Daniel Stenberg recounts the story of how he and two friends reverse engineered the firmware of early 2000s MP3 players to create their own improved open source firmware called Rockbox. They grew bored with the poor quality firmware on early players like the Archos and were able to analyze, disassemble and modify the firmware through techniques like examining the hardware, analyzing differences between firmware versions, using debug tools, and eventually developing workarounds to load custom code. Their open source Rockbox firmware now runs on over 100 different MP3 player models and helped spur innovation in the portable music player market.

1. Яev3rse eng1neering
Daniel Stenberg, May 21st
2014

2. Agenda
A saga about a bunch of people
who grew bored with a factory
installed firmware in consumer
electronics and wrote their own
implementation

3. Daniel Stenberg
Email: daniel@haxx.se
Twitter: @bagder
Web: daniel.haxx.se
Blog: daniel.haxx.se/blog
hacker at

4. Please ask!
Feel free to interrupt and ask at any time!

5. Let me tell you a story...
•this story begins in the early 2001
•3 men in their best years
•embedded systems hackers
•the dawn of the mp3 player revolution

6. Archos mp3 players
•December 2000
•First mp3 player with a HDD
•Probably the crappiest firmware
in the world
•Surely we could do better? How
hard can it be?

7. What's inside?
•Read the onboard circuits and search or ask
•Used to be a good away
•… barely gives away anything nowadays
•Can it be hacked?
•Almost universally: yes

8. Firmware upgrade option!
•Look you can upgrade firmware...
•What's the architecture again?
•Collect several firmware files
•Analyze differences
•Throw everything and everyone at it
•This can take a lot of time

9. Intermission: legality
•Were we allowed to do this?
•The world is full of jurisdictions
•We are Swedish, what does EU laws say?
•The 1991 EU Computer Programs Directive, article 6:
•The 2009 EU Computer Program Directive:
The authorization of the rightholder shall not be required
where reproduction of the code and translation of its form ...
are indispensable to obtain the information necessary to
achieve the interoperability of an independently created
computer program with other programs
performance of the acts of reproduction and translation by or
on behalf of a person having a right to use a copy of the
program is legitimate and compatible with fair practice and
must therefore be deemed not to require the authorisation of
the rightholder. An objective of this exception is to make it
possible to connect all components of a computer system,
including those of different manufacturers, so that they can
work together

10. XORing a fixed string
•By guessing parts of the decrypted file some “encryption”
is easier than others
•Early devices just scrambled firmwares like this

11. How do things work in there?
•Figure out how your architecture works
•Master things like the CPU instruction set and how a
stack works
•These days things are “always” ARM
•The CPU core is not the problem, the peripherals, busses
and associated HW are the challenges

12. Disassembly
•Objdump is an excellent tool
•Is not as hard as you might think
•Look for register addresses / memory layout patterns
•Error messages/strings/bitmaps or pointers to them

13. Disassembly even easier
•IDApro is an advanced tool to automate the task even
more
• Detects lots of C/C++ magic by itself
• Stack frames
• Bitmaps
• Structs
• Strings
• Memory layout

14. Scan the bare PCB and beep them
BGA
removed!
Surprisingly many
clues printed on the
PCB

15. Hiding what's inside
•Rub off all markings
•Use chips without public documentation
•Use chips unsupported by gcc

16. Hardware debuggers
•Many devices leave debug points for BDM / JTAG or
debug uarts
•Like this:

17. Attaching stuff on their HW
•Hm, what do we have here...

18. oh so useful

19. Stripped and attached

20. Anti-bricking measures
•Have multiple devices
•Primary boot-loader feature: run the original
•If possible, load and run from RAM only until tested

21. Software decoding targets
•Our first targets had mp3 decoding hardware
•New architectures entered
•More XOR and checksums
•Detect code patterns and search online for data sheets.
•Partial matches may give “similar chips” that have
documentation

22. Early ipods
•Used hidden hard-drive partitions
•Used completely (publicly) undocumented chips
•Took a long time to master

23. Later target examples
•Boot-loader that loads and decrypts firmware
•Magic constants in the boot-loader code revealed
algorithm.
•Boot-loader also upgradable (plain) gave away crypto
keys
•Known flags in digital signature algorithm (DSA) offered
shortcut in code signing

24. So what about true cryptography?

25. Loading encrypted payloads
•Target loads only encrypted files
•User finds flaw in one firmware version that crashes the
device
•A buffer overflow in the HTML reader
•Look, if we add crafted data in that HTML file we can
execute code
•When we used a loop to write in memory we managed to
toggle the backlight

26. A backlight reading device

27. From backlight to restored key
• dump memory using a videocamera and toggle backlight
• 32MB contents took many hours
• analyze what's in memory
• code, data, clues
• see, there's a pattern of USB registers
• rewrite the memory dump program to send contents over USB, insert
the whole thing into a HTML file, load it on target device
• there seems to be code referencing an SRAM
• dump SRAM too
• Look, there's something that looks like a crypto key!

28. It takes time and people
•Many volunteers
•Skilled volunteers
•Devoted volunteers
•Lots of time
•> 1 year from buffer overflow to running code

29. Getting it done faster?
•What if there was (lots of) money to gain?
•More clever people spending more of their time
•More computers cracking crypto
•More hardware analyzers
•If you can upgrade the device, reverse engineering it will
be possible

30. From tiny to Android
2001
2MB RAM
6GB HDD
12 MHz CPU
2010
64MB RAM
120GB HDD
500 MHz CPU
Today: the dedicated mp3 player market is dead or dying
and everyone is going Android
Reverse engineering is still done mostly the same

31. Linux-based reverse engineering
•More devices use full-fledged Linux
•More flaws, more drivers, less ways to have “unique”
solutions hard to figure out
•Manufacturers stick to undocumented hardware
•… and booting encrypted blobs
•Once “hacked”, putting your own SW can be much easier:
• familiar APIs (libc, u-boot, standard libs)
• familar drivers (even if binary blobs will be there)
• gcc!

32. Rockbox
A complete and very portable open source mp3 player
firmware replacement, including multi-tasking operating
system and application suite...
Started by me and my two friends Linus and Björn.
Runs on almost 100 different mp3 players from brands such
as Toshiba, Tatung, Sony, SanDisk, Samsung, Philips, Pandora,
Onda, Olympus, MPIO, Creative, Apple, Archos, Cowon,
HifiMan, Meizu, iRiver, Packard Bell, iAudio and more...

33. Tower of Rockbox

34. Thank you!

35. Learn more!
•Rockbox http://www.rockbox.org/
•“Reverse Engineering for Beginners”
http://yurichev.com/writings/RE_for_beginners-en.pdf

36. Doing good is part of our code