GAP (Advertising and Connections) - Getting Started with Bluetooth Low Energy - page 46

Level 3
Authenticated encryption.
Security mode 2
This mode enforces security by means of data signing (see “Security Manager
(SM)” on page 28 ), and it contains two levels:
Level 1
Unauthenticated data signing.
Level 2
Authenticated data signing.
Each connection starts its lifetime in security mode 1, level 1, and can later be upgraded
to any of the security modes by means of encryption or data signing. It is important to
know that a link can be downgraded from mode 1, level 3, to mode 1, level 2 by switching
encryption keys, but encryption can never be disabled in the lower layers, making it
impossible to go down from security mode 1, level 2.
Security Modes and Procedures
Along with all the modes and procedures detailed in previous sections, GAP also defines
additional ones related to security establishment and enforcement.
In this section, the term mode refers back to the temporary state to
which a device can switch in order to perform a procedure or to allow
a procedure to be performed.
This section briefly describes the security modes and procedures, which complement
and build upon the basis set in “Security Manager (SM)” on page 28 .
Non-bondable mode
A device in this mode does not allow a bonding procedure to take place, although
it can freely permit pairing procedures to execute. In this mode, a device cannot
distribute, accept, or store keys, limiting all security level upgrades to the lifetime
of the connection.
Bondable mode
This mode enables the device to create a bond with a peer, permanently storing
security keys.
Bonding procedure
The central can initiate the bonding procedure described in “Security Proce‐
dures” on page 29 at any time (even if the devices are already bonded, in which cases
new keys would be generated and the old ones would be replaced). However, it is