pycryptodome/Util/number.py

#
#   number.py : Number-theoretic functions
#
#  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: number.py,v 1.11 2003-04-04 15:15:13 akuchling Exp $"

bignum = long
try:
    from Crypto.PublicKey import _fastmath
except ImportError:
    _fastmath = None

# Commented out and replaced with faster versions below
## def long2str(n):
##     s=''
##     while n>0:
##         s=chr(n & 255)+s
##         n=n>>8
##     return s

## import types
## def str2long(s):
##     if type(s)!=types.StringType: return s   # Integers will be left alone
##     return reduce(lambda x,y : x*256+ord(y), s, 0L)

def getRandomNumber(N, randfunc):
    """getRandomNumber(N:int, randfunc:callable):long
    Return an N-bit random number."""

    S = randfunc(N/8)
    odd_bits = N % 8
    if odd_bits != 0:
        char = ord(randfunc(1)) >> (8-odd_bits)
        char |= (1<<odd_bits)
        S = chr(char) + S
    return bytes_to_long(S)

def GCD(x,y):
    """GCD(x:long, y:long): long
    Return the GCD of x and y.
    """
    x = abs(x) ; y = abs(y)
    while x > 0:
        x, y = y % x, x
    return y

def inverse(u, v):
    """inverse(u:long, u:long):long
    Return the inverse of u mod v.
    """
    u3, v3 = long(u), long(v)
    u1, v1 = 1L, 0L
    while v3 > 0:
        q=u3 / v3
        u1, v1 = v1, u1 - v1*q
        u3, v3 = v3, u3 - v3*q
    while u1<0:
        u1 = u1 + v
    return u1

# Given a number of bits to generate and a random generation function,
# find a prime number of the appropriate size.

def getPrime(N, randfunc):
    """getPrime(N:int, randfunc:callable):long
    Return a random N-bit prime number.
    """

    number=getRandomNumber(N, randfunc) | 1
    while (not isPrime(number)):
        number=number+2
    return number

def isPrime(N):
    """isPrime(N:long):bool
    Return true if N is prime.
    """
    if N == 1:
        return 0
    if N in sieve:
        return 1
    for i in sieve:
        if (N % i)==0:
            return 0

    # Use the accelerator if available
    if _fastmath is not None:
        return _fastmath.isPrime(N)

    # Compute the highest bit that's set in N
    N1 = N - 1L
    n = 1L
    while (n<N):
        n=n<<1L
    n = n >> 1L

    # Rabin-Miller test
    for c in sieve[:7]:
        a=long(c) ; d=1L ; t=n
        while (t):  # Iterate over the bits in N1
            x=(d*d) % N
            if x==1L and d!=1L and d!=N1:
                return 0  # Square root of 1 found
            if N1 & t:
                d=(x*a) % N
            else:
                d=x
            t = t >> 1L
        if d!=1L:
            return 0
    return 1

# Small primes used for checking primality; these are all the primes
# less than 256.  This should be enough to eliminate most of the odd
# numbers before needing to do a Rabin-Miller test at all.

sieve=[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]

# Improved conversion functions contributed by Barry Warsaw, after
# careful benchmarking

import struct

def long_to_bytes(n, blocksize=0):
    """long_to_bytes(n:long, blocksize:int) : string
    Convert a long integer to a byte string.

    If optional blocksize is given and greater than zero, pad the front of the
    byte string with binary zeros so that the length is a multiple of
    blocksize.
    """
    # after much testing, this algorithm was deemed to be the fastest
    s = ''
    n = long(n)
    pack = struct.pack
    while n > 0:
        s = pack('>I', n & 0xffffffffL) + s
        n = n >> 32
    # strip off leading zeros
    for i in range(len(s)):
        if s[i] != '\000':
            break
    else:
        # only happens when n == 0
        s = '\000'
        i = 0
    s = s[i:]
    # add back some pad bytes.  this could be done more efficiently w.r.t. the
    # de-padding being done above, but sigh...
    if blocksize > 0 and len(s) % blocksize:
        s = (blocksize - len(s) % blocksize) * '\000' + s
    return s

def bytes_to_long(s):
    """bytes_to_long(string) : long
    Convert a byte string to a long integer.

    This is (essentially) the inverse of long_to_bytes().
    """
    acc = 0L
    unpack = struct.unpack
    length = len(s)
    if length % 4:
        extra = (4 - length % 4)
        s = '\000' * extra + s
        length = length + extra
    for i in range(0, length, 4):
        acc = (acc << 32) + unpack('>I', s[i:i+4])[0]
    return acc

# For backwards compatibility...
import warnings
def long2str(n, blocksize=0):
    warnings.warn("long2str() has been replaced by long_to_bytes()")
    return long_to_bytes(n, blocksize)
def str2long(s):
    warnings.warn("str2long() has been replaced by bytes_to_long()")
    return bytes_to_long(s)