Question

For a class assignment, I’m analyzing how Amazon’s Kindle digital rights management implementation works as well as how to defeat it. In my research, I came across a set of Python scripts that extract out the book data from the encryption. It fits my needs for explaining the encryption-cracking part of my paper.

Problem is, I’m not fluent in Python or have any experience other than print 'Hello World'.

When working my way through the source code, I came across this snippet

def __init__(self, infile):
    # initial sanity check on file
    self.data_file = file(infile, 'rb').read()
    self.mobi_data = ''
    self.header = self.data_file[0:78]

    if self.header[0x3C:0x3C+8] != 'BOOKMOBI' and self.header[0x3C:0x3C+8] != 'TEXtREAd':
        raise DrmException("invalid file format")

    self.magic = self.header[0x3C:0x3C+8]
    self.crypto_type = -1

My interpretation of the code goes like this:

1. self.data_file is a byte array that is returned by read() on the file(infile, 'rb') call.
2. self.header is the value of the first 79 bytes of the data file

The problem I’m having is just what does self.header[0x3C:0x3C+8] mean?

Answer 1

It’s a normal slicing, like self.data_file[0:78], except uses hex literal as the offset. 0x3C is 60 in base10, so it’s the same as self.header[60:60+8].