Bluetooth & BLE · Study deck
BLE Encryption: Secure Pairing
A sensor with no screen still needs safe pairing.
Radio Remi is your guide for this deck.
After studying this chapter
Learning objectives
You will be able to:
- Test pitfall: using "just works" pairing because the device has no display with a concrete scenario and pass criteria.
- Validate deep dive: key lifecycle, privacy addresses, and signed data evidence with a concrete scenario and pass criteria.
- 'test pitfall: using "just works" pairing because the device has no display with a concrete scenario and pass criteria'
- 'validate deep dive: key lifecycle, privacy addresses, and signed data evidence with a concrete scenario and pass criteria'
Major section
Worked Example: Quantifying BLE Pairing Attack Surfaces
A medical device manufacturer is designing a BLE-connected insulin pump.
- The pump communicates with a companion smartphone app to receive bolus dose commands.
- The design team must choose the appropriate BLE security level.
- Threat Analysis -- What Happens If Pairing is Compromised?: An attacker who performs a MITM during pairing becomes the bonded device.
Major section
Worked Example: Quantifying BLE Pairing Attack Surfaces (continued)
Passkey with Legacy pairing: The risk is a captured pairing exchange combined with low-entropy passkey material.
- OOB using NFC or QR: The attacker must compromise the out-of-band channel as well as the BLE exchange, which materially raises the bar.
- Key rotation: The pump re-pairs weekly.
- Compromised keys expire automatically.
Major section
Worked Example: Quantifying BLE Pairing Attack Surfaces (continued)
An attacker must be physically within 4 cm -- essentially impossible without being noticed in a clinical setting.
- An attacker who somehow bonds still cannot issue a valid dose command without the clinician's PIN.
- OOB pairing: Adds hardware or provisioning-flow complexity, but removes the remote pairing MITM path.
- Application-layer PIN or role check: Adds a small user step for high-risk commands, but blocks unauthorized commands even if a bond is misused.
Major section
Worked Example: Quantifying BLE Pairing Attack Surfaces (continued)
Periodic re-authentication: Adds occasional friction, but limits how long a compromised bond remains useful.
- Combined result: Pairing protects the link, authorization protects the action, and revocation limits persistence.
- Key Insight: OOB pairing removes the primary remote MITM path, while application-layer authorization adds an independent barrier for dose commands.
- Neither layer alone is sufficient for a life-safety device, but together they provide defense-in-depth with manageable user friction.
Major section
Concept Relationships:
Key hierarchy and bonding: Session keys and stored keys determine whether reconnection can happen securely without re-pairing.
- Pairing methods and key generation: Authentication strength during pairing determines whether encryption protects the right peer.
- CSRK and data integrity: Signature keys verify authenticity on unencrypted LE links, but they are not a substitute for encryption where confidentiality matters.
- AES-CCM-128 and link encryption: Link encryption protects post-pairing traffic once trusted keys are available.
Major section
Deep Dive: Key Lifecycle, Privacy Addresses, and Signed Data Evidence
Encryption is only part of the BLE security record.
- Purpose-specific derivation limits what one key can authorize; rotation or revocation then leads to verified destruction and audit evidence, with the loop reopening when ownership or risk changes.
- Resolvable Private Addresses make the IRK role concrete.
Major section
Deep Dive: Key Lifecycle, Privacy Addresses, and Signed Data Evidence (continued)
For BLE bonds, the inventory should therefore name each LTK, IRK, and CSRK owner, purpose, state, storage boundary, and revocation path.
- A sensor can rotate its over-the-air address periodically so a passive scanner sees unrelated-looking identifiers, while a bonded gateway uses the stored IRK to resolve those addresses as the same trusted device.
- CSRK signed data is a different tradeoff.
- Bond deletion must remove all relevant keys.
Deck summary
Key takeaways
A medical device manufacturer is designing a BLE-connected insulin pump.
- Passkey with Legacy pairing: The risk is a captured pairing exchange combined with low-entropy passkey material.
- An attacker must be physically within 4 cm -- essentially impossible without being noticed in a clinical setting.
- Periodic re-authentication: Adds occasional friction, but limits how long a compromised bond remains useful.
- Key hierarchy and bonding: Session keys and stored keys determine whether reconnection can happen securely without re-pairing.
Retrieval practice
Recall check 1 of 4
Radio Remi says: answer from memory, then check your reasoning.
Q1What is the role of the Long Term Key (LTK) in BLE security?
Show answer
Answer: B The LTK is a 128-bit key saved during bonding so that two devices can re-establish an encrypted link in future sessions without repeating the full pairing ceremony.
Retrieval practice
Recall check 2 of 4
Radio Remi says: answer from memory, then check your reasoning.
Q2Which pairing method should be used for a BLE-connected medical glucose monitor?
Show answer
Answer: C OOB pairing requires physical access to pair (NFC tap or QR scan), providing strong MITM protection.
Retrieval practice
Recall check 3 of 4
Radio Remi says: answer from memory, then check your reasoning.
Q3What key improvement does LE Secure Connections (BLE 4.2+) provide over Legacy Pairing?
Show answer
Answer: B LE Secure Connections uses ECDH on the P-256 curve for key agreement, so a passive attacker who records the pairing exchange cannot derive the shared keys.
Retrieval practice
Recall check 4 of 4
Radio Remi says: answer from memory, then check your reasoning.
Q4Place each BLE security item where it lives so you can choose the right key job and avoid treating every security control as link encryption.
Show answer
Answer: A Separate relationship keys, signed-data authentication, and packet protection so you can explain which BLE property each item supplies and which property it does not.
Q5Complete the authorization logic for a high-risk BLE command after bonding:
Show answer
Answer: A Bonding and link encryption are necessary but not sufficient for high-risk commands.
Print reference
Answers 1 of 2
Answer key.
- B · The LTK is a 128-bit key saved during bonding so that two devices can re-establish an encrypted link in future sessions without repeating the full pairing ceremony.
- C · OOB pairing requires physical access to pair (NFC tap or QR scan), providing strong MITM protection.
- B · LE Secure Connections uses ECDH on the P-256 curve for key agreement, so a passive attacker who records the pairing exchange cannot derive the shared keys.
Print reference
Answers 2 of 2
Answer key.
- A · Separate relationship keys, signed-data authentication, and packet protection so you can explain which BLE property each item supplies and which property it does not.
- A · Bonding and link encryption are necessary but not sufficient for high-risk commands.