We sincerely thank the reviewers for their detailed feedback and will carefully address the issues in the revised paper. Here, we respond to key comments and look forward to further clarifying any remaining issues during the rebuttal phase. We would love to work with a shepherd on the final version of the paper.

Presentation (RevABD)

As suggested we will fix redundant, vague, incorrect statements (e.g., improving the lessons learned). We welcome that the BT SIG has finally embraced more inclusive terminology (i.e., central/peripheral) and will update the paper accordingly.

CTKD implementation (RevACD)

While it is true that each evaluated device has a concrete implementation of CTKD, there is no easily accessible public source implementation of CTKD. Our open-source implementation enables researchers to verify the correctness of observed protocol traces, allowing them to check messages and validate third-party CTKD implementations both in benign and adversarial settings. Moreover, our implementation can be easily integrated into other open-source projects related to Bluetooth security. For example, it can be used within a Bluetooth attack simulator.

CTKD Reverse engineering (RevABCD)

The Bluetooth standard only provides partial information, making reverse engineering CTKD necessary. Specifically, we had to infer how CTKD is negotiated from BT and BLE and how they differ by dynamically monitoring CTKD executions on actual devices. We will clarify in Section 3.B exactly which aspects we had to recover (e.g., what/how/when flags and parameters were used) to distinguish our analysis from the standard. We base our information about CTKD from the Bluetooth standard version 5.2, pages 280, 1401, 1634, 1658, 1721. These sections miss the above-mentioned aspects. If RevB is aware of any document/aspects we missed in the standard, we would appreciate references so that we can update the paper accordingly.

Accepting pairing requests when a Bluetooth device is on (RevB)

The fact that a device accepts pairing requests while it is powered on depends on several design and implementation factors. For example, by design, a BLE device can only accept pairing requests if it is a BLE peripheral device. For BT this rule does not hold as any device supporting pairing can send or receive pairing requests. For concrete implementations, the vendor decides when a device is pairable (or not). For example, Linux devices are, by default, pairable but not discoverable, the user sets the discoverability flag in a UI and experts may set the pairability flag in a configuration file. On the Apple BT stack, devices are only pairable while the settings dialog remains open (see

Disabling JustWorks with CTKD as mitigation (RevB)

We are not considering disabling CTKD+JustWorks as a viable mitigation for two reasons. First, the mitigation does not prevent all attacks as the attacker can bypass association methods that are stronger than JustWorks. For example, leveraging the “Method Confusion Attack on Bluetooth Pairing” from S&P 21, the attacker can still perform a BLUR man-in-the-middle attack even if the victims disallow CTKD+JustWorks. Second, the mitigation introduces backward compatibility issues. Bluetooth devices that only support JustWorks association (e.g., devices with no I/O capabilities) could no longer use CTKD. Concretely, all CTKD-compatible headsets and speakers could no longer leverage CTKD as a result of this mitigation. We will clarify these aspects in the paper.

Standard compliance (RevB)

We thank the reviewer for pointing out the repetitive argument on standard compliance and will tone down the paper accordingly. The vulnerabilities are rooted in the (lack of) specification of CTKD and are not “just” vendor-specific CTKD implementation bugs. Thanks to our initial report, the standard version 5.2 now includes a note on the BLUR attack vector but does not fully mitigate it. We are continuing our disclosure process with the Bluetooth SIG to ensure it is well discussed and mitigated. For the paper, we will follow RevB’s suggestion of carefully rephrasing this claim. Implementations suffer from an under-specified CTKD protocol. Ultimately the Bluetooth standard is responsible for properly specifying security mechanisms and discussing their associated threats. Currently, this is not the case for CTKD.

Formal verification (RevB)

We considered formal verification through Tamarin. While Tamarin has been used to analyze Bluetooth classic, formalizing the BT, BLE, and CTKD specifications given the lack of formal details in the specification would have resulted in a large set of assumptions. We, therefore, chose a reverse engineering approach at the binary protocol level for the first inter-protocol analysis. While formal verification would provide a more complete analysis, it also requires a complete specification. We consider deriving such a concrete specification as orthogonal work.

Research Contributions (RevC)

Our key research contribution is the first analysis of CTKD, a cross-protocol key generation technique. Both formal and manual analysis of protocol specifications (and their implementations) have a long history at Oakland and other top-tier venues. Under-specifications that enable such severe vulnerabilities like BLUR immediately apply to billions of devices with broad impact. Our attack paper is the first to evaluate cross-transport issues on Bluetooth and analyzes the security of CTKD, an opaque but critical cross-transport security mechanism baked in the Bluetooth standard. Additionally, this paper concretely demonstrates an original class of attacks allowing, for the first time, to exploit BT and BLE by just targeting one of the two.

There are multiple lessons to be learned. Firstly, a technology introducing cross-transport security mechanisms must update its threat model to consider a cross-transport attacker. This is not the case for Bluetooth and the paper provides actionable recommendations. Another important lesson is about threat model equivalence. When designing a cross-transport security mechanism, it is crucial that its entry points have comparable threat models and associated security properties. This is not the case for Bluetooth, where the entry points are BT and BLE pairing which are different protocols with different threat models and associated security properties. This is an actionable lesson for future cross-transport protocols.

Lack of rigor and incorrect claims (RevD)

We are aware that some devices do not support pairing. The Background paragraph about “all devices support Just Works” starts with “While pairing” to contextualize the text on devices supporting pairing. We will rephrase the sentence to clarify this message. Thank you, RevD, for pointing out the imprecision about “the first that can be conducted in the absence of one of the victims” statement; we will drop “the first” and clarify/rephrase this sentence to highlight the contribution of BLUR.

CTI1 is CTKD specific and silent pairing (RevD)

CTI1 is CTKD specific as CTKD enables a novel way to pair devices. For example, with the advent of CTKD, we observed devices that use BT for their main functionality (e.g., headphones and speakers) to advertise over BLE, using BLE pairing+CTKD to improve user experience. The fact that two devices can pair silently is not related to CTKD but depends on the authentication requirements and I/O capabilities of the devices. For example, you can silently pair two devices that only support BT or BLE.

Novelty of the CTIs, especially CTI3 and CTI4 (RevD)

Other Bluetooth attacks are based on issues related to the CTIs, however, none of those issues are cross-transport. For example, “key tampering” does not equal “cross-transport key tampering (CTI 3)”; in the first case, the issue is limited to one transport (either BT or BLE), while in the second case the issue affects multiple transports (BT and BLE) and related to how keys are managed across transports. In other words, cross-transport key tampering introduces an alternative, novel way to overwrite keys for Bluetooth. Similar reasoning holds for cross-transport manipulation of associations (CTI 4). We will clarify this important distinction and refer to other key overwrite attacks in more detail.

Regarding the clarification attack example with a laptop and a pair of headphones, our attack scenario assumes that the headset supports both BT and BLE and therefore CTKD (see our system model), contrary as assumed by RevD in their review.

About finding examples where legacy attacks cannot accomplish what the BLUR attacks can, we provide concrete examples comparing our attacks to Nino and BLESA as suggested by the reviewer. The BLUR unintended session attacks presented in 4.E cannot be achieved using Nino and BLESA as Nino is a MitM attack and BLESA is a spoofing attack. If we focus on spoofing and MitM attacks, unlike the BLUR spoofing and MitM attacks, Nino and BLESA even when combined cannot achieve a persistent compromise of BT and BLE. In particular, only with the BLUR spoofing or MitM attacks the attacker can arbitrarily spoof and MitM any BT or BLE device and gain persistence by overriding BT and BLE pairing keys. With Nino the attacker can only temporarily MitM a BT pairing session, cannot spoof single devices, and cannot override BT pairing keys. With BLESA the attacker can only temporarily spoof a BLE server, cannot spoof the BLE client, cannot establish a BLE MitM position, and cannot override BLE pairing keys.