bignum.py

#!/usr/bin/env python3
#
# Distributed under the MIT software license, see the accompanying
# file COPYING or http://www.opensource.org/licenses/mit-license.php.
"""Big number routines.

This file is copied from python-bitcoinlib.
"""

import struct


# generic big endian MPI format

def bn_bytes(v, have_ext=False):
    ext = 0
    if have_ext:
        ext = 1
    return ((v.bit_length()+7)//8) + ext

def bn2bin(v):
    s = bytearray()
    i = bn_bytes(v)
    while i > 0:
        s.append((v >> ((i-1) * 8)) & 0xff)
        i -= 1
    return s

def bn2mpi(v):
    have_ext = False
    if v.bit_length() > 0:
        have_ext = (v.bit_length() & 0x07) == 0

    neg = False
    if v < 0:
        neg = True
        v = -v

    s = struct.pack(b">I", bn_bytes(v, have_ext))
    ext = bytearray()
    if have_ext:
        ext.append(0)
    v_bin = bn2bin(v)
    if neg:
        if have_ext:
            ext[0] |= 0x80
        else:
            v_bin[0] |= 0x80
    return s + ext + v_bin

# bitcoin-specific little endian format, with implicit size
def mpi2vch(s):
    r = s[4:]           # strip size
    r = r[::-1]         # reverse string, converting BE->LE
    return r

def bn2vch(v):
    return bytes(mpi2vch(bn2mpi(v)))