Explaination of Python code snippet

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:

self.data_file is a byte array that is returned by read() on the file(infile, 'rb') call.
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?

The length of [0:78] is 78, not 79. A slice doesn't include the ending value, so it's 0..77. — Mark Ransom
– Mark Ransom, Commented May 2, 2012 at 22:45
@MarkRansom, most code I work with has the last value inclusive, not exclusive. Thanks for the tip. — Jason
– Jason, Commented May 2, 2012 at 22:50

Cat Plus Plus · Accepted Answer · 2012-05-02 22:42:56Z

8

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].

answered May 2, 2012 at 22:42

Cat Plus Plus

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Jonathon Reinhart · Accepted Answer · 2012-05-02 22:43:06Z

2

self.header[0x3C:0x3C+8] will get a string of 8 bytes from the header, starting at offset 0x3C.

answered May 2, 2012 at 22:43

Jonathon Reinhart

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