Message Authentication Code

Message streams are used to configure Audio switch, see Audio switch messages. For these important configurations, the Provider needs to ensure that the message is sent by GMSCore (Fast Pair module) and not any other app on the Seeker.

Generate MAC (message authentication code)

FP Seeker adds a message authentication code for device configuration messages using HMAC-SHA256. The MAC of the message consists of the first 8 bytes of:

 sha256(concat((K ^ opad), sha256(concat((K ^ ipad), concat(nonce, message)))))


  1. K is generated by concat(account key, 48-byte ZEROs).
  2. message is the additional data of Message stream.
  3. nonce is generated by concat(session_nonce, message_nonce); session nonce and message nonce are defined in the following section.
  4. opad is 64 bytes of outer padding, consisting of repeated bytes valued 0x5C.
  5. ipad is 64 bytes of inner padding, consisting of repeated bytes valued 0x36.

Session nonce and message nonce

To prevent a replay attack, the Provider needs to ensure that a nonce is not repeated. Since maintaining clock or counter synchronization on both Provider and Seeker is not straightforward, the Provider generates the session nonce (per connection), which is shared with all messages during the connection, while the Seeker generates the message nonce (per message), which is randomly generated for each message. The nonce for generating the MAC of each message is the combination of session nonce and message nonce, i.e. concat(session_nonce, message_nonce).

We add a session nonce to the Device information event group:

Message Group Name Value
Device information event 0x03
Message Code Name Value
Session nonce 0x0A

The session nonce should be generated and sent to the Seeker when RFCOMM connects:

Octet Data Type Description Value
0 uint8 Device information event 0x03
1 uint8 Session nonce 0x0A
2 - 3 uint16 Additional data length 0x0008
4 - 11 session nonce varies

To send a message when a MAC is required, the Seeker will send a message nonce and the MAC together with the message.

Octet Data Type Description Value
0 uint8 Message group varies
1 uint8 Message code varies
2 - 3 uint16 Additional data length(the additional data length + 16) varies
4 - n Additional data varies
n + 1 - n + 8 Message nonce varies
n + 9 - n + 16 Message authentication code varies

Verify MAC (message authentication code)

Upon receiving a message with the message authentication code, the Provider shall verify it by using the same function as the generating function. That is, the received MAC should be equal to the first 8 bytes of

 sha256(concat((K ^ opad), sha256(concat((K ^ ipad), concat(section_nonce, message_nonce, message)))))


  1. K is generated by concat(account key, 48-byte ZEROs), and the Provider shall traverse all stored account keys to verify the MAC.
  2. message is the additional data (excluding message nonce and MAC) of the Message stream.

If the MAC is correct, then the Provider shall follow the instruction of the message. Otherwise, the Provider shall send a NAK with the error reason, 0x3 - not allowed due to incorrect message authentication code.