Bluetooth has been around now for 20 years; over that time the number of security features in the specification has greatly increased, but many of those security features are optional and may or may not be implemented by device vendors. This talk gives a brief overview of the evolution of Bluetooth security features, then going into detail on how the security characteristics of a Bluetooth connection are determined, depending on which features are implemented by the vendors of the devices involved. We conclude by covering the questions that vendors need to answer, to determine whether the security of their device is adequate for its intended purpose.

1. Bluetooth Security:
Where are we now and how did we get here?
Craig Heath
@heathcr
29 Oct 2019 DC4420

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Bluetooth Security: Topics
29 Oct 2019 2
 (very) brief history of Bluetooth
 (very) high-level architecture overview
 Security features by version
 How device security capabilities interact
 What vendors should tell you, but don’t
 What security-sensitive users should do

3. © Franklin Heath Ltd c b CC BY 4.0
Bluetooth Core Specification History
29 Oct 2019 3
Year Version Main Features Pages
1999 1.0 Basic Rate (BR) 1082
2001 1.1 interoperability fixes 1084
2003 1.2 frequency hopping 1200
2004 2.0 Enhanced Data Rate (EDR) 1230
2007 2.1 Secure Simple Pairing (SSP) 1420
2009 3.0 High Speed (HS / AMP) 1712
2010 4.0 Low Energy (LE) 2302
2013 4.1 LTE-friendly, IoT enhancements 2684
2014 4.2 Low power IPv6 support 2772
2017 5.0 Improved range/speed, connectionless beacons 2822
2019 5.1 Indoor positioning, power optimisation 2985

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Bluetooth Architecture Overview
29 Oct 2019 4
HCI
HostAudio
L2CAP
BNEP
RFCOMM
SDP
OBEX
PAN
Applications
TCSAVCTP
GATT
SMP
OPP
Other BR/EDR
profiles …
A2DP,
AVCRP
LE profiles …
Baseband
Radio
Link Controller
Link Manager (LMP)
Controller
ATT
Generic
Access Profile
Generic
protocols
Application
profiles
SPP
Application
Presentation
Session
Transport
Network
Data Link
Physical
Software/Hardware Component View Network Layer View
(approximate)

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Security Features of Bluetooth Versions
29 Oct 2019 5
Version Security Enhancements
<= 2.0 BR/EDR Security Modes 1, 2 and 3
2.1 Secure Simple Pairing (SSP) + Security Mode 4
3.0 no significant changes
4.0 LE Security Modes 1 and 2
4.1 BR/EDR “Secure Connections”
4.2 LE Secure Connections, LE Privacy
5.0 no significant changes
5.1 no significant changes

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How Security Modes and Levels are Determined
29 Oct 2019 6
1. A pairing “association model” is chosen based on the
capabilities of the 2 devices
 Out-of-Band authentication tokens
 I/O capabilities (e.g. keyboard input)
2. Either or both of the devices may request Secure
Connections and/or Man-in-the-Middle protection
3. The key establishment protocol is negotiated
4. A persistent shared secret link key is then established for
the pair of devices

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Pairing Association Models
29 Oct 2019 7
 legacy PIN
 Just Works
 Passkey Entry
 Numeric Comparison
 Out-of-Band

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Bluetooth Security Algorithms
29 Oct 2019 8
BR/EDR Versions LE Versions
1.0 2.1 4.1 4.0 4.2
(legacy)
Secure Simple
Pairing
Secure
Connections
LE Legacy
Pairing
LE Secure
Connections
Key Exchange
E21/E22
(SAFER+)
ECDH P-192,
HMAC-SHA-256
ECDH P-256,
HMAC-SHA-256
AES-128
ECDH P-256,
AES-CMAC
Authentication E1 (SAFER+) HMAC-SHA-256 AES-CCM
Encryption E0 (Massey-Rueppel) AES-CCM

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Security Modes After Pairing
29 Oct 2019 9
BR/EDR
Versions
BR/EDR
Security Modes
LE
Versions
LE Security Modes
legacy (PIN) All Mode 2 or 3 None -
Just Works 2.1+
Mode 4
Levels 0, 1 or 2
4.0+
Mode 1 Level 1 or
2, or Mode 2 Level
1
Passkey
Entry
2.1+
Mode 4
all levels*
4.0+
Mode 1 or Mode 2
all levels*
Out of Band 2.1+
Mode 4
all levels*
4.0+
Mode 1 or Mode 2
all levels*
Numeric
Comparison
2.1+
Mode 4
all levels*
4.2+
Mode 1 or Mode 2
all levels*
* BR/EDR Mode 4 Level 4 and LE Mode 1 Level 4 are only available if both devices
support Secure Connections

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BR/EDR Security Mode 4 Levels
29 Oct 2019 10
Mode 4
Security Level
Man-in-the-
Middle Protection
User Interaction
During Pairing
Data
Confidentiality
0 No No No
1 No Minimal No
2 No Minimal Standard
3 Yes Yes Standard
4 Yes Yes Strong

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LE Security Modes and Levels
29 Oct 2019 11
Bluetooth LE Security
Mode and Level
Authenticated
Pairing
Data
Integrity
Data
Confidentiality
Mode 1 Level 1 No No No
Mode 2 Level 1 No Yes No
Mode 1 Level 2 No Yes Standard
Mode 2 Level 2 Yes Yes No
Mode 1 Level 3 Yes Yes Standard
Mode 1 Level 4 Yes Yes Strong

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Bluetooth Secure Simple Pairing Association Models
29 Oct 2019 12
Table
1A
Initiator: OOB flag
Yes No
Responder:
OOBflag
Yes
OOB OOB
No
OOB
Table
1B
Table
1B
Initiator: MITM flag
Yes No
Responder:
MITMflag
Yes
Table
1C
Table
1C
No
Table
1C
Just
Works
Table
1C
Initiator IO Capability
KeyboardOnly
DisplayYesNo
DisplayOnly
NoInputNoOutput
ResponderIOCapability
KeyboardOnly
Passkey
Entry
Passkey
Entry
Passkey
Entry
Just
Works
DisplayYesNo
Passkey
Entry
Numeric
Compar­-
ison
Just
Works
Just
Works
DisplayOnly
Passkey
Entry
Just
Works
Just
Works
Just
Works
NoInputNoOutput
Just
Works
Just
Works
Just
Works
Just
Works

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Bluetooth LE Pairing Association Models
29 Oct 2019 13
Table
2A
Initiator: OOB flag
On Off
SC flag SC flag
On Off On Off
Responder:OOBflag
On
SCflag
On
OOB OOB OOB
Table
2B
Off
OOB OOB
Table
2B
Table
2B
Off
SCflag
On
OOB
Table
2B
Table
2B
Table
2B
Off
Table
2B
Table
2B
Table
2B
Table
2B
Table
2B
Initiator:
MITM flag
On Off
Responder:
MITMflag
On
Table
2C
Table
2C
Off
Table
2C
Just
Works
Table
2C
Initiator: IO Capability
KeyboardOnly
KeyboardDisplay DisplayYesNo
DisplayOnly
NoInputNoOutput
SC flag SC flag
On Off On Off
Responder:IOCapability
KeyboardOnly Passkey
Entry
Passkey
Entry
Passkey
Entry
Passkey
Entry
Passkey
Entry
Passkey
Entry
Just
Works
KeyboardDisplay
SCflag
On
Passkey
Entry
Numeric
Compar-
ison
Passkey
Entry
Numeric
Compar-
ison
Passkey
Entry
Passkey
Entry
Just
Works
Off
Passkey
Entry
Passkey
Entry
Passkey
Entry
Passkey
Entry
Passkey
Entry
Passkey
Entry
Just
Works
DisplayYesNo
SCflag
On
Passkey
Entry
Numeric
Compar-
ison
Passkey
Entry
Numeric
Compar-
ison
Just
Works
Just
Works
Just
Works
Off
Passkey
Entry
Passkey
Entry
Passkey
Entry
Just
Works
Just
Works
Just
Works
Just
Works
DisplayOnly Passkey
Entry
Passkey
Entry
Passkey
Entry
Just
Works
Just
Works
Just
Works
Just
Works
NoInputNoOutput
Just
Works
Just
Works
Just
Works
Just
Works
Just
Works
Just
Works
Just
Works

14. © Franklin Heath Ltd c b CC BY 4.0
What does “Qualification” Mean?
29 Oct 2019 14
 What Bluetooth SIG conformance testing is called
 Conformance test suites
 Implementation eXtra Information for Testing (IXIT)
 Implementation Conformance Statement (ICS)
 Public listing
 Older specification versions are deprecated, and as of this
year are starting to be completely withdrawn

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Questions Bluetooth Device Vendors Should Answer
29 Oct 2019 15
 Is it qualified against a Bluetooth Core Specification version of 4.2 or
better?
 Does it include the optional Secure Connections feature?
 (If LE or dual-mode) Does it include the optional LE Privacy feature?
 Does it have the “Secure Connections Only” mode? How selected?
 If not, what minimum security attributes are implemented for each
Bluetooth service offered?
 What minimum effective encryption key length is enforced?
 Is there a maximum effective encryption key length enforced?
 Does it request Man-in-the-Middle protection during pairing?

16. © Franklin Heath Ltd c b CC BY 4.0
What Else Can Security-Sensitive Device Admins Do?
29 Oct 2019 16
 Devices *and accessories* must be maintained with regular security
patches.
 When connecting two devices (or a device and an accessory), at least
one of the pair must have Secure Connections Only mode turned on.
 Tethering of devices using the Bluetooth PAN profile should be
discouraged.
 Users of Bluetooth BR/EDR accessories should be made aware that they
may be exposing trackable device identification.
 If it *really* matters: Vendor claims should be validated by independent
testing to verify that appropriate security modes, levels and key lengths
are being used in practice.

17. © Franklin Heath Ltd c b CC BY 4.0
Thank You!
29 Oct 2019
craig@franklinheath.co.uk
@heathcr
@franklinheath
17