Bluetooth Low Energy GATT Security Levels

Question

I am investigating the types of security available in Bluetooth Low Energy (BLE) related to GATT. More specifically, what kind of operations are done when using gatttool with different security levels specified (low, med, high)? My understanding is that the Security Manager in BLE supports 4 different security properties:

• no pairing
• pairing with an association model that doesn't support man-in-the-middle (MitM) protections (JustWorks)
• pairing with MitM protections (passkey entry, numeric comparison, OOB)
• LE Secure Connections pairing.

Are these security properties related to the security levels specified with gatttool or is there some other security feature I missed while reading the Bluetooth Specification?

Edit: I would like to extend my question in order to clarify the issue. How does the 4.2 Bluetooth stack determine whether to use legacy pairing or not? That is to say, if I have a packet capture of two BLE 4.2 devices pairing, how can I tell whether legacy pairing is being used vs pairing that uses ECDH? Does the Secure Connections flag indicate that legacy pairing should not be used or is it just its own mode that ensures FIPS approved algorithms are used?

Answer 1

You are correct but you forget one main threat in BLE communication. Here are the three basic threats :

Man In The Middle (MITM) :

A MITM requires an attacker to have the ability to both monitor and alter or inject messages into a communication channel

Eavesdropping :

Passive Eavesdropping is secretly listening (by using a sniffing device) to the private communication of others without consent

Privacy/Identity tracking :

Since most of the Bluetooth LE advertisement and data packets have the source addresses of the devices that are sending the data, third-party devices could associate these addresses to the identity of a user and track the user by that address

The quotes come from developer.bluetooth.org.

You already mentioned the protections against MitM and Eavesdropping, however there is still the problem of identity tracking.

The protection against identity tracking is to use a MAC address that cannot be linked to the same device through time, i.e. a MAC address that changes (typically every 15 minutes). There are four types of MAC address :

1. Public address : This address is unencrypted and contains your company unique ID and your device ID. It's unsafe since it does not change through time.
2. Random static address : This address is random (and known as random thanks to flags inside) and unencrypted. Once it does change, you loose the ability to reconnect with the devices that already knows you, you've got to restart the connection from scratch.
3. Random resolvable private address : This address can be resolved by the devices that know its IRK, a shared secret between the devices. As for the static random address it changes often but is always resolvable. It's the most common option since it preserves privacy and allow to restore a connection.
4. Random non-resolvable private address : This address cannot be resolved. The Core Spec doesn't say that much about it and it seems not to be very common. The difference with the static address is that it is not stored since it's a private address (i.e. a device doesn't expect to be able to restore a connection with a private address device).

This is explained in BLE Core Spec 4.2 Vol. 3 Part C 15.1.1 Bluetooth Device Address Types.

Concerning the security level, I don't know gatttool but I will assume it's somehow similar to nRF Connect/Master Control Panel or LightBlue. What you see here is probably the security level associated with each attribute. There are four security levels and they can be different for each attribute :

Mode 1 Level 1 :

No encryption required. The attribute is accessible on a plain-text, non-encrypted connection.

Mode 1 Level 2 :

Unauthenticated encryption required. The connection must be encrypted to access this attribute, but the encryption keys do not need to be authenticated (although they can be).

Mode 1 Level 3 :

Authenticated encryption required. The connection must be encrypted with an authenticated key to access this attribute.

Mode 1 Level 4 :

Authenticated LE Secure Connections pairing with encryption. The connection must be encrypted using the Secure Connection Pairing, which was introduced in Bluetooth LE since version 4.2.

The definitions of modes 1 level 1-3 come from 'Getting Started with Bluetooth Low Energy' by Robert Davidson, Akiba, Carles Cufi, Kevin Townsend.

The device can also be in a mode called Secure Connection Only in which all its services, except the one in Mode 1 Level 1, can only be accessed in Mode 1 Level 4.