Taekyoung Kwon

Subin Song, Michael K. Reiter, Taekyoung Kwon

5G base stations broadcast unauthenticated system information (SI) that every user equipment (UE) reads during cell selection. This enables attackers to broadcast forged SI from a fake base station (FBS), deceiving UEs into camping on it. Prior approaches require UEs to authenticate System Information Block 1 (SIB1) using digital signatures. This necessitates computation-heavy verification for every SIB1 reception, imposing a significant burden on resource-constrained UEs. We propose TESLA-for-5G (TF5), a broadcast authentication protocol for 5G SIB1 that combines TESLA with GG09 Schnorr-like identity-based signatures (IBS). In the steady state, TF5 enables UEs to authenticate each SIB1 message using a symmetric MAC and delayed key disclosure, eliminating the need for per-message digital signatures. Initial trust is bootstrapped during cell entry using a lightweight GG09 IBS over the TESLA parameters, avoiding certificate distribution overhead. We formally verify TF5 in Tamarin under a Dolev-Yao adversary and demonstrate its favorable computation, communication, and storage costs through both an implementation on the OpenAirInterface 5G stack and trace-driven analysis.

Subin Song, Taekyoung Kwon

End-to-end encrypted messaging systems depend on authentic public key distribution to prevent man-in-the-middle (MitM) attacks. Current solutions present a stark trade-off: out-of-band (OOB) verification provides strong security but lacks scalability for large contact lists, while key transparency (KT) systems enable automated verification at high storage costs and operational complexity. We propose DKVE, a protocol that validates public keys through privacy-preserving cross-validation within users' social graphs. When obtaining a contact's public key from a key server, clients query mutual contacts to verify they hold the same key, combining Oblivious Pseudorandom Functions (OPRF) and Oblivious Key-Value Stores (OKVS) to preserve privacy of both queries and contact lists. DKVE employs a Sequential Probability Ratio Test (SPRT) to aggregate responses and detect server misbehavior with user-configurable error bounds. We evaluate DKVE through simulations on real social network datasets, demonstrating DKVE can detect MitM attacks with exceeding 97% for strong-to-moderate-tie networks. The remaining 3% of cases require validation through alternative methods such as KT and OOB verification. Our proof-of-concept implementation confirms feasibility for background operation on commodity hardware, in terms of the latency and bandwidth. As DKVE can reduce the frequency of KT queries by two orders of magnitude, it enables fundamental architectural shifts: KT directories can migrate from fast but space-inefficient Merkle trees to space-efficient data structures like RSA accumulators. While DKVE cannot replace existing methods entirely -- suffering from bootstrapping problems and degraded performance on weak-tie networks -- it provides a practical complementary key validation mechanism, making secure messaging more deployable for billion-user systems.