Bluetooth: Host: Add legacy pairing test config

Adds the `CONFIG_BT_SMP_ENFORCE_LEGACY_PAIRING` Kconfig option to force
devices to use legacy pairing. This has a dependency on
`CONFIG_BT_TESTING` as it is only intended for testing purposes, and use
of legacy pairing is discouraged.

Signed-off-by: Håvard Reierstad <haavard.reierstad@nordicsemi.no>

Author: HaavardRei

subsys/bluetooth/host/Kconfig

@@ -1163,6 +1163,15 @@ config BT_CONN_DISABLE_SECURITY
 	  WARNING: This option enables anyone to snoop on-air traffic.
 	  Use of this feature in production is strongly discouraged.
 
+config BT_SMP_ENFORCE_LEGACY_PAIRING
+	bool "Enforce legacy pairing"
+	depends on BT_TESTING
+	depends on !(BT_SMP_SC_PAIR_ONLY || BT_SMP_SC_ONLY)
+	help
+	  This option enforces legacy pairing. This is required for testing
+	  legacy pairing between two Zephyr Bluetooth devices, as without this
+	  option the devices will default to using Secure Connections pairing.
+
 rsource "./classic/Kconfig"
 
 config BT_HCI_VS_EVT_USER

subsys/bluetooth/host/smp.c

@@ -93,21 +93,21 @@ LOG_MODULE_REGISTER(bt_smp);
 #if defined(CONFIG_BT_CLASSIC)
 
 #define BT_SMP_AUTH_MASK_SC	0x2f
-#if defined(CONFIG_BT_SMP_OOB_LEGACY_PAIR_ONLY)
+#if defined(CONFIG_BT_SMP_OOB_LEGACY_PAIR_ONLY) || defined(CONFIG_BT_SMP_ENFORCE_LEGACY_PAIRING)
 #define BT_SMP_AUTH_DEFAULT (BT_SMP_AUTH_BONDING_FLAGS | BT_SMP_AUTH_CT2)
 #else
 #define BT_SMP_AUTH_DEFAULT (BT_SMP_AUTH_BONDING_FLAGS | BT_SMP_AUTH_CT2 |\
 			     BT_SMP_AUTH_SC)
-#endif /* CONFIG_BT_SMP_OOB_LEGACY_PAIR_ONLY */
+#endif /* CONFIG_BT_SMP_OOB_LEGACY_PAIR_ONLY || CONFIG_BT_SMP_ENFORCE_LEGACY_PAIRING */
 
 #else
 
 #define BT_SMP_AUTH_MASK_SC	0x0f
-#if defined(CONFIG_BT_SMP_OOB_LEGACY_PAIR_ONLY)
+#if defined(CONFIG_BT_SMP_OOB_LEGACY_PAIR_ONLY) || defined(CONFIG_BT_SMP_ENFORCE_LEGACY_PAIRING)
 #define BT_SMP_AUTH_DEFAULT (BT_SMP_AUTH_BONDING_FLAGS)
 #else
 #define BT_SMP_AUTH_DEFAULT (BT_SMP_AUTH_BONDING_FLAGS | BT_SMP_AUTH_SC)
-#endif /* CONFIG_BT_SMP_OOB_LEGACY_PAIR_ONLY */
+#endif /* CONFIG_BT_SMP_OOB_LEGACY_PAIR_ONLY || CONFIG_BT_SMP_ENFORCE_LEGACY_PAIRING */
 
 #endif /* CONFIG_BT_CLASSIC */
 
@@ -322,7 +322,8 @@ static struct {
 
 static bool le_sc_supported(void)
 {
-	if (IS_ENABLED(CONFIG_BT_SMP_OOB_LEGACY_PAIR_ONLY)) {
+	if (IS_ENABLED(CONFIG_BT_SMP_OOB_LEGACY_PAIR_ONLY) ||
+	    IS_ENABLED(CONFIG_BT_SMP_ENFORCE_LEGACY_PAIRING)) {
 		return false;
 	}