PM0237 datasheet(20/88 Pages) STMICROELECTRONICS

20 / 88 page

Bluetooth low energy technology

PM0237

20/88

DocID027104 Rev 2

1.7

Security Manager (SM)

The Bluetooth low energy link layer supports encryption and authentication by using the

Cipher Block Chaining-Message Authentication Code (CCM) algorithm and a 128-bit AES

block cipher. When encryption and authentication are used in a connection, a 4-byte

Message Integrity Check (MIC) is appended to the payload of the data channel PDU.

Encryption is applied to both the PDU payload and MIC fields.

When two devices want to encrypt the communication during the connection, the Security

Manager uses the pairing procedure. This procedure allows to authenticate two devices

and creates a common link key that can be used as a basis for a trusted relationship or a

(single) secure connection.

Pairing procedure is a three-phase process.

Phase 1: pairing feature exchange

•

The two connected devices communicates their input/output capabilities by using the

Pairing request message. This message also contains a bit stating if out-of-band data

is available and the authentication requirements.

•

There are three input capabilities:

no input;

the ability to select yes/no;

the ability to input a number by using the keyboard.

•

There are two output capabilities:

–

No output;

–

Numeric output: ability to display a six-digit number

The information exchanged in Phase 1 is used to select which STK generation method is

used in Phase 2.

Phase 2: short term key (STK) generation:

•

The pairing devices first define a Temporary Key (TK), by using one of the following

methods.

The out-of-band (OOB) method which uses out of band communication (example:

NFC) for the TK agreement (it is selected if the out-of-band bit is set);

Passkey Entry method: user passes six numeric digits as the TK between the

devices;

Just Works: this method is not authenticated, and it does not provide any

protection against man-in-the-middle (MITM) attacks.

Table 10. Combination of Input/Output capabilities on a BLE device

No output

Display

No input

No Input No output

Display Only

Yes/No

No Input No output

Display Yes/No

Keyboard

Keyboard only

Keyboard Display

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