bluetooth: services: fast_pair: add new storage api for finding a key

Added a new fp_storage_account_key_find API to the internal header
of the Fast Pair Storage module. This API allows the user to search
for a specific Account Key that satisfies certain conditions. These
conditions are defined via callback function.

The new Fast Pair Storage API is now used in the Keys module.

Signed-off-by: Kamil Piszczek <[email protected]>

Author: kapi-no

subsys/bluetooth/services/fast_pair/fp_keys.c

@@ -38,6 +38,11 @@ struct fp_procedure {
 	uint8_t aes_key[FP_CRYPTO_ACCOUNT_KEY_LEN];
 };
 
+struct fp_key_gen_account_key_check_context {
+	struct bt_conn *conn;
+	struct fp_keys_keygen_params *keygen_params;
+};
+
 static bool user_pairing_mode = true;
 static struct fp_procedure fp_procedures[CONFIG_BT_MAX_CONN];
 
@@ -155,35 +160,41 @@ static int key_gen_public_key(struct bt_conn *conn, struct fp_keys_keygen_params
 	return err;
 }
 
-static int key_gen_account_key(struct bt_conn *conn, struct fp_keys_keygen_params *keygen_params)
+static bool key_gen_account_key_check(const uint8_t *account_key, void *context)
 {
 	int err;
+	uint8_t req[FP_CRYPTO_AES128_BLOCK_LEN];
+	struct fp_key_gen_account_key_check_context *ak_check_context = context;
+	struct bt_conn *conn = ak_check_context->conn;
+	struct fp_keys_keygen_params *keygen_params = ak_check_context->keygen_params;
 	struct fp_procedure *proc = &fp_procedures[bt_conn_index(conn)];
 
-	uint8_t req[FP_CRYPTO_AES128_BLOCK_LEN];
-	uint8_t ak[CONFIG_BT_FAST_PAIR_STORAGE_ACCOUNT_KEY_MAX][FP_CRYPTO_ACCOUNT_KEY_LEN];
-	size_t ak_cnt = CONFIG_BT_FAST_PAIR_STORAGE_ACCOUNT_KEY_MAX;
+	memcpy(proc->aes_key, account_key, FP_CRYPTO_ACCOUNT_KEY_LEN);
 
-	err = fp_storage_account_keys_get(ak, &ak_cnt);
+	err = fp_keys_decrypt(conn, req, keygen_params->req_enc);
 	if (err) {
-		return err;
+		return false;
 	}
 
-	for (size_t i = 0; i < ak_cnt; i++) {
-		memcpy(proc->aes_key, ak[i], FP_CRYPTO_ACCOUNT_KEY_LEN);
-
-		err = fp_keys_decrypt(conn, req, keygen_params->req_enc);
-		if (!err) {
-			err = keygen_params->req_validate_cb(conn, req, keygen_params->context);
-		}
-
-		if (!err) {
-			/* Key was found. */
-			break;
-		}
+	err = keygen_params->req_validate_cb(conn, req, keygen_params->context);
+	if (err) {
+		return false;
 	}
 
-	return err;
+	return true;
+}
+
+static int key_gen_account_key(struct bt_conn *conn, struct fp_keys_keygen_params *keygen_params)
+{
+	struct fp_key_gen_account_key_check_context context = {
+		.conn = conn,
+		.keygen_params = keygen_params,
+	};
+
+	/* This function call assigns the Account Key internally to the Fast Pair Keys
+	 * module. The assignment happens in the provided callback method.
+	 */
+	return fp_storage_account_key_find(NULL, key_gen_account_key_check, &context);
 }
 
 int fp_keys_generate_key(struct bt_conn *conn, struct fp_keys_keygen_params *keygen_params)

subsys/bluetooth/services/fast_pair/fp_storage.c

@@ -239,6 +239,33 @@ int fp_storage_account_keys_get(uint8_t buf[][FP_CRYPTO_ACCOUNT_KEY_LEN], size_t
 	return 0;
 }
 
+int fp_storage_account_key_find(uint8_t account_key[FP_CRYPTO_ACCOUNT_KEY_LEN],
+				fp_storage_account_key_check_cb account_key_check_cb,
+				void *context)
+{
+	if (!atomic_get(&settings_loaded)) {
+		return -ENODATA;
+	}
+
+	if (!account_key_check_cb) {
+		return -EINVAL;
+	}
+
+	for (size_t i = 0; i < account_key_count; i++) {
+		if (account_key_check_cb(account_key_list[i], context)) {
+			if (account_key) {
+				memcpy(account_key,
+				       account_key_list[i],
+				       FP_CRYPTO_ACCOUNT_KEY_LEN);
+			}
+
+			return 0;
+		}
+	}
+
+	return -ESRCH;
+}
+
 int fp_storage_account_key_save(const uint8_t *account_key)
 {
 	if (!atomic_get(&settings_loaded)) {

subsys/bluetooth/services/fast_pair/include/fp_storage.h

@@ -21,6 +21,19 @@ extern "C" {
 #include <sys/types.h>
 #include "fp_crypto.h"
 
+/**
+ * @typedef fp_storage_account_key_check_cb
+ * @brief Callback used to check if a given Account Key satisfies user-defined
+ *        conditions.
+ *
+ * @param[in] account_key 128-bit (16-byte) Account Key to be checked.
+ * @param[in] context     Pointer used to pass operation context.
+ *
+ * @return True if Account Key satisfies user-defined conditions.
+ *         False otherwise.
+ */
+typedef bool (*fp_storage_account_key_check_cb)(const uint8_t *account_key, void *context);
+
 /** Save Account Key.
  *
  * @param[in] account_key 128-bit (16-byte) Account Key to be saved.
@@ -50,6 +63,21 @@ int fp_storage_account_key_count(void);
  */
 int fp_storage_account_keys_get(uint8_t buf[][FP_CRYPTO_ACCOUNT_KEY_LEN], size_t *key_count);
 
+/** Iterate over stored Account Keys to find a key that matches user-defined conditions.
+ *  If such a key is found, the iteration process stops and this function returns.
+ *
+ * @param[out] account_key Found Account Key. It is possible to pass NULL pointer if the found
+ *                         key value is irrelevant.
+ * @param[in] account_key_check_cb Callback for determining if a given Account Key satisfies
+ *                                 user-defined conditions.
+ * @param[in] context Pointer used to pass operation context.
+ *
+ * @return 0 If the Account Key was found. Otherwise, a (negative) error code is returned.
+ */
+int fp_storage_account_key_find(uint8_t account_key[FP_CRYPTO_ACCOUNT_KEY_LEN],
+				fp_storage_account_key_check_cb account_key_check_cb,
+				void *context);
+
 #ifdef __cplusplus
 }
 #endif