Atmel SAM7XC Crypto Co-Processor key recovery (with bonus Mifare DESFire hack)

[Nytro]

[h=3]Atmel SAM7XC Crypto Co-Processor key recovery (with bonus Mifare DESFire hack)[/h]The problem with crypto is that it is processor intensive (i.e. slow), so it's common, these days, to offload these functions to a dedicated hardware co-processor which will leave the main processor free to do whatever it is that it's supposed to be doing and not faffing about with crypto. This is good in theory, as it means that cryptographic protection can be added to more or less any system without having to worry that it's going to add an unacceptable level of load to the processor. It is also theoretically more secure as the crypto keys are safely tucked away in a dedicated secure store. Much is made of this additional level of security in marketing literature.

But theory is all well and good - the bitch is getting it right in practice...

The following is the result of an audit performed for a client and is published with their full knowledge and consent.

There are a couple of ways you can attack a crypto system: go after the algorithm itself, or go after a specific implementation. To crack the algorithm, you generally need to be a very good mathematician (and more likely a cryptographer). Crypto algorithms are thought about long and hard, and specifically designed to be impervious to this kind of attack, so, although they don't always get it right, it is certainly not going to be an easy task to find the fundamental flaw (if any exists).

The other way, to go after a specific implementation, is much more likely to bear fruit and will usually be a whole lot easier, and can therefore be attempted by the likes of you and me. This is because in crypto the devil is in the detail, and as a system gets more complicated there is a lot more detail to take care of and therefore a lot more avenues of attack.

One of the specific issues crypto brings with it is the problem of managing the keys. The keys are literally the keys to your kingdom, and if you lose them you lose everything, so my first question is always: what keys are we after, and where can I find them?

Our target today is an access control system that uses a Mifare DESFire EV1 card as it's token. The microprocessor in the control unit is the Atmel AT91SAM7XC256, which is part of Atmel's ARM based SAM7XC series. It has a crypto co-processor, which includes a write-only memory for the keys, and the whole of the chip's memory can also be locked so that the code cannot be read out.

DESFire is a proprietary standard, but if you dig deep enough you'll find everything you're looking for, so it's relatively easy to write code to interrogate these tags and figure out what's on them (libfreefare would be a good place to start). I used my own RFIDIOt library to do so (note that I will always publish code if I can, but unfortunately as the DESFire standard is not open, and some of this tool is under NDA, I cannot in this case. If / when the restriction is lifted I'll be more than happy to do so, but in the meantime please don't ask. Sorry.)

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