OTA MAC Rotation
Unencrypted predictable frame fields in IEEE 802.11-23 can lead to client tracking, compromising privacy. This document proposes a method to prevent tracking of associated clients across epoch boundaries. OTA randomization is suggested to re-randomize MAC addresses at each epoch. The approach involves negotiation between APs and STAs, using HKDF for constructing new OTA MACs. By aligning hash algorithms during association, client tracking can be minimized effectively.
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Oct 2023 doc.: IEEE 802.11-23/1818r4 OTA MAC Rotation Date: 2023-10-19 Authors: Name Stephen Rodriguez Affiliations Cisco Systems Inc Address ::1 Phone email stephen.rodriguez@ieee.org sorr@cisco.com Stephen Orr Cisco Systems Inc Submission Slide 1 Stephen Rodriguez, Cisco
Oct 2023 doc.: IEEE 802.11-23/1818r4 Summary Background Unencrypted predictable frame fields can enable client tracking by identifying a set of frames for which a single client is transmitter or intended receiver. This can allow tracking presence or location of the person using that client, compromising privacy. Proposal Goal: prevent tracking associated clients across epoch boundaries, within each epoch is acceptable. OTA Randomization: Prevents client frame tracking across epoch boundaries Re-randomize MAC address at every epoch Does not prevent client frame tracking within an epoch Submission Slide 2 Stephen Rodriguez, Cisco
Oct 2023 doc.: IEEE 802.11-23/1818r4 Approach At association AP STA and non-AP STA shall negotiate if they both support OTA rotation; AP STA and non-AP STA shall use HKDF to construct the new OTA MAC prior to epoch HKDF-Extract requires a salt, and IKM (Input Keying Material) value to derive the PRK (pseudo random key) For the salt value we can use an FTM value the PMK derived from the 4WHS shall be used as the IKM Because FTM (Fine Time Measurement) and PMK are known to both AP and non-AP STA the new IRM can be validated by the AP There is no need to send the new IRM over the air, STA can just start using it PRK = HMAC-Hash (FTM, PMK) Hash algorithm would be aligned with the Hash algorithm used with the AKM during association (SHA256, 384,512) Submission Slide 3 Stephen Rodriguez, Cisco
Oct 2023 doc.: IEEE 802.11-23/1818r4 Approach HKDF-Expand (PRK, info, L) -> OKM Pseudo Random Key calculated from HKDF-Extract function (PRK) Info context, in this case New OTA MAC L length of output keying material in octets (<= 255*HashLen) IRM_List = HMAC-Hash(PRK, "New OTA MAC , L) Hash would be aligned with the Hash algorithm used with the AKM during association (SHA256, 384,512) L = Num_Macs * 6(bytes) Num_Macs must be an integer between 1 and 42 Mac1 = IRM_List [first 6 bytes:] | Mac2 = IRM_List [second 6 bytes:] etc; second bit force to 26 A | E to align with IEEE Submission Slide 4 Stephen Rodriguez, Cisco
Oct 2023 doc.: IEEE 802.11-23/1818r4 Client Initiated Rotation Before the defined epoch, allow the client to rotate the OTA mac Announce time Tell AP-STA when we will start to use new OTA IRM sent in a protected frame e.g. in 1 minute we will use new OTA IRM Once announce time has elapsed, countdown to Sunset time starts. Sunset time After old mac shall not respond and AP shall not forward packets for old IRM Fully Untrusted - DTIM * 5 Semi Untrusted DTIM * 5 Trusted DTIM * 5 Submission Slide 5 Stephen Rodriguez, Cisco
Oct 2023 doc.: IEEE 802.11-23/1818r4 Straw Poll Would you agree that HKDF could solve the OTA mac rotation issue? Would you agree that computing multiple future OTA at one time would be beneficial? Submission Slide 6 Stephen Rodriguez, Cisco
