pastebin - collaborative debugging

# This is a python script. You need a Python interpreter to run it.
# For example, ActiveState Python, which exists for windows.
#
# Changelog
#  0.01 - Initial version
#  0.02 - Huffdic compressed books were not properly decrypted
#  0.03 - Wasn't checking MOBI header length
#  0.04 - Wasn't sanity checking size of data record
#  0.05 - It seems that the extra data flags take two bytes not four

import sys,struct,binascii

class DrmException(Exception):
        pass

#implementation of Pukall Cipher 1
def PC1(key, src, decryption=True):
    sum1 = 0;
    sum2 = 0;
    keyXorVal = 0;
    if len(key)!=16:
        print "Bad key length!"
        return None
    wkey = []
    for i in xrange(8):
        wkey.append(ord(key[i*2])<<8 | ord(key[i*2+1]))

    dst = ""
    for i in xrange(len(src)):
        temp1 = 0;
        byteXorVal = 0;
        for j in xrange(8):
            temp1 ^= wkey[j]
            sum2  = (sum2+j)*20021 + sum1
            sum1  = (temp1*346)&0xFFFF
            sum2  = (sum2+sum1)&0xFFFF
            temp1 = (temp1*20021+1)&0xFFFF
            byteXorVal ^= temp1 ^ sum2
        curByte = ord(src[i])
        if not decryption:
            keyXorVal = curByte * 257;
        curByte = ((curByte ^ (byteXorVal >> 8)) ^ byteXorVal) & 0xFF
        if decryption:
            keyXorVal = curByte * 257;
        for j in xrange(8):
            wkey[j] ^= keyXorVal;
        dst+=chr(curByte)
    return dst

def checksumPid(s):
        letters = "ABCDEFGHIJKLMNPQRSTUVWXYZ123456789"
        crc = (~binascii.crc32(s,-1))&0xFFFFFFFF
        crc = crc ^ (crc >> 16)
        res = s
        l = len(letters)
        for i in (0,1):
                b = crc & 0xff
                pos = (b // l) ^ (b % l)
                res += letters[pos%l]
                crc >>= 8
        return res

def getSizeOfTrailingDataEntries(ptr, size, flags):
        def getSizeOfTrailingDataEntry(ptr, size):
                bitpos, result = 0, 0
                if size <= 0:
                        return result
                while True:
                        v = ord(ptr[size-1])
                        result |= (v & 0x7F) << bitpos
                        bitpos += 7
                        size -= 1
                        if (v & 0x80) != 0 or (bitpos >= 28) or (size == 0):
                                return result
        num = 0
        flags >>= 1
        while flags:
                if flags & 1:
                        num += getSizeOfTrailingDataEntry(ptr, size - num)
                flags >>= 1
        return num


class DrmStripper:
        def loadSection(self, section):
                if (section + 1 == self.num_sections):
                        endoff = len(self.data_file)
                else:
                        endoff = self.sections[section + 1][0]
                off = self.sections[section][0]
                return self.data_file[off:endoff]

        def patch(self, off, new):
                self.data_file = self.data_file[:off] + new + self.data_file[off+len(new):]

        def patchSection(self, section, new, in_off = 0):
                if (section + 1 == self.num_sections):
                        endoff = len(self.data_file)
                else:
                        endoff = self.sections[section + 1][0]
                off = self.sections[section][0]
                assert off + in_off + len(new) <= endoff
                self.patch(off + in_off, new)

        def parseDRM(self, data, count, pid):
                pid = pid.ljust(16,'\0')
                keyvec1 = "\x72\x38\x33\xB0\xB4\xF2\xE3\xCA\xDF\x09\x01\xD6\xE2\xE0\x3F\x96"
                temp_key = PC1(keyvec1, pid, False)
                temp_key_sum = sum(map(ord,temp_key)) & 0xff
                found_key = None
                for i in xrange(count):
                        verification, size, type, cksum, cookie = struct.unpack('>LLLBxxx32s', data[i*0x30:i*0x30+0x30])
                        cookie = PC1(temp_key, cookie)
                        ver,flags,finalkey,expiry,expiry2 = struct.unpack('>LL16sLL', cookie)
                        if verification == ver and cksum == temp_key_sum and (flags & 0x1F) == 1:
                                found_key = finalkey
                                break
                return found_key


        def __init__(self, data_file, pid):

                if checksumPid(pid[0:-2]) != pid:
                        raise DrmException("invalid PID checksum")
                pid = pid[0:-2]

                self.data_file = data_file
                header = data_file[0:72]
                if header[0x3C:0x3C+8] != 'BOOKMOBI':
                        raise DrmException("invalid file format")
                self.num_sections, = struct.unpack('>H', data_file[76:78])

                self.sections = []
                for i in xrange(self.num_sections):
                        offset, a1,a2,a3,a4 = struct.unpack('>LBBBB', data_file[78+i*8:78+i*8+8])
                        flags, val = a1, a2<<16|a3<<8|a4
                        self.sections.append( (offset, flags, val) )

                sect = self.loadSection(0)
                records, = struct.unpack('>H', sect[0x8:0x8+2])
                mobi_length, = struct.unpack('>L',sect[0x14:0x18])
                extra_data_flags = 0
                if mobi_length >= 0xE4:
                        extra_data_flags, = struct.unpack('>H', sect[0xF2:0xF4])


                crypto_type, = struct.unpack('>H', sect[0xC:0xC+2])
                if crypto_type != 2:
                        raise DrmException("invalid encryption type: %d" % crypto_type)

                # calculate the keys
                drm_ptr, drm_count, drm_size, drm_flags = struct.unpack('>LLLL', sect[0xA8:0xA8+16])
                found_key = self.parseDRM(sect[drm_ptr:drm_ptr+drm_size], drm_count, pid)
                if not found_key:
                        raise DrmException("no key found. maybe the PID is incorrect")

                # kill the drm keys
                self.patchSection(0, "\0" * drm_size, drm_ptr)
                # kill the drm pointers
                self.patchSection(0, "\xff" * 4 + "\0" * 12, 0xA8)
                # clear the crypto type
                self.patchSection(0, "\0" * 2, 0xC)

                # decrypt sections
                print "Decrypting. Please wait...",
                for i in xrange(1, records+1):
                        data = self.loadSection(i)
                        extra_size = getSizeOfTrailingDataEntries(data, len(data), extra_data_flags)
                        self.patchSection(i, PC1(found_key, data[0:len(data) - extra_size]))
                print "done"
        def getResult(self):
                return self.data_file

print "MobiDeDrm v0.05. Copyright (c) 2008 The Dark Reverser"
if len(sys.argv)<4:
        print "Removes protection from Mobipocket books"
        print "Usage:"
        print "  mobidedrm infile.mobi outfile.mobi PID"
else:
        infile = sys.argv[1]
        outfile = sys.argv[2]
        pid = sys.argv[3]
        data_file = file(infile, 'rb').read()
        try:
                file(outfile, 'wb').write(DrmStripper(data_file, pid).getResult())
        except DrmException, e:
                print "Error: %s" % e