Clarification on CCID and HID Usage in Pico FIDO Firmware on RP2040 (No Secure Element)

[devi1207]

Hi @polhenarejos,

I'm currently using this firmware with the RP2040 board (without a Secure Element), and while debugging, I noticed that CCID APDU debug messages are being printed. This led to some confusion as I initially assumed only the HID protocol would be active in the absence of a Secure Element.

Could you please help me understand a few things?

What is the purpose of the CCID interface in this project when no Secure Element is connected?

Under what conditions does the firmware decide to use CCID vs HID?

Are the CCID-related debug messages indicative of actual CCID functionality running, or are they placeholders/simulated behavior?

Is there a fallback mechanism that handles CCID requests in software if a Secure Element is not present?

I’m trying to get a full picture of how both protocols are architected and when they're used, so I can better understand.

Thanks for your time and help.

[polhenarejos]

CCID it’s not related with Secure Element. CCID is a protocol over USB used as interface. Any software can use this interface to communicate with the board with any purpose. It depends on the vendor how this interface is used.
in the case of Pico Fido, CCID is used as interface of Yubico software, since they use CCID to manage the board (mainly configuration) and to provide OATH functionalities. Nitrokey use CCID also for management.
I use CCID to manage the board through these apps.