An algorithm to calculate a chained hashing schema suitable for streamed file chunks.

streamhash.py

# -*- coding:utf-8 -*-

"""An algorithm to calculate a hashing schema described in:
https://class.coursera.org/crypto-004/quiz/feedback?submission_id=193325

"""

import io
import sys
import hashlib


def split_file(fname, block_size=1024):
    """Open and split file fname into byte strings of length block_size.
    
    Return a generator of such strings, with the last block having the
    length in (0, block_size].

    """
    with io.open(fname, mode='rb') as f:
        while True:
            block = f.read(block_size)
            if not block:
                return
            yield block
            f.seek(0, io.SEEK_CUR)


def calculate_hash(blocks):
    # calculate the hash of the last block
    h = hashlib.sha256()
    h.update(blocks[-1])
    hstr = h.digest()

    # for the rest, append the hash of the "next" block then hash
    for block in reversed(blocks[:-1]):
        h = hashlib.sha256()
        h.update(block)
        h.update(hstr)
        hstr = h.digest()

    return hstr


if __name__ == '__main__':
    fname = sys.argv[1]
    fblocks = [block for block in split_file(fname)]
    print calculate_hash(fblocks).encode('hex')