Return formatted SID string given list of integers containing SID from byte array

sid.py

#!/usr/bin/env python
"""
Module containing class to parse and return formatted SID string given list of integers containing SID from byte array

This was made for formatting the CreatorSID from the Microsoft Windows CIM (WMI) repository database in the standard 
Windows SID format ("S-1-5-21-<RID>-<RID>...). For instance, if using a script such as [`python-cim`](https://github.com/fireeye/flare-wmi/tree/master/python-cim) 
[filter-to-consumer bindings](https://github.com/fireeye/flare-wmi/blob/master/python-cim/samples/show_filtertoconsumerbindings.py), 
to extract CreatorSID using that script, you would add `'CreatorSID'` to the filter or consumer properties like so:

    filter_sid = filter.properties["CreatorSID"].value
    consumer_sid = consumer.properties["CreatorSID"].value

The output from either of these statements could then be used to initialize a SID object to get the formatted Windows 
SID string:

    s = SID(filter_sid)
    print(s)  # S-1-5-21-3111613574-2524581245-2586426736-500

## MIT License

Copyright (c) 2018 Dan O'Day <[email protected]>

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation 
files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, 
modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the 
Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE 
WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR 
COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, 
ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
"""

import struct


class SID(object):
    """
    Return normalized Windows SID string given byte array (list of ints) containing SID

    See:
    - https://technet.microsoft.com/en-us/library/cc962011.aspx
    - https://msdn.microsoft.com/en-us/library/windows/desktop/aa379597(v=vs.85).aspx
    - https://blogs.msdn.microsoft.com/oldnewthing/20040315-00/?p=40253
    - https://www.nirsoft.net/kernel_struct/vista/SID.html
    - https://www.nirsoft.net/kernel_struct/vista/SID_IDENTIFIER_AUTHORITY.html

    NirSoft C representation of structs:

        typedef struct _SID
        {
             UCHAR Revision;
             UCHAR SubAuthorityCount;
             SID_IDENTIFIER_AUTHORITY IdentifierAuthority;
             ULONG SubAuthority[1];
        } SID, *PSID;

        typedef struct _SID_IDENTIFIER_AUTHORITY
        {
             UCHAR Value[6];
        } SID_IDENTIFIER_AUTHORITY, *PSID_IDENTIFIER_AUTHORITY;

    ## Example Usage

    Example usage. Drop module into import path and use like so:

        from sid import SID

        # ba = byte array containing SID
        ba = [1, 5, 0, 0, 0, 0, 0, 5, 21, 0, 0, 0, 134, 116, 119, 185, 125, 13, 122, 150, 112, 189, 41, 154, 244, 1, 0, 0]
        s = SID(ba)
        print s  # S-1-5-21-3111613574-2524581245-2586426736-500

    """
    def __init__(self, sid_byte_array):
        self.sid_byte_array = sid_byte_array
        self.revision_level = self.sid_byte_array[0]
        self.subauthority_count = self.sid_byte_array[1]
        self.identifier_authority = self._unpack_bytes_big_endian(self.sid_byte_array[2:8])
        self.subauthorities = []

        # current index in sid byte array
        i = 8  # initial starting position
        for rid in xrange(self.subauthority_count):
            self.subauthorities.append(struct.unpack('<L', ''.join([chr(e) for e in self.sid_byte_array[i:i+4]])))
            i += 4

    def _unpack_bytes_big_endian(self, n):
        """
        Convert arbitrary number of bytes to int (big endian)

        The struct unpack() method only works with bytes provided with lengths divisible by powers of 2. I could pad
        the 48-bit (6-byte) identifier authority value but instead I'm just reimplementing unpack to work with any
        number of bytes since it's easy math

        :param n: list containing bytes to be converted to int (big endian)
        :return: int value of bytes n (big endian)
        """
        r = 0
        for b in n:
            r = r * 256 + int(b)
        return r

    def __str__(self):
        """
        Prints SID in standard format

        :return: SID string in standard format
        """
        sid = "S-{0}-{1}".format(self.revision_level, self.identifier_authority)
        for rid in self.subauthorities:
            sid += "-{0}".format(str(rid[0]))
        return sid