Bluetooth & BLE · Study deck
Pairing Methods and MITM Protection
A resident pairs a phone with a new door lock while another person waits in the hallway.
Radio Remi is your guide for this deck.
After studying this chapter
Learning objectives
You will be able to:
- Explain the BLE Pairing Protocol: Describe the three-phase pairing sequence and what keys are exchanged in each phase
- Compare Pairing Methods: Analyze Just Works, Passkey Entry, Numeric Comparison, and OOB pairing across security level and I/O requirements
- Diagnose Just Works Vulnerabilities: Justify why TK=0 in legacy Just Works provides no MITM protection and when it is acceptable
- Select Appropriate Pairing: Evaluate device capabilities and threat model to choose and configure the correct pairing method
Major section
Start With the Story
A resident pairs a phone with a new door lock while another person waits in the hallway.
- The important question is whether the resident confirmed the intended lock or merely accepted the first nearby request.
- A secure link cannot repair a weak choice of partner made at the start.
- This test does not protect a device whose screen, code, or stored keys are already under an attacker's control.
Major section
Key Concepts
Numeric Comparison (NC): BLE 4.2+ LESC pairing method displaying a 6-digit number on both devices; user confirms they match, providing MITM protection.
- Passkey Entry Display: One device shows a 6-digit passkey; user enters it on the other device; protects against passive eavesdropping; 1-in-10^6 brute-force probability.
- IO Capability Matrix: How the pairing method is selected based on the IO capabilities of both devices; e.g., DisplayYesNo + KeyboardOnly → Passkey Entry; NoInputNoOutput + any → Just Works.
- LE Secure Connections Confirmation: LESC pairing sends a cryptographic commitment (EC-DAKE protocol) before revealing the confirmation value, preventing the TK=0 attack that breaks legacy Just Works.
Major section
Why Bluetooth Security Matters
Analogy:: Bluetooth security is like locking your front door.
- No security = Leaving door wide open (anyone can walk in).
- Wearables: unencrypted or weakly authenticated links can expose private health or location-adjacent data.
- Medical devices: default or static passkeys weaken the pairing ceremony and can put treatment workflows at risk.
Major section
"Just Works" Pairing: A Critical Security Flaw
Attacker completes "Just Works" pairing (no user verification required).
- BLE's "Just Works" pairing mode provides zero authentication and is vulnerable to Man-in-the-Middle (MITM) attacks.
- Attacker intercepts pairing request before legitimate user connects.
- Attacker gains full access - can read data, send commands, impersonate user.
Major section
Alternative View: Pairing Security as Defense Layers
Think of pairing methods as concentric security layers. "Just Works" is an unlocked door - attackers walk right through.
- Each stronger method (Passkey, Numeric Comparison, OOB) adds a verification barrier that creates progressively more resistance to MITM attackers.
Major section
BLE Pairing Process
The BLE pairing process establishes secure communication through distinct decisions.
- Phase II authenticates according to that method and establishes protection for the current exchange.
- Phase III distributes the keys needed for later encryption, private-address resolution, or signed data when bonding is requested.
Major section
Academic Reference: BLE Numeric Comparison Authentication (University of Edinburgh)
Pairing proceeds only after the user verifies that the displays match.
- An active relay establishing two different key exchanges cannot silently make both authenticated transcripts agree.
Major section
Common Beginner Mistakes
Mistake 1: "My device has Bluetooth, so it's secure".
- Wrong!: Many BLE devices use "Just Works" pairing (no MITM protection during setup).
- Wrong!: If pairing mode is "Just Works", password doesn't help.
- Mistake 3: "Bluetooth only works 10 meters, so I'm safe from hackers".
Major section
Common Misconception: "My Device Has Encryption, So It's Secure"
The misconception: "If BLE link encryption is enabled, my system is secure.".
- The reality:: Link encryption protects confidentiality/integrity after two devices agree on keys.
- The pairing method and your authorization logic determine who gets those keys and what they can do.
- BLE link-layer encryption is AES-CCM with 128-bit keys (you do not "choose AES-256" at the BLE link layer).
Major section
Dig Deeper and Practice
BLE pairing security is a setup ceremony, not just a cipher choice.
- Just Works can encrypt a link but gives no user-authenticated MITM protection.
- LE Secure Connections uses P-256 ECDH to protect against passive key recovery, while Numeric Comparison, Passkey Entry, or OOB evidence is still needed for active MITM resistance.
- Bonding stores LTK, IRK, and CSRK material for reconnection, so stored-key protection and revocation are part of the pairing design.
Deck summary
Key takeaways
A resident pairs a phone with a new door lock while another person waits in the hallway.
- Numeric Comparison (NC): BLE 4.2+ LESC pairing method displaying a 6-digit number on both devices; user confirms they match, providing MITM protection.
- Analogy:: Bluetooth security is like locking your front door.
- Attacker completes "Just Works" pairing (no user verification required).
- Think of pairing methods as concentric security layers. "Just Works" is an unlocked door - attackers walk right through.
Retrieval practice
Recall check 1 of 5
Radio Remi says: answer from memory, then check your reasoning.
Q1A reviewer is checking the Bluetooth security decision for this topic. Which evidence keeps the claim bounded?
Show answer
Answer: C Bluetooth security review should separate pairing, encryption, key management, authorization, and recovery evidence.
Retrieval practice
Recall check 2 of 5
Radio Remi says: answer from memory, then check your reasoning.
Q2A BLE medical glucose monitor has a small display and two physical buttons. Which pairing method should the designer configure, and why?
Show answer
Answer: B With a display and Yes/No buttons, the monitor satisfies the I/O requirements for Numeric Comparison.
Retrieval practice
Recall check 3 of 5
Radio Remi says: answer from memory, then check your reasoning.
Q3What is the primary security weakness of BLE 'Just Works' pairing?
Show answer
Answer: B Just Works has zero authentication bits -- there is no verification step during pairing.
Retrieval practice
Recall check 4 of 5
Radio Remi says: answer from memory, then check your reasoning.
Q4What is the key security trade-off when enabling BLE bonding?
Show answer
Answer: B Bonding saves LTK, IRK, and CSRK for future use.
Retrieval practice
Recall check 5 of 5
Radio Remi says: answer from memory, then check your reasoning.
Q5A BLE smart lock has no display but includes an NFC chip. Which pairing method provides the strongest security?
Show answer
Answer: C OOB pairing via NFC requires the user to physically tap their phone against the lock (within ~4 cm), ensuring physical presence.
Print reference
Answers
Answer key.
- C · Bluetooth security review should separate pairing, encryption, key management, authorization, and recovery evidence.
- B · With a display and Yes/No buttons, the monitor satisfies the I/O requirements for Numeric Comparison.
- B · Just Works has zero authentication bits -- there is no verification step during pairing.
- B · Bonding saves LTK, IRK, and CSRK for future use.
- C · OOB pairing via NFC requires the user to physically tap their phone against the lock (within ~4 cm), ensuring physical presence.