AVR Freaks

Groan.

Ok, what's the best option for someone developing on a mac who's never done either of them before? The hardware is a blank slate - I can set it up for either or both (assuming it doesn't take up pins I need for something else). What I do *not* want to do is do programming over USB. If you want to inject rogue firmware in, you're gonna have to crack the case open. :)

What's the PDI equivalent of AVRDude? I will check if my avr-gcc accepts an mmcu argument for xmega. If I can figure out how to get a .hex into the chip, the rest I think I can manage.

Well, it turns out the C4 doesn't have the AES subsystem.  The C3 does... maybe. The datasheet says that "selected devices only" have it, but I don't see where it says which ones.

EC is the wrong tool for the job. I need a symmetric algorithm for this.

I've ordered prototype boards for the xxA4U and have some 32A4U parts on the way. I also bought an AtmelICE and the plan is to use avrdude -c atmelice_pdi for programming. I haven't exhaustively started porting the firmware yet, but LUFA does seem to at least compile when properly set up for the A4U, and it looks like I can use the internal 32 MHz clock with USB SOF correction for the system clock and the PLL to turn that into 48 MHz for USB. Finally, it looks like I can eventually try USART0 in MSPI mode to improve the throughput. The eventual hope is to break through 500 kB/sec (the 32u2 with no crypto is doing 200 now, but that's at 8 MHz SPI instead of 16 and no SPI double-buffering). It's not going to be a speed demon, but that will at least be minimally usable, I think. Schematic: https://cdn.hackaday.io/files/20772888709248/Orthrus%20v2.0.pdf

So far the SOF calibration system seems to be working ok, albeit with a very small sample size. One thing I wonder about is that the sample code in LUFA uses the 32 MHz oscillator raised up to 48 MHz as the USB clock with the SOF correction and separately uses the 2 MHz oscillator with the PLL to make the 32 MHz CPU clock. I, instead, clock the 32 MHz oscillator at 32 MHz for the CPU clock, SOF correct it, and then use the PLL from that to get 48 MHz for the USB clock. I figured the results would be the same, but I'd be running one less oscillator and save some power. Might there be an accuracy difference there that you're speaking of?

The idea of overclocking the CPU at 48 MHz is very, very tempting. I was getting around 225 KB/sec at 32 MHz, but that was with AES counter mode, which is cryptographically weaker for this purpose, so I've switched to XEX mode. But XEX mode can't be precomputed in the background with the DMA system, so I'm now down to 135 KB/sec. I'm contemplating moving to a AT32UC3A4128S so that I can get a faster core, high speed USB, 4 bit SD I/O and AES-256. There's no question that I'll get results that are within an order of magnitude of the current state of the art for ordinary SD card readers. But that chip is going to require a whole lot of "firsts" from me - BGA, AVR32, 4 bit SD, JTAG programming... It's giving me pause to have that many new things to work through at one time.

I was using counter mode (perform ECB on 0, 1, 2... and XOR with plaintext). When you're doing WDE, there's no place to store an IV, so you can't use individual ones for every ciphertext. You can only use a nonce (you can tweak the nonce on a per-disk-block basis, but within a single disk block it's still a fixed nonce). Counter mode allowed me to precalculate the cryptostream in the background and in my testing it literally had no impact on the throughput compared to turning the crypto completely off. The downside, though, is that like all streaming modes (without varying IVs), there are known plaintext, known ciphertext  and bit-flip attacks that make it weak for this purpose.

XEX blunts those weaknesses and adds 16 byte diffusion on bit-flipping attacks, but there's no way to use DMA practically that I can find because you need the plaintext to perform the encryption, and you only get that in the pipeline between USB and the SD card.

Oops. Will fix. Fixed.