PySecretHandshake/secret_handshake/util.py

import struct

NONCE_SIZE = 24
MAX_NONCE = (8 * NONCE_SIZE)


def inc_nonce(nonce):
    num = bytes_to_long(nonce) + 1
    if num > 2 ** MAX_NONCE:
        num = 0
    bnum = long_to_bytes(num)
    bnum = b'\x00' * (NONCE_SIZE - len(bnum)) + bnum
    return bnum


def split_chunks(seq, n):
    """Split sequence in equal-sized chunks.
    The last chunk is not padded."""
    while seq:
        yield seq[:n]
        seq = seq[n:]


# Stolen from PyCypto (Public Domain)
def b(s):
    return s.encode("latin-1")  # utf-8 would cause some side-effects we don't want


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 = b('')
    pack = struct.pack
    while n > 0:
        s = pack('>I', n & 0xffffffff) + s
        n = n >> 32
    # strip off leading zeros
    for i in range(len(s)):
        if s[i] != b('\000')[0]:
            break
    else:
        # only happens when n == 0
        s = b('\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) * b('\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 = 0
    unpack = struct.unpack
    length = len(s)
    if length % 4:
        extra = (4 - length % 4)
        s = b('\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