Crate hsm_usb_protocol 

USB-HID protocol used between the mini-HSM dongle and the host.

This crate is no_std by default so the firmware can depend on it. The std feature toggles a handful of niceties (currently nothing, kept as a future extension point).

Wire format

Every exchange consists of fixed-size HID reports of HID_REPORT_SIZE bytes. The same 3-byte header is used in both directions:

Byte 0           : Opcode (command opcode for IN reports, status byte for OUT)
Byte 1..3        : Payload length, little-endian u16
Byte 3..3+len    : Payload
Byte 3+len..end  : Zero padding (sent zero, ignored on receive)

len is the host-meaningful payload size, not the wire size. The wire size is always HID_REPORT_SIZE regardless. len must not exceed MAX_PAYLOAD_SIZE.

Layering

Frame encodes/decodes the byte layout. It is opcode-agnostic and deals only with (u8, &[u8]). Both the firmware and hsm-host build one Frame per HID report, then interpret the payload with helpers defined in commands and responses. There is no big enum Command { Info, Sign { slot, digest }, ... }: that pattern scales poorly with size and pays runtime cost for static information.

Re-exports

pub use commands::CommandOpcode;

pub use frame::Frame;

pub use frame::FrameParseError;

pub use responses::ResponseStatus;

Modules

commands

Commands sent from the host to the token.

frame

Byte-level encoding and decoding of HID frames.

hid_descriptor 🔒

HID report descriptor for the mini-HSM.

responses

Status bytes returned by the token in every response report.

Constants

HEADER_SIZE 🔒

Size of the 3-byte header (opcode | len_lo | len_hi).

HID_REPORT_DESCRIPTOR

HID report descriptor, exactly as sent to the host during USB enumeration.

HID_REPORT_SIZE

Fixed size of every HID report in either direction.

MAX_PAYLOAD_SIZE 🔒

Maximum payload size in a report (HID_REPORT_SIZE - HEADER_SIZE).

USB_PID

USB product identifier for this project.

USB_VID

USB vendor identifier for this project. 0xCAFE is a community convention for open-source / hobby devices (no USB-IF assignment).