bento_devmode.h

Go to the documentation of this file.

/*******************************************************************************
 * File Name: bento_devmode.h
 *
 * Description: Developer-mode unlock state for the Bento Desktop Buddy bridge.
 *              Gates bento.exec and other privileged verbs behind an
 *              HMAC-SHA256 challenge/response.
 *
 *              Lifecycle:
 *                1. Desktop sends bento.devmode.nonce → firmware returns a
 *                   fresh 16-byte nonce (64-ms TTL).
 *                2. Desktop computes HMAC-SHA256(shared_secret, nonce) and
 *                   sends bento.devmode.unlock.
 *                3. Firmware verifies and sets the unlocked flag.
 *                4. bento.devmode.lock clears the flag.
 *
 *              Shared secret storage:
 *                Primary — OPTIGA Trust M arbitrary-data slot 0xE120
 *                Fallback — RAM-only, regenerated every boot (means desktop
 *                           must re-provision after every reflash).
 *                v1 uses the RAM-only fallback unconditionally; OPTIGA
 *                persistence lands in a follow-up along with the proper
 *                first-pair provisioning UX.
 *
 ******************************************************************************/
 
#ifndef BENTO_DEVMODE_H
#define BENTO_DEVMODE_H
 
#include <stdbool.h>
#include <stddef.h>
#include <stdint.h>
 
#ifdef __cplusplus
extern "C" {
#endif
 
#define BENTO_DEVMODE_NONCE_LEN   (16u)
#define BENTO_DEVMODE_SECRET_LEN  (32u)
#define BENTO_DEVMODE_HMAC_LEN    (32u)    /* SHA-256 output */
#define BENTO_DEVMODE_NONCE_TTL_MS (60000u)
 
/* Initialise the secret (RAM-only fallback — seeds from boot entropy +
 * TickCount on first call). Idempotent. */
void bento_devmode_init(void);
 
/* Generate a fresh 16-byte nonce and return it via `nonce_out`. Overwrites
 * any previous pending nonce (only one in-flight challenge). Sets the
 * internal ttl clock. */
void bento_devmode_nonce_issue(uint8_t nonce_out[BENTO_DEVMODE_NONCE_LEN]);
 
/* Verify an HMAC-SHA256 hex string against the current pending nonce + the
 * shared secret. Returns true on match AND ttl window unexpired, in which
 * case the internal unlocked flag is set. Consumes the nonce on any call
 * (one-shot, prevents replay). */
bool bento_devmode_unlock(const char *hmac_hex, size_t hmac_hex_len);
 
/* Manually clear the unlocked flag. */
void bento_devmode_lock(void);
 
/* Query whether exec is currently permitted. */
bool bento_devmode_is_unlocked(void);
 
/* Expose the provisioning secret as a hex string for the first-pair
 * emit-once flow. Returns length written (64 hex chars
 * + NUL) or 0 if the out buffer is too small. RAM-only fallback; the
 * caller must NOT log this to UART. */
size_t bento_devmode_secret_hex(char *out, size_t out_sz);
 
/* Emit `{"evt":"bento.devmode.provision","secret":"<64 hex>"}` over BLE
 * NUS once per boot so the desktop side can store the secret in its
 * keychain. Without this emit the desktop holds a randomly-generated
 * placeholder secret that will never match firmware-side state and
 * every `bento.devmode.unlock` will return `not_permitted`. Wired into
 * the lazy BLE bring-up's CONNECTED transition, paired with
 * `bento_fw_emit_boot_complete()`. Internally guards with a static
 * "already sent this boot" flag — multiple BLE connect/disconnect
 * cycles within one boot will not re-send (the secret in firmware
 * doesn't change without a reset). */
void bento_devmode_emit_provision(void);
 
/* First 4 bytes of SHA-256(secret) as 8 hex chars + NUL — safe to log,
 * used to cross-check that desktop and firmware are operating on the
 * same secret without leaking it. Matches the `secret_fp` tracing on
 * the Rust side (`verbs/devmode.rs::unlock`). */
size_t bento_devmode_secret_fp_hex(char *out, size_t out_sz);
 
#ifdef __cplusplus
}
#endif
 
#endif /* BENTO_DEVMODE_H */