[dm-crypt] Quick dm-crypt questions

Arno Wagner arno at wagner.name
Wed Oct 29 15:59:01 CET 2014

On Wed, Oct 29, 2014 at 11:33:24 CET, Ralf Ramsauer wrote:
> On 29.10.2014 11:24, Cpp wrote:
> > The thing is I planned to use a microcontroller to store an encryption
> > key in its RAM, and I see the device uses SRAM, so this might be a
> > problem?
> > http://www.atmel.com/Images/Atmel-8271-8-bit-AVR-Microcontroller-ATmega48A-48PA-88A-88PA-168A-168PA-328-328P_datasheet_Summary.pdf
> Yes, comments :-)
> First of all: are you going to store the Masterkey or the Passphrase /
> Keyfile which is used for key derivation?
> If you're going to store the master key, you don't need Luks at all,
> this would also be a solution for your detached-header problem.
> But....
> How do you want to realize the communication between the µC and you
> Linux Box? Over Uart? (Uart communication can _easily_ be sniffed, so be
> aware of that....)

If an attacker has access on that level, they can probaly just do
a memory-freeze attack or a fire-wire attack. Remember that 
disk encryption does not protect data while the system is running
and has the data decrypted. 

> Also don't forget to deactivate the JTAG interface. Otherwise the µC
> could get debugged... And don't forget to set the correct FUSE bits
> (disallow reading / writing from / to flash / EPROM memory, ....)
> And did you know, that CPU operations can be reconstructed by small
> fluctuations in current[1]? How do you want to solve this issue?
> How does the key get to the µC?
> Aah, almost forgot to mention: you talked about to use a RNG on your AVR
> to move the key around. RNG on AVR? From where do you get your entropy?
> I don't know much about this project, but maybe this helps you [2].
> There are *so* many traps... Do you really think this is a good idea?

I think this is mostly intended as a project to learn. As such
it should do well. But do not expet this to be secure against a
competent attacker.


> [1] http://en.wikipedia.org/wiki/Power_analysis
> [2] http://www.das-labor.org/wiki/AVR-Crypto-Lib //
> http://www.das-labor.org/wiki/AVR-Crypto-Lib#PRNGs
> Regards
>   Ralf
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Arno Wagner,     Dr. sc. techn., Dipl. Inform.,    Email: arno at wagner.name
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