Suggested RP235x for new board design

[juliadin]

Hi,

I am looking into designing a custom RP2350 board because the ones that exist seem to be quite big and not ideally suited for everyday use (flimsy button presses etc). I'd like to use as little room as possible. Could your pico-fido, pico-fido2 or pico-openpgp firmwares run on the 2354 chip without external flash? Do you see any limitations regarding the usability of resident credentials and the internal filesystem size?

Best wishes,
Julia

[juliadin]

I should mention that this is an Open Hardware project. And if this should work, I'd like to release this as an open hardware kit (with everything needed to order your own with prototyping fabs like JLCPCB or PCBWay.

I don't like that many of the sticks available are not really well suited for use as FIDO key because they are quite flimsy, slip around in the port and have top mounted buttons that often cause problems for me... also they are bigger than needed because of the pin headers usually found on them.

And to reduce the size of the hardware i'd love to use the internal flash of the 2354 only.

if the picokeys firmwares don't work I'll most likely still create it but i'd love to use this as a truly open source Security Key.

[polhenarejos]

It should work but I guess some additional development to support the internal flash will be required. However currently I do not own a board with a mounted RP2354 but as soon I get one I'll add support.

[juliadin]

If the board design works out (idk how soon I can get things rolling and how many iterations it will take) I will send one or two your way if you like.

[juliadin]

If you need anything more to debug on a board than the SWD headers and an exposed UART, let me know. I want to keep it as small and simple as possible but then again, I have never done this before :D. I expect to run into some rough weather. Thanks for the reply :)

[PeterFalken]

@juliadin, I'm interested in your project and would be willing to collaborate.
I have a similar idea with an ESP32-S3 or an RP2350 with enough flash memory to probably run a version with FIDO2, HSM & PGP at the same time. I believe that for now, we need to switch/flash between the firmwares - the data from each App/Function is kept as long as you don't re-initialize the device after changing the firmware.

[juliadin]

I'd be interested to know what you mean by 'switching flash between firmwares'. I'd imagine, Pol having already made a firmware with the GPG and FIDO functions merged can comment on the feasibility of having all three functions in one firmware, no switcheroo would be necessary.

My device mainly focuses on having a low component count and small outline and I am currently not doing that well with the outline. But after finishing the BOM I should be ready for ordering a first batch to test. It will definitely forego the external flash and use an RP2354 for now. But exploring a single sided ESP-S3 board could be a fun exercise.

[polhenarejos]

Rapsberry guys gave me some models of Pico 2 with mounted RP2354 and it worked with minor fixes. Development branch works with rp2354 with the internal flash (2MB).

[PeterFalken]

I'd be interested to know what you mean by 'switching flash between firmwares'. I'd imagine, Pol having already made a firmware with the GPG and FIDO functions merged can comment on the feasibility of having all three functions in one firmware, no switcheroo would be necessary.

It was my understanding that we can switch between the pico-fido2(~795KB) and pico-hsm(~570KB) firmwares, to get complementary functionality. Not sure if making a unified firmware would enable the YubiHSM Auth application for example. I took these two screenshots the one that has NFC is the one from a real Yubikey the other one is from a Xiao RP2040.

That being said, combining all functionalities would take from the available space to store certificates & keys. Devices with less than 2MB of storage would be a tight fit, unless the common code overlaps enough that it becomes a none issue.

@polhenarejos do you know if this is possible?

[polhenarejos]

Not really. YubiHSM uses a propietary standard. It’s not based on anything known, so it would require implementing a full stack solely for this app. It’s not difficult, but IMHO it doesn’t worth it.

[PeterFalken]

Not really. YubiHSM uses a propietary standard. It’s not based on anything known, so it would require implementing a full stack solely for this app. It’s not difficult, but IMHO it doesn’t worth it.

That makes sense, thanks for the input @polhenarejos.

[PeterFalken]

@juliadin, I guess an RP2354 design with an option to expand the storage with an external storage chip might also be an alternative.

[PeterFalken]

@polhenarejos - is it feasible to have a version of the firmware that combines pico-FIDO2, pico-HSM and pico-PGP if there was enough storage?

[zequinha-taveira]

@juliadin, I guess an RP2354 design with an option to expand the storage with an external storage chip might also be an alternative.

Sim, o RP2354 com opção de expansão de storage externa é uma excelente alternativa!

[polhenarejos]

@polhenarejos - is it feasible to have a version of the firmware that combines pico-FIDO2, pico-HSM and pico-PGP if there was enough storage?

Purposes are completely different, albeit putting altogether would result in a heavy firmware.

[PeterFalken]

Purposes are completely different, albeit putting altogether would result in a heavy firmware.

Thank you for the answer @polhenarejos, in that case a key with enough storage would be needed to combine all the functionality and also have enough free space available to store the data. I'm guessing that all the code would use about 2MB (it might be a tight fit), so a device with at least 4MB of storage would probably be needed to try this out.