PM0237 datasheet(21/88 Pages) STMICROELECTRONICS

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DocID027104 Rev 2

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PM0237

Bluetooth low energy technology

The selection between PassKey and Just Works method is done based on the following

table:

Phase 3: transport specific key distribution

•

Once the Phase 2 is completed, up to three 128-bit keys can be distributed by

messages encrypted with the STK key:

Long-term key (LTK): it is used to generate the 128-bit key used for Link Layer

encryption and authentication;

Connection signature resolving key (CSRK): it is used for the data signing

performed at the ATT layer;

Identity resolving key (IRK): it is used to generate a private address on the basis of

a device public address.

When the established encryption keys are stored in order to be used for future

authentication, the devices are bonded.

Another security mechanism supported from BLE is the use of private addresses. A private

address is generated by encrypting the public address of the device. This private address

can be resolved by a trusted device that has been provided with the corresponding

encryption key.This allows the device to use a private address for a more secure

communication and to change it frequently (only devices with the related IRK are able to

recognize it).

It is also possible to transmit authenticated data over an unencrypted Link Layer connection

by using the CSRK key: a 12-byte signature is placed after the data payload at the ATT

layer.

The signature algorithm also uses a counter which allows to provide protection against

replay attacks (an external device which can simply capture some packets and send them

later as they are without any understanding of packet content: the receiver device simply

checks the packet counter and discards it since its frame counter is less than the latest

received good packet).

Table 11. Methods used for calculating the Temporary Key (TK)

Display only

Display

Yes/No

Keyboard

only

No Input No

Output

Keyboard

display

Display

Only

Just Works

Passkey Entry

Just Works

Passkey Entry

Display

Yes/No

Just Works

Passkey Entry

Just Works

Passkey Entry

Keyboard

Only

Passkey Entry

Passkey Entry Passkey Entry

Just Works

Passkey Entry

No Input No

Output

Just Works

Keyboard

Display

Passkey Entry

Passkey Entry Passkey Entry

Just Works

Passkey Entry

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