On 23/02/17 13:36, Gerd v. Egidy wrote: > So I think that this would move the bar for a possible user of paperbackup.py > higher than I want to.
Yes, it should be easy to use. In fact, I've sometimes heard the complaint that "paperkey is not easy to install and/or use". That's really too bad that those people feel that way. > Ideally it is a tool or combination of tools already deployed widely, like > sed > and sort I used in paperrestore. This would make the checksums still usable > even when the source to paperbackup.py isn't available anymore. It took me some fiddling... but using CRC RevEng[1] I got a checksum in Python that is compatible to POSIX cksum. The following Python: >>> from posixcksum import PosixCkSum >>> from base64 import b64encode >>> crc, _ = PosixCkSum.sum_whole(bytearray(b'123456789')) >>> b64encode(crc.to_bytes(4,byteorder='big'))[:4] b'N3pg' generates the same checksums as the following Bash code: $ printf $(printf '%08x' $(echo -n 123456789 | cksum | cut -d' ' -f1) | sed 's/../\\x\0/g')|base64|cut -b-4 N3pg This is done with the attached Python code. It is written for compactness rather than speed. Just re-implementing the crc function would probably be quicker. HTH, Peter. [1] <http://reveng.sourceforge.net/> -- I use the GNU Privacy Guard (GnuPG) in combination with Enigmail. You can send me encrypted mail if you want some privacy. My key is available at <http://digitalbrains.com/2012/openpgp-key-peter>
#!/usr/bin/python3
# Copyright (c) 2017 Peter Lebbing <pe...@digitalbrains.com>
# All rights reserved.
#
# Redistribution and use in source and binary forms, with or without
# modification, are permitted provided that the following conditions are met:
#
# 1. Redistributions of source code must retain the above copyright notice,
# this list of conditions and the following disclaimer.
#
# 2. Redistributions in binary form must reproduce the above copyright notice,
# this list of conditions and the following disclaimer in the documentation
# and/or other materials provided with the distribution.
#
# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
# AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
# IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
# ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
# LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
# CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
# SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
# INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
# CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
# ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
# POSSIBILITY OF SUCH DAMAGE.
import binascii
class PosixCkSum(object):
"""Create a POSIX cksum-compliant checksum."""
def __init__(self):
"""Create new checksumming instance."""
self.n = 0
self.crc = 0xffffffff
def sum(self, bs):
"""Checksum another part of data.
IMPORTANT: the data in bs is DESTROYED.
"""
self.bit_reverse(bs)
self.crc = binascii.crc32(bs, self.crc)
self.n += len(bs)
def finish(self):
"""Finish checksum computation and return result.
After this, the object should no longer be used.
"""
if not self.n:
return self.crc, self.n
bs = bytearray(self.n.to_bytes((self.n.bit_length() + 7) // 8,
byteorder='little'))
self.bit_reverse(bs)
self.crc = binascii.crc32(bs, self.crc)
crcb = bytearray(self.crc.to_bytes(4, byteorder='little'))
self.bit_reverse(crcb)
crc = int.from_bytes(crcb, byteorder='big')
return crc, self.n
@classmethod
def sum_whole(cls, bs):
"""Checksum a single piece of data.
IMPORTANT: the data in bs is DESTROYED.
"""
c = cls()
c.sum(bs)
return c.finish()
@staticmethod
def bit_reverse(bs):
"""Reverse the bits in every byte of the argument in-place"""
for i,b in enumerate(bs):
r = 0
t = 0x80
s = 0x01
while t:
if (b & t):
r |= s
t >>= 1;
s <<= 1;
bs[i] = r
if __name__ == '__main__':
# Checksum all files passed as arguments, or stdin if none
from sys import argv,stdin
if len(argv) == 1:
bs = bytearray(stdin.buffer.read())
crc, n = PosixCkSum.sum_whole(bs)
print(crc, n)
else:
for fn in argv[1:]:
if fn == '-':
bs = bytearray(stdin.buffer.read())
else:
with open(fn, 'rb') as f:
bs = bytearray(f.read())
crc, n = PosixCkSum.sum_whole(bs)
print(crc, n, fn)
signature.asc
Description: OpenPGP digital signature
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