin_test_count = 0