Bluetooth: host: crypto: Add a config to select how rand is generated

Considering that in most scenarios, bt_rand will not be called
frequently, but the current implementation of tinycrypt will
occupy more than 300 bytes of RAM space. Its existence is to
optimize the frequent call of bt_rand.

Therefore, it is considered to put it into a config
(`BT_HOST_CRYPTO_RANDOM`), when this config has been selected,
will use tinycrypt library for random. Otherwise will call bt random
hci command.

Signed-off-by: Lingao Meng <[email protected]>

Author: LingaoM

subsys/bluetooth/host/Kconfig

@@ -114,16 +114,29 @@ rsource "../mesh/Kconfig"
 rsource "../audio/Kconfig"
 
 config BT_HOST_CRYPTO
-	# Hidden option that compiles in random number generation and AES
-	# encryption support using TinyCrypt library if this is not provided
-	# by the controller implementation.
+	# Hidden option that compiles in AES encryption support using TinyCrypt
+	# library if this is not provided by the controller implementation.
 	bool
 	default y if !BT_CTLR_CRYPTO
 	select TINYCRYPT
 	select TINYCRYPT_AES
+
+config BT_HOST_CRYPTO_PRNG
+	bool "Use Tinycrypt library for random number generation"
+	default y
 	select TINYCRYPT_SHA256
 	select TINYCRYPT_SHA256_HMAC
 	select TINYCRYPT_SHA256_HMAC_PRNG
+	depends on BT_HOST_CRYPTO
+	help
+	  When selected, will use tinycrypt library for random number generation.
+	  This will consume additional ram, but may speed up the generation of random
+	  numbers.
+
+	  Otherwise, random numbers will be generated through multiple HCI calls,
+	  which will not consume additional resources, but may take a long time,
+	  depending on the length of the random data.
+	  This method is generally recommended within 16 bytes.
 
 config BT_SETTINGS
 	bool "Store Bluetooth state and configuration persistently"

subsys/bluetooth/host/crypto.c

@@ -94,6 +94,7 @@ int prng_init(void)
 	return prng_reseed(&prng);
 }
 
+#if defined(CONFIG_BT_HOST_CRYPTO_PRNG)
 int bt_rand(void *buf, size_t len)
 {
 	int ret;
@@ -114,6 +115,44 @@ int bt_rand(void *buf, size_t len)
 
 	return -EIO;
 }
+#else /* !CONFIG_BT_HOST_CRYPTO_PRNG */
+int bt_rand(void *buf, size_t len)
+{
+	int ret, size;
+	size_t i = 0;
+
+	/* Check first that HCI_LE_Rand is supported */
+	if (!BT_CMD_TEST(bt_dev.supported_commands, 27, 7)) {
+		return -ENOTSUP;
+	}
+
+	while (len) {
+		struct bt_hci_rp_le_rand *rp;
+		struct net_buf *rsp;
+
+		ret = bt_hci_cmd_send_sync(BT_HCI_OP_LE_RAND, NULL, &rsp);
+		if (ret) {
+			return ret;
+		}
+
+		rp = (void *)rsp->data;
+		if (rp->status) {
+			return -EIO;
+		}
+
+		size = MIN(len, sizeof(rp->rand));
+
+		(void)memcpy((uint8_t *)buf + i, rp->rand, size);
+
+		net_buf_unref(rsp);
+
+		i += size;
+		len -= size;
+	}
+
+	return 0;
+}
+#endif /* CONFIG_BT_HOST_CRYPTO_PRNG */
 
 int bt_encrypt_le(const uint8_t key[16], const uint8_t plaintext[16],
 		  uint8_t enc_data[16])

subsys/bluetooth/host/hci_core.c

@@ -2655,7 +2655,7 @@ static int common_init(void)
 	read_supported_commands_complete(rsp);
 	net_buf_unref(rsp);
 
-	if (IS_ENABLED(CONFIG_BT_HOST_CRYPTO)) {
+	if (IS_ENABLED(CONFIG_BT_HOST_CRYPTO_PRNG)) {
 		/* Initialize the PRNG so that it is safe to use it later
 		 * on in the initialization process.
 		 */