GAP Bond Manager and LE Secure Connections

The GAP Bond Manager (GAPBondMgr) is a configurable module that offloads most of the Pairing & Bonding security mechanisms associated with the Security Manager (SM) protocol from the application. The GAPBongMgr executes in the protocol stack task’s context. Table 10. lists the terminology.

Table 10. GAP Bond Manager Terminology

Term	Description
Pairing	The process of generating & exchanging keys. Not to be confused with forming or establishing a BLE connection between two devices
Encryption	Data is encrypted after pairing, or re-encryption (a subsequent connection where keys are looked up from nonvolatile memory).
Association	The pairing method used based on the I/O Capabilities of both devices. For LE devices, Just Works, Numeric Comparison, Passkey Entry and Out-Of-Band are supported.
Authentication	The pairing process using an association method that supports MITM (Man in the Middle) protection.
Bonding	Storing the keys generated during the pairing process in nonvolatile memory to use for the next encryption sequence.
Authorization	An additional application level verification in addition to authentication.
OOB	Out of Band. Pairing keys are not exchanged over the air, but rather over some other source such as serial port or NFC. This also provides MITM protection.
MITM	Man in the Middle protection. MITM provides authentication during the pairing process which helps prevent a malicious attacker from impersonating the peer device during the key exchange.
Just Works	Unauthenticated pairing association method where keys are exchanged without MITM protection.

The general process that the GAPBondMgr uses is as follows:

1. The pairing process: exchange keys through the methods described in Selection of Pairing Method.

2. The encryption process: Encrypt the link using keys Step 1.

3. The bonding process: store keys in secure flash (Simple Non Volatile memory, SNV).

4. Reconnecting: Use the keys stored in SNV to encrypt the link.

Tip

Performing all of these steps is not necessary. For example, two devices may choose to pair but not bond.

Selection of Pairing Method

Bluetooth Core Specification Version 4.2 added support for the LE Secure Connections feature to enable additional strength to the BLE pairing process. For a detailed description of the algorithms used for LE Secure Connections, see the Security Architecture section of the Bluetooth Core Specification Version 5.1. The previous pairing methods used in the Bluetooth Core Specification Versions 4.1 and 4.0 are still available, and are now defined as LE Legacy Pairing. The main difference is that Secure Connection uses Elliptic Curve Diffie-Hellman (ECDH) cryptography, while LE Legacy Pairing does not.

There are four types of pairing methods which are referred to as “Association Models” in the core specification. Each pairing method is described in detail in GAPBondMgr Examples for Different Pairing Methods.

• Just Works (LE Secure Connections or LE Legacy)
• Passkey Entry (LE Secure Connections or LE Legacy)
• Numeric Comparison (LE Secure Connections)
• Out Of Band (LE Secure Connections or LE Legacy)

Which pairing method is selected, and whether or not pairing will succeed depends on the following parameters from both peer devices during the pairing process:

• Out Of Band (OOB) set / not set
• Man In The Middle (MITM) set / not set
• Input/Output (IO) Capabilities
• LE Secure Connections supported / not supported

The GAPBondMgr parameters, as they map to the table parameters below are listed here. For more information on these parameters, see BLE Stack API Reference (GAPBondMgr section).

• GAPBOND_LOCAL_OOB_SC_ENABLED: Out of band (OOB) set / not set

• GAPBOND_MITM_PROTECTION: Man in the middle (MITM) set / not set

• GAPBOND_IO_CAPABILITIES: In/out (IO) Capabilities

• GAPBOND_SECURE_CONNECTION: Secure connections supported / not supported

  Beyond what the Bluetooth Core Specification Version 5.1 defines, this parameter also affects whether or not pairing succeeds, as described in BLE Stack API Reference (GAPBondMgr section).

The tables below are from the Selecting Key Generation Method section ([Vol 3], Part H, Section 2.3.5.1) of the Bluetooth Core Specification Version 5.1. Use these tables to determine which pairing mode is selected for any set of parameters.

If both devices support LE Secure Connections, use Figure 55. to decide upon the next step.

Figure 55. GAPBondMgr parameters when LE Secure Connections is supported by both devices.

If at least one device does not support LE Secure Connections, use Figure 56. to decide upon the next step.

Figure 56. GAPBondMgr parameters when LE Secure Connections is not supported by one or both devices.

If, based on one of the previous tables, IO capabilities are to be used to determine the association model, use Figure 57.

Figure 57. GAPBondMgr parameters with IO capabilities

Using GAPBondMgr

This section describes what the application must do to configure, start, and use the GAPBondMgr. The GAPRole handles some of the GAPBondMgr functionality. The GAPBondMgr is defined in gapbondmgr.c and gapbondmgr.h. BLE Stack API Reference (GAPBondMgr section) describes the full API including commands, configurable parameters, events, and callbacks.

The general steps to use the GAPBondMgr module are as follows:

1. Configure the stack to include GAPBondMgr functionality by defining the following in build_config.opt in the stack project:

• -DGAP_BOND_MGR

2. The stack must also be configured to use 1 or 2 SNV pages, by defining OSAL_SNV=1 or OSAL_SNV=2 as a preprocessor-defined symbol in the stack project.

3. If using LE Secure Connections, the PDU size must be >= 69. This can be set by defining the following preprocessor symbol in the application project: MAX_PDU_SIZE=69. Also, the minimum heap size that can be used with LE Secure Connections is 3690. (See Dynamic Memory Allocation for heap size management.)

4. Configure the GAPBondMgr by initializing its parameters as desired. See BLE Stack API Reference (GAPBondMgr section) for a complete list of parameters with functionality described. There are examples of this for the various pairing/bonding modes in GAPBondMgr Examples for Different Pairing Methods.

5. Register application callbacks with the GAPBondMgr, so that the application can communicate with the GAPBondMgr and be notified of events.

// Register with bond manager after starting device
GAPBondMgr_Register(&bondmanager_callbacks);

Here bondmanager_callbacks is defined as a structure containing the GAPBondMgr Callbacks. A passcode callback function is mandatory.

// Bond Manager Callbacks
static gapBondCBs_t SimpleBLECentral_bondCB =
{
    (pfnPasscodeCB_t)SimpleBLECentral_passcodeCB, // Passcode callback
    SimpleBLECentral_pairStateCB                  // Pairing / Bonding state Callback
};

6. Once the GAPBondMgr is configured, it operates mostly autonomously from the perspective of the application. When a connection is established, the GAPBondMgr manages pairing and bonding depending on the configuration parameters set during initialization. It also communicates with the application as needed through the defined callbacks.

A few parameters can be set and functions can be called asynchronously at any time from the application. See BLE Stack API Reference (GAPBondMgr section) for more information.

Most communication between the GAPBondMgr and the application at this point occurs through the callbacks which were registered in Step 5. Figure 58. is a flow diagram example of the GAPBondMgr notifying the application that pairing has been completed. The same method occurs for various other events and will be expanded upon in the following section.

Figure 58. GapBondMgr Callback Example.

GAPBondMgr Examples for Different Pairing Methods

This section provides message diagrams for the types of security that can be implemented. These modes assume acceptable I/O capabilities are available for the security mode, and that the selection of whether or not to support LE Secure Connections allows for the pairing method. See the Selection of Pairing Method on how these parameters affect pairing. These examples only consider the pairing aspect. Bonding can be added to each type of pairing in the same manner and is shown in the next section.

Caution

The code snippets here are not complete functioning examples, and are only intended for illustration purposes.

Disabling Pairing

With pairing mode set to GAPBOND_PAIRING_MODE_NO_PAIRING, the BLE stack automatically rejects any attempt at pairing. Configure the GAPBondMgr as follows to disable pairing:

// Pairing is not allowed
uint8_t pairMode = GAPBOND_PAIRING_MODE_NO_PAIRING;
GAPBondMgr_SetParameter(GAPBOND_PAIRING_MODE, sizeof(uint8_t), &pairMode);

Enabling Pairing

To start or allow the pairing process after a connection is formed, the GAPBondMgr can be configured to automatically request pairing or wait for pairing request from the peer device. The actual behavior depends on the device’s GAP role (Central or Peripheral) and the setting of the GAPBondMgr pairing mode (GAPBOND_PAIRING_MODE).

To initiate the pairing process on Peripheral role devices, GAPBOND_PAIRING_MODE_INITIATE will send a Slave Security Request shortly after the GAPBondMgr is informed that the connection is formed. For Central role devices, GAPBOND_PAIRING_MODE_INITIATE will send a Pairing Request or request the Link Layer to encrypt the link if the device has previously paired/bonded:

// Initiate pairing request
uint8_t pairMode = GAPBOND_PAIRING_MODE_INITIATE;
GAPBondMgr_SetParameter(GAPBOND_PAIRING_MODE, sizeof(uint8_t), &pairMode);

The Peripheral can be configured to wait for a Pairing Request from the Central when the pairing mode is set to GAPBOND_PAIRING_MODE_WAIT_FOR_REQ. When this pairing mode is selected, the GAPBondMgr will automatically respond with a Pairing Response based on other GAPBondMgr configured parameters.

// Wait for a pairing request
uint8_t pairMode = GAPBOND_PAIRING_MODE_WAIT_FOR_REQ;
GAPBondMgr_SetParameter(GAPBOND_PAIRING_MODE, sizeof(uint8_t), &pairMode);

Tip

When pairing with smartphone Central devices, it is recommended to use GAPBOND_PAIRING_MODE_WAIT_FOR_REQ as undefined behavior may occur when a Slave Security Request is sent by the Peripheral. Both iOS and Android will initiate pairing when the peripheral responds with an Insufficient Authentication error response when a GATT secure characteristic is accessed.

LE Secure Connections

LE Secure Connections (LESC) is enabled by default in BLE-Stack. If you don’t want to use LE Secure Connections, set the GAPBOND_SECURE_CONNECTION variable to GAPBOND_SECURE_CONNECTION_NONE during the GAPBondMgr initialization.

uint8_t gapbondSecure = GAPBOND_SECURE_CONNECTION_NONE;
GAPBondMgr_SetParameter(GAPBOND_SECURE_CONNECTION, sizeof(uint8_t), &gapbondSecure);

It is important when trying to decipher over-the-air sniffer logs with Secure Connections enabled, you need to use a specific “debug” key as defined by Vol 3 Part H section 2.3.5.6.1 of the Bluetooth Core Specification Version 5.1. In the BLE-Stack, this key is enabled when the GAPBondMgr has SC_HOST_DEBUG defined in the stack project. When either the initiating or non-initiating device uses this specific debug key, it enables over-the-air sniffer equipment that supports LE Secure Connections to determine the LTK and therefore monitor/decrypt encrypted traffic throughout the connection.

LESC Limitations & Recommendations

LE Secure Connections uses an ECDH public-private key pair as part of the pairing process. See Selection of Pairing Method and the LE Secure Connections Pairing Phase 2 section of the Bluetooth Core Specification Version 5.1 for more information about how these keys and how they are used in the pairing process.

To summarize, as part of LESC pairing Phase 1 each device will generate its own ECDH public-private key pair. As part of LESC pairing Phase 2 each device will compute the Diffie-Hellman (DH) Key based on the public keys that are exchanged.

ECC public-private and DH key generation is implemented in the ROM software on the CC2640R2F. These routines are blocking by nature and take ~160ms to execute. During LESC pairing, the ECC ROM will be accessed twice by the link layer (LL) once to generate the public/private key pair and again to generate the DH key. This means the user application and other stack OSAL tasks will be blocked for 160ms twice during pairing.

Because of this, TI recommends the following when using LESC:

• Supervision timeout >= 165ms
• Expect/design for at least 160ms of blocking during LESC pairing

To alleviate the amount of blocking required, the user application can generate the public-private key pair ahead of the pairing process, or it can define when the keys should be recycled by using the following parameters of the GapBondMgr. These options are mutually exclusive, as generation of keys by the application bypasses the recycle parameter.

• GAPBOND_ECC_KEYS
• GAPBOND_ECCKEY_REGEN_POLICY

Just Works Pairing

Just Works pairing allows encryption without man in the middle (MITM) authentication and is vulnerable to MITM attacks. Just Works pairing can be LE Legacy or LE Secure Connections pairing. The GAPBondMgr does not need any additional input from the application for Just Works pairing. Configure the GAPBondMgr for Just Works pairing as follows.

uint8_t mitm = FALSE;
GAPBondMgr_SetParameter( GAPBOND_MITM_PROTECTION, sizeof (uint8_t), &mitm);

Figure 59. describes the interaction between the GAPBondMgr and the application for Just Works pairing. As shown, the application receives a GAPBOND_PAIRING_STATE_STARTED event once the pairing request has been sent, and a GAPBOND_PAIRING_STATE_COMPLETE event once the pairing process has been completed. At this time, the link is encrypted.

Figure 59. Just Works Pairing.

Passcode Entry

Passkey entry is a type of authenticated pairing that can prevent man in the middle (MITM) attacks. It can be used with either LE Legacy pairing or Secure Connections pairing. In this pairing method, one device displays a 6-digit passcode, and the other device enters the passcode. As described in Selection of Pairing Method, the IO capabilities decide which device performs which role. The passcode callback registered with the GAPBondMgr when it was started is used to enter or display the passcode. The following is an example of initiating Passcode Entry pairing where the passcode is displayed.

1. Define passcode callback

Listing 58. Define passcode callback.

// Bond Manager Callbacks
  static gapBondCBs_t SimpleBLECentral_bondCB =
  {
      (pfnPasscodeCB_t)SimpleBLECentral_passcodeCB, // Passcode callback
      SimpleBLECentral_pairStateCB                  // Pairing / Bonding state Callback
  };

  /*********************************************************************
  * @fn      SimpleBLECentral_passcodeCB
  *
  * @brief   Passcode callback.
  *
  * @return  none
  */
  static void SimpleBLECentral_passcodeCB(uint8_t *deviceAddr, uint16_t connHandle, uint8_t uiInputs, uint8_t uiOutputs)
  {
      uint8_t *pData;

      // Allocate space for the passcode event.
      if ((pData = ICall_malloc(sizeof(uiOutputs))))
      {
          *pData = uiOutputs;

          // Enqueue the event.
          SimpleBLECentral_enqueueMsg(SBC_PASSCODE_NEEDED_EVT, 0, pData);
      }
  }

2. Configure GAPBondMgr

Listing 59. Configure GAPBondMgr for MITM.

uint8_t pairMode = GAPBOND_PAIRING_MODE_INITIATE;
uint8_t mitm = TRUE;
GAPBondMgr_SetParameter(GAPBOND_PAIRING_MODE, sizeof(uint8_t), &uint8_t pairMode = GAPBOND_PAIRING_MODE_INITIATE;
GAPBondMgr_SetParameter(GAPBOND_PAIRING_MODE, sizeof(uint8_t), &mitm);

3. Process passcode callback and send the response to the stack.

Listing 60. Process passcode and send the response.

//! BLE Default Passcode
#define B_APP_DEFAULT_PASSCODE                    123456

static void SimpleBLECentral_processPasscode(uint16_t connectionHandle, uint8_t uiOutputs)
{
  // This app uses a default passcode. A real-life scenario would handle all
  // pairing scenarios and likely generate this randomly.
  uint32_t passcode = B_APP_DEFAULT_PASSCODE;

  // Display passcode to user
  if (uiOutputs != 0)
  {
      Display_print1(dispHandle, 4, 0, "Passcode: %d", passcode);
  }

  // Send passcode response
  GAPBondMgr_PasscodeRsp(connectionHandle, SUCCESS, passcode);
}

Depending on the uiInputs and uiOutputs returned from the GAPBondMgr, the passcode must either be displayed or entered. The passcode is then sent to the GAPBondMgr using GAPBondMgr_PasscodeRsp(), so that pairing can continue. In this case, the password is statically set to 123456. In a real product, the password will likely be randomly generated, and the device must expose a way for the user to enter the passcode, then send it to the GAPBondMgr using GAPBondMgr_PasscodeRsp(). An example interaction between the GAPBondMgr and the application is shown in Figure 60..

Figure 60. Interaction Between the GAPBondMgr and the Application when exchanging a passcode.

Numeric Comparison

Numeric comparison is a type of authenticated pairing that protects from MITM attacks. It is only possible as a LE Secure Connections pairing; not LE legacy. For numeric comparison pairing, both devices display a 6-digit code. Each device must then indicate, through a button press or some other yes-no input, whether the codes match. The passcode callback registered with the GAPBondMgr when it was started is used to display the 6-digit code. The following is an example of initiating Numeric Comparison pairing where the passcode is displayed. The IO capabilities must be set appropriately to select numeric comparison (that is, Display/Yes-No on both sides).

1. Define passcode callback to display code.

Listing 61. Define passcode callback to display code.

// Bond Manager Callbacks
static gapBondCBs_t SimpleBLECentral_bondCB =
{
  (pfnPasscodeCB_t)SimpleBLECentral_passcodeCB, //Passcode callback
  SimpleBLECentral_pairStateCB                  //Pairing state callback
};

static void SimpleBLECentral_passcodeCB (uint8_t *deviceAddr, uint16_t connHandle, uint8_t uiInputs, uint8_t uiOutputs, uint32_t numComparison)
{
  gapPasskeyNeededEvent_t *pData;

  // Allocate space for the passcode event.
  if ((pData = ICall_malloc(sizeof(gapPasskeyNeededEvent_t))))
  {
    memcpy(pData->deviceAddr, deviceAddr, B_ADDR_LEN);
    pData->connectionHandle = connHandle;
    pData->numComparison = numComparison;

    // Enqueue the event.
    SimpleBLECentral_enqueueMsg(SEC_PASSCODE_NEEDED_EVT, 0, (uint8_t *) pData);

2. Configure GAPBondMgr

Listing 62. Configure GAPBondMgr For Secure Connections + Authentication.

uint8_t pairMode = GAPBOND_PAIRING_MODE_WAIT_FOR_REQ;
uint8_t scMode = GAPBOND_SECURE_CONNECTION_ONLY;
uint8_t mitm = TRUE;
uint8_t ioCap = GAPBOND_IO_CAP_DISPLAY_YES_NO;

GAPBondMgr_SetParameter(GAPBOND_IO_CAPABILITIES, sizeof(uint8_t), &ioCap);
GAPBondMgr_SetParameter(GAPBOND_PAIRING_MODE, sizeof(uint8_t), &pairMode);
GAPBondMgr_SetParameter(GAPBOND_MITM_PROTECTION, sizeof(uint8_t), &mitm);
GAPBondMgr_SetParameter(GAPBOND_SECURE_CONNECTION, sizeof(uint8_t), &scMode);

3. Process passcode callback and display code.

Listing 63. Process passcode callback and display code.

static void SimpleBLECentral_processPasscode (uint16_t connectionHandle, gapPasskeyNeededEvent_t *pData)
{

  if (pData->numComparison) //numeric comparison
  {

    //Display passcode
    DISPLAY_WRITE_STRING_VALUE("Num Cmp: %d", pData->numComparison, LCD_PAGE4);
  }
}

4. Accept Yes-No input from user and send response to GAPBondMgr.

Listing 64. Accept Yes-No input from user and send response to GAPBondMgr.

if (keys & KEY_RIGHT)
{
  GAPBondMgr_PasscodeRsp(connHandle, SUCCESS, TRUE);
  DISPLAY_WRITE_STRING("Codes Match!", LCD_PAGE5);
  return;
}

In this case, the third parameter of GAPBondMgr_PasscodeRsp, which usually accepts a passcode, is overloaded to send TRUE to the stack to indicate that the codes match and to continue with pairing. The process of numeric comparison is illustrated in Figure 61..

Figure 61. Numeric Comparison.

GAPBondMgr Example With Bonding Enabled

Bonding can enabled or disabled for any type of pairing through the GAPBOND_BONDING_ENABLED parameter, and occurs after the pairing process is complete. To enable bonding, configure the GAPBondMgr as follows:

uint8_t bonding = TRUE;
GAPBondMgr_SetParameter(GAPBOND_BONDING_ENABLED, sizeof(uint8_t), &bonding);

With bonding enabled, the GAPBondMgr stores the long-term key transferred during the pairing process to SNV. See GAPBondMgr and SNV for more information. After this is completed, the application is notified through the GAPBOND_PAIRING_STATE_COMPLETE event. GAPBOND_PAIRING_STATE_BOND_SAVED is only passed to the application pair state callback when initially connecting, pairing, and bonding. For future connections to a bonded device, the security keys are loaded from flash, thus skipping the pairing process. In this case, only GAPBOND_PAIRING_STATE_BONDED is passed to the application pair state callback. This is illustrated in Figure 62.

Figure 62. GAPBondMgr Example With Bonding Enabled.

GAPBondMgr and SNV

This section describes how the GAPBondMgr uses the SNV flash area for storing bonding information. For more information on SNV itself, see Flash. The amount of bonds that can be stored is set by the GAP_BONDINGS_MAX definition, which is set to 10 by default in gapbondmgr.h. The functionality of the GAPBondMgr when there are no more available bonds varies based on whether the “least recently used” scheme is enabled. See BLE Stack API Reference (GAPBondMgr section) for more information on the GAPBOND_LRU_BOND_REPLACEMENT parameter. If this parameter is set to false, it is not possible to add any more bonds without manually deleting a bond. If the parameter is set to true, the least recently used bond is deleted to make room for the new bond.

The following components comprise one bonding entry:

1. Bond Record: this consists of the peer’s address, address type, privacy reconnection address, and state flags. This comprises 14 bytes and is defined as such:

typedef struct
{
  uint8   publicAddr[B_ADDR_LEN];     // Peer's address
  uint8   publicAddrType;             // Peer's address type
  uint8   reconnectAddr[B_ADDR_LEN];  // Privacy Reconnection Address
  uint8   stateFlags;                 // State flags: @ref GAP_BONDED_STATE_FLAGS
} gapBondRec_t;

2. Client Characteristic Configurations (CCC): the amount of CCCs stored in each entry are set by the GAP_CHAR_CFG_MAX define. This is set to 4 by default. Each CCC is comprised of 4-bytes and is defined as follows:

// Structure of NV data for the connected device's characteristic configuration
typedef struct
{
  uint16 attrHandle;  // attribute handle
  uint8  value;       // attribute value for this device
} gapBondCharCfg_t;

3. Local Long Term Key (LTK) info: this stores the local device’s encryption information. This comprises 28 bytes and is composed as such:

typedef struct
{
  uint8   LTK[KEYLEN];              // Long Term Key (LTK)
  uint16  div;  //lint -e754        // LTK eDiv
  uint8   rand[B_RANDOM_NUM_SIZE];  // LTK random number
  uint8   keySize;                  // LTK key size
} gapBondLTK_t;

4. Connected Device Long Term Key Info: this stores the connected device’s encryption information. This is also a gapBondLTK_t and comprises 28 bytes.
5. Connected Device Identity Resolving Key (IRK): this stores the IRK generated during pairing. This is a 16-byte array.
6. Connected Device Sign Resolving Key (SRK): this stores the SRK generated during pairing. This is a 16-byte array.
7. Connected Device Sign counter: this stores the sign counter generated during pairing. This is a 4-byte word.

Increasing Number of Bonding Entries

The amount of bonds that can be stored is set by the GAP_BONDINGS_MAX definition, which is set to 10 by default in gapbondmgr.h. However, due to the structure of SNV, if a GAP_BONDINGS_MAX value of more than 13 is needed, then there are some additional required changes to support storage of the larger number of bonds:

1. Modify GAP_BONDINGS_MAX to the desired maximum number of bonding entries to store before requiring deletion of old bonds. The value of GAP_BONDINGS_MAX should not exceed 32.
2. In the file bcomdef.h, modify the start and end ranges of Bonding NV Items, GATT Configuration NV Items, and Customer NV Items. This is done by modifying BLE_NVID_GAP_BOND_END, BLE_NVID_GATT_CFG_START, BLE_NVID_GATT_CFG_END, BLE_NVID_CUST_START, and BLE_NVID_CUST_END. The modifications must follow these rules:
   • For GAP: (BLE_NVID_GAP_BOND_END - BLE_NVID_GAP_BOND_START) >= GAP_BONDINGS_MAX*6
   • For GATT: (BLE_NVID_GATT_CFG_END - BLE_NVID_GATT_CFG_START) >= GAP_BONDINGS_MAX.
   • No overlap can exist between any of the ranges.
   • All indexes are 1 Byte values and so should note exceed 0xFF or 255.

Note

Any change in the bonding configuration, such as increasing the max number of bonding entries, must be followed by a full erase of the NV. Since there is no API in the stack for doing this, the erase must be performed using a programming tool such as CCS or Uniflash.