STM32WBA Bluetooth® LE – Security Levels Characteristic

1. Security Level Characteristic feature

Bluetooth® Security Levels Characteristic feature

1.1. Security Levels Characteristic (SLC) feature

1.1.1. Principles

LE GATT Security Levels characteristic shall contain the highest security requirements among all server's GATT characteristic while operating on a LE connection.
The value of the LE GATT Security Level characteristic shall be static during a connection.

Security Levels characteristic (UUID: 0x2BF5) contains a sequence of one or more security level requirements:

*security level requirement: uint8_t[2]: security mode, security level

The Attribute Value is a sequence of Security Level Requirements, each with the type uint8[2]. Each Security Level Requirement consists of a Security Mode field followed by a Security Level field.
The Security Mode and Security Level shall be expressed as the same number as used in their definitions; e.g., mode 1 is represented as 0x01 and level 4 is represented as 0x04.

Bluetooth® Read Security Level Characteristic

1.2. How to implement SLC on STM32WBA applications

1.2.1. Peripheral

Add a dedicated SLC characteristic (UUID = 0x2BF5) to GAP service

Char_UUID_t  uuid;
uint16_t gap_SecuLevelsCharac_handle = 0U;
static const uint8_t p_additional_svc_record[1] = {0x03};
  
ret = aci_hal_write_config_data(CONFIG_DATA_GAP_ADD_REC_NBR_OFFSET,
                                 CONFIG_DATA_GAP_ADD_REC_NBR_LEN,
                                 (uint8_t*) p_additional_svc_record);
if (ret != BLE_STATUS_SUCCESS)
{
  LOG_INFO_APP("  Fail   : aci_hal_write_config_data command - CONFIG_DATA_GAP_ADD_REC_NBR_OFFSET, result: 0x%02X\n", ret);
}
else
{
  LOG_INFO_APP("  Success: aci_hal_write_config_data command - CONFIG_DATA_GAP_ADD_REC_NBR_OFFSET\n");
}
uint16_t SizeSecuLevelsCharac_C = sizeof(tab_ModeLevel);
uuid.Char_UUID_16 = LE_GATT_SECURITY_LEVELS_UUID;
ret = aci_gatt_add_char(gap_service_handle,
                         UUID_TYPE_16,
                         (Char_UUID_t *) &uuid,
                         SizeSecuLevelsCharac_C,
                         CHAR_PROP_READ,
                         ATTR_PERMISSION_NONE,
                         GATT_DONT_NOTIFY_EVENTS,
                         0x10,
                         CHAR_VALUE_LEN_CONSTANT,
                         &gap_SecuLevelsCharac_handle);
if (ret != BLE_STATUS_SUCCESS)
{
  APP_DBG_MSG("  Fail   : aci_gatt_add_char command   : security_levels_char, error code: 0x%2X\n", ret);
}
else
{
  APP_DBG_MSG("  Success: aci_gatt_add_char command   : security_levels_char\n");
}
 
/* write key/vector in characteristic -fixed value for the moment */  
ret = aci_gatt_update_char_value(gap_service_handle, gap_SecuLevelsCharac_handle, 0, SizeSecuLevelsCharac_C, (uint8_t *)&tab_ModeLevel);
if (ret != BLE_STATUS_SUCCESS)
{
  APP_DBG_MSG("  Fail   : aci_gatt_update_char_value - security_levels_char, result: 0x%02X\n", ret);
}
else
{
  APP_DBG_MSG("  Success: aci_gatt_update_char_value - security_levels_char\n");
}

1.2.2. Central

Discover Security Levels Characteristic added to GAP service (among all characteristics).

2. Example of Security Levels Characteristic feature on STM32WBA

2.1. Advertising data

At startup, Security Levels Characteristic Server application starts Advertising.
Data advertised are composed as follows:

Security Levels Characteristic (SLC) Server Advertising packet
Description Length AD Type Value Device Name 8 0x09 P2PSLC_XX (XX: last byte of BD address) Manufacturer Data 15 0xFF See table below Flags 2 0x01 0x06 (GeneralDiscoverable, BrEdrNotSupported)

Manufacturer data are encoded following STMicroelectronicsBlueST SDK v2 as described below:

STMicroelectronics Manufacturer Advertising data
Byte Index 0 1 2-3 4 5 6 7 8 9 10-15 Function Length Manufacturer ID Company BlueST SDK Version Device ID Firmware ID Option 1 Option 2 Option 3 Device Address Value 0x0F 0xFF 0x0030 STMicro 0x02 0x8B Nucleo-WBA 0xE4 - P2P SLC Server 0x00 0x00 0x00 0x08E12Axxxx

2.2. On-board buttons configuration

Button configuration for Bluetooth® LE connection subrating application on Nucleo-WBA55CG boards
Application Condition B1 Click B1 Long Press B2 Click B2 Long Press B3 Click B3 Long Press P2P SLC Server Idle Connected - Starts/Stops notification / / / / / P2P SLC Client Idle Connected Starts scan then connects Starts/Stops write data / / / Enable Char3 notification /

2.3. Security Levels Characteristic P2P Central and Peripheral applications

• Connection established at 50 ms
• Service, characteristics discovery
• Read Security Levels Characteristic values: mode, level
• If security level reported is higher than Level 1 (no security is required): new pairing process is requested

Bluetooth® Example of flow diagram between p2pServer SLC and p2pClient SLC

3. Code Example

An STM32WBA Security Level Characteristic code example (peripheral and central) is available from the STM32-Hotspot GitHub.