Jamming-resistant Broadcast Communication without Shared Keys

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1 1/18 Jamming-resistant Broadcast Communication without Shared Keys Christina Pöpper Joint work with Mario Strasser and Srdjan Čapkun System Security Group ETH Zürich August 2009

2 Broadcast Communication Jamming Attacks Problem Statement Our Solution 2/18 Broadcast Communication Setting: Broadcast of (authenticated) messages to a (large) number of receivers Wireless RF communication Receivers may be unknown and/or untrusted Broadcast Applications: Alarm broadcast Broadcast of navigation signals...

Broadcast Communication Jamming Attacks Problem Statement Our Solution 3/18 Jamming Attacks Jamming Attacks: Jamming devices are cheap and easy to obtain Anti-Jamming Techniques: Spread

3 Broadcast Communication Jamming Attacks Problem Statement Our Solution 3/18 Jamming Attacks Jamming Attacks: Jamming devices are cheap and easy to obtain Anti-Jamming Techniques: Spread Spectrum Techniques, e.g., Frequency Hopping Spread Spectrum Direct-Sequence Spread-Spectrum (DSSS) Rely on a secret key (or code) pre-shared between sender and receivers before the communication

4 Broadcast Communication Jamming Attacks Problem Statement Our Solution 4/18 Jamming Attacks Anti-Jamming Techniques in Broadcast Settings: Pre-sharing keys is complex or infeasible Public key cryptography does not help Even if secret keys are pre-shared, receivers still need to be trusted Anti-jamming Broadcast Problem

5 Broadcast Communication Jamming Attacks Problem Statement Our Solution 5/18 Problem Statement Problem Statement: How can we enable jamming-resistant broadcast communication if the sender does not share secret keys with (all the) receivers? In [Desmedt et al., ICON99] and [Chiang et al., InfoCom08], solutions were proposed for jamming-resistant broadcast, but they rely on shared secret information

6 Broadcast Communication Jamming Attacks Problem Statement Our Solution 6/18 Our Solution Anti-jamming Broadcast without Shared Secrets Scheme called (UDSSS) Achieve communication to an unknown/untrusted set of receivers in the presence of communication jamming Key Idea: Base the communication on DSSS but release the requirement of shared secret keys by randomization Key Observation: Whatever has arrived unjammed at the receiver can be decoded A B M M M M t t

7 7/18 (UDSSS) DSSS

8 7/18 (UDSSS) DSSS UDSSS

9 7/18 (UDSSS) - Public set C of spreading sequences Sender randomly selects sequence c s C to spread message M Receivers record signal and despread M by applying sequences from C using a trial-and-error method UDSSS

10 8/18 UDSSS Sender Side Message repetitions, due to lacking synchronization between sender and receivers the possibility of successful jamming attacks A M 1 M 1 M 1 M 1 M 1 M 2 M 2 M 2 M 2 t buffer buffer B M 1 M 2 sampling (st m ) decoding (T d ) t

11 9/18 UDSSS Code Set & Despreading Code set C composed of n code sequences Each code sequence is composed of l spreading codes containing N chips l codes per code sequence E.g., N = 100 chips 20 db processing gain Auto-correlation and cross-correlation properties c 1 c 1,1 c 1,2 c 1,l c 2 c 2,1 c 2,2 c 2,l N chips per code c s c s,1 c s,2 c s,l c n c n,1 c s,1 M[1] M[2] c n,2 c n,l c s,l M[l] M n code sequences Successful despreading requires to hit the correct spreading sequence and the correct synchronization

12 10/18 Attacker Analysis Attacker goal: To prevent communication Attacker types Non-reactive jammers blindly jam part of the spectrum Reactive jammers sense for ongoing transmissions Decoding jammers: try to find the used spreading codes and construct the corresponding jamming signal Repeater jammers: intercept the signal and re-radiate it without knowledge of the used spreading codes Attacker strength: Jamming probability p j (with respect to a given message transmission)

13 11/18 Evaluation metric: Message transmission time One receiver: Expected time for message recovery at a receiver with jamming (p j > 0) and without jamming (p j = 0) Multiple receivers: Expected time until all l receivers have received the message (for independent receptions) under p j One receiver: T r T s + T d = 2 M N R + n 2 kqn M + M Λ B (N) R = 1/Tc chip rate q samples per chip Λ B (N): # bit despreading operations that the receiver can perform per second despread k bits before decision on code sequence, etc.

14 12/18 Analytical Evaluation and Simulation Multiple (l) receivers Probability that a message is received by all l = 100 receivers 0.4 m = 1 p j = p j = 0.2 p j = 0.5 p j = number of message decodings (i) Time (in sec) after which all l receivers have decoded the message T d = 2s m = 1 p j = 0.0 p j = 0.2 p j = 0.5 p j = number of receivers (l) UDSSS can be enhanced to yield the same performance as (non-synchronized) DSSS in the absence of jamming by two parallel signal transmission using C 1 = {c 1 } and C 2

15 13/18 Implementation Prototype implementation of UDSSS on USRP/GnuRadio Carrier frequency of 2.4 GHz (8,4)-Hamming-code ECC 2 USRPs positioned indoors at a distance of around 5 m message sender ECC encoding bit scrambling bit spreading usrp sink USRP message receiver ECC decoding bit unscrambling bit despreading usrp source USRP

16 14/18 Implementation Results Duration (in sec) to receive and decode a message M = 256 M = 512 M = 1024 M = 1536 M = 2048 n IPS = 100 = code length N per bit Duration (in sec) to receive and decode a message M = 256 M = 512 M = 1024 M = 1536 M = 2048 N IPS = 256 = number of code sequences n Increasing the processing gain (i.e., N) is more harmful to the latency/throughput than increasing the code set (i.e., n)

17 15/18 UDSSS Optimization Idea: Use UDSSS to transmit the spreading key only Trick: First transmit message M using a random spreading code K, then transmit the spreading code K using UDSSS A K {0, 1} M c s C K t B M K UDSSS despreading t K Advantages: Smaller spreading code set. Quicker decoding. Longer messages. More flexible security level.

Navigation Signals : Navigation Signal Broadcasts For positioning and/or time-synchronization Requirements: signals from three to four different base stations precise time-stamping of signal

18 Navigation Signals : Navigation Signal Broadcasts For positioning and/or time-synchronization Requirements: signals from three to four different base stations precise time-stamping of signal reception A1 J t1, pos1 t2, pos2 A2 Receiver buffer A3 t3, pos3 t4, pos4 UDSSS A4 t, pos received power tr broadband recording UDSSS signals UDSSS provides: anti-jamming transmission of multiple signals in parallel precise time-stamping of signal reception (despite delayed recovery) & updated time-stamps in each transmitted message anti-spoofing protection of authenticated messages 16/18 tr + Tr noise level t

19 17/18 Concluding Remarks We tackled the anti-jamming broadcast problem: antijamming broadcast communication without pre-shared secrets such that devices cannot jam the reception of other receivers Uncoordinated Spread Spectrum techniques are a solution to the anti-jamming broadcast problem UDSSS ZPK-DSSS [Jin et al, MobiHoc09] UFH [Strasser et al., S&P08], [Strasser et al., MobiHoc09], and [Slater et al., WiSec09] Basic idea: randomize the spreading operation (random code selection) Application: e.g., anti-jamming navigation signal broadcasts

20 18/18 Questions

Jamming-resistant Broadcast Communication without Shared Keys

Jamming-resistant Broadcast Communication without Shared Keys Christina Pöpper System Security Group ETH Zurich, Switzerland poepperc@inf.ethz.ch Mario Strasser Communication Systems Group ETH Zurich,