Techniques for Identifying and Preventing SPAM over IP Telephony (SPIT)

1. Felipe Santos Manoj Deshpande ECE 4112 – Internetwork Security Georgia Institute of Technology SPAM over IP Telephony (SPIT) Identification and prevention Techniques

2. Background • SPAM considered one of biggest problems in Internet • SPIT is expected to become a major issue in the next few years with increasing deployment of VoIP solutions • Potential for productivity disturbance is much greater than SPAM

3. Background • Definition: The transmission of unsolicited calls over Internet telephony (VoIP) • “SPITTERS” will forge their identities • SPITTING agent capable of placing hundreds of simultaneous automated calls • SIP is not voice only, but applies to Instant Messaging and video as well

4. SPAM vs. SPIT

5. SPIT Prevention Framework • Goals: • Minimize false positives & negatives • Minimize callee interaction in identifying SPIT • Minimize inconvenience to caller • General enough to work in different environments (work, home, etc) and cultures

6. SPIT Prevention Framework • 5 Stage Approach: • Stage 1: no interaction w/ users • Blacklist, Whitelist, Graylisting, Circles of Trust, Pattern / AnomalyDetection • Stage 2: caller interaction • Computational Puzzles, SenderChecks, Audio CAPTCHAS (Turing Tests)

7. SPIT Prevention Framework • 5 Stage Approach (continued): • Stage 3: feedback before call • Manual authorization to receivecall and/or authenticate user • Stage 4: during the call • Content analysis (not currentlyviable) • Stage 5: feedback after call • Reputation System, Limited-Use Address, Payments at Risk, Litigation

8. SPIT Prevention Techniques • Blacklists & Whitelists • Pros: • Simple implementation • Effective (users in whitelist will always be allowed through and vice versa) • Cons: • Manual data gathering by user or global service required to build such lists • SPITTERS can easily spoof identity and bypass lists

9. SPIT Prevention Techniques • Circles of Trust • Inter-domain connections are checked before a call is forwarded. Each domain control its users • Pros: • Efficient • Even if a user misbehaves, easy to identify user • Cons: • Requires a priori inter-domain agreements/validation • Relatively complex implementation

10. SPIT Prevention Techniques • Pattern/Anomaly Detection • Statistical analysis of a user’s calling behavior based on studies that identify “normal” call behavior. • Pros: • Potentially most acurate • Mature methodology • Cons: • Requires monitoring agent to keep track of user behavior • Never before implemented to voice calls

11. SPIT Prevention Techniques • Graylisting • Consists of calculating a gray level for each and every caller • Gray level determines how likely a caller is to be a SPITTER

12. SPIT Prevention Techniques • Graylisting (continued) • Progressive Multi Gray-Leveling (PMG) • Considers two levels per caller: short-term level and long-term level • Short-term level • considers the number of calls a given user places within a short period of time (i.e. 10 min) • Level changes rapidly - Prevents DoS attacks • Long-term level • considers the number of calls a given user places within a long period of time (i.e. 10 hours) • Level changes slowly – prevents SPITTER from regaining calling rights

13. SPIT Prevention Techniques • Graylisting (continued) • Progressive Multi Gray-Leveling (PMG) (continued) • A threshold is established, such that if (short-term level + long-term level) > ThresholdA user’s outgoing call is blocked

14. SPIT Prevention Techniques • Graylisting (continued) • Pros: • Effective caller limiting approach • Relatively simple implementation • Makes a SPITTER’s task much harder • Cons: • Legitimate users can potentially have calls blocked just for placing too many calls within a given time frame.

15. SPIT Prevention Techniques • Computational Puzzle • Verify a caller’s “willingness” to place the call by imposing that the client solves a digital puzzle/calculation prior to call establishment • Caller must spend at least a given minimum period of time to ensure solution is not “guessed” • Pros: • Limit a SPITTER’s calling rate by adding required computational overhead to establish • Cons: • Increased overhead for call establishment • Could be relatively easily circumvented

16. SPIT Prevention Techniques • Sender Check • Verify/authenticate a caller by actively consulting its domain • Equivalent of Sender Policy Framework (SPF) and Sender ID in email • Pros: • Originating domain certifies its users • Prevents user ID spoofing • Cons: • Relies on remote domain information that may not be correctly implemented or updated

17. SPIT Prevention Techniques • Turing Test • Differentiate between automated computer placed calls (likely SPIT) and calls placed by human beings • Uses Audio Completely Automated Public Turing test to tell Computers and Humans Apart (CAPTCHAS) • Pros: • Quickly and easily identify automated vs. human calls • Cons: • Increased overhead for connection establishment • Could potentially block non-SPIT automated calls (banks, package delivery notifications, reverse 911, etc)

18. SPIT Prevention Techniques • Consent-Based Communication • User authentication / identity verification • Calleeauthoizes caller a priori with a previously exchanged key or passphrase • Pros: • SPIT is completely blocked, since only authorized callers can place call to user • Cons: • Any new caller who wishes to contact a user must request and receive the shared key a priori

19. SPIT Prevention Techniques • Content Filtering • Process call content to detect SPIT as done in SPAM filters • Pros: • If viable, would be the most accurate technique • Cons: • Not viable / implementable. Although there exist DSP algorithms to analyze audio data and convert audio waveforms to ASCII text, process is not real-time and call contents are not available for processing until after the call is actually placed.

20. SPIT Prevention Techniques • Reputation System • Centralized reputation score based on user behavior and other users’ feedback • Pros: • Centralized global resource to identify SPITTERS • Cons: • Requires protocol standardization for feedback framework

21. SPIT Prevention Techniques • Payments at Risk • Require a refundable payment for each call from an unknown party. The payment is only refunded if the caller was not a SPITTER. • Pros: • Increase cost / decrease profitability of SPIT • Cons: • Quite unrealistic scenario, since a standardized framework would be required for feedback and payment charging and many VoIP services are free and fully p2p

22. Lab VoIP Testbed

23. Lab Exercises • Students will: • Configure and setup the VoIP testbed • Establish an authenticated VoIP call and notice a SPITTER’s inability to contact a user that requires caller authentication • Create a SPIT message • Place an automated SPIT call by capturing and replaying the SPIT message created above • Place an automated SPIT call with a spoofed ID

24. Exercise Results • User Authentication (with shared keys)

25. Exercise Results • User Authentication (no shared keys)

26. Exercise Results • Creating SPIT Message & Generating Automated SPIT Call

27. Exercise Results • Spoofing Caller ID

28. References • J. Quittek, S. Niccolini, S. Tartarelli, and R. Schlegel, “Prevention of Spam over IP Telephony,” NEC Technical Journal, vol. 1, no. 2, Feb., pp. 114-119, 2006. • D. Shin and C. Shim, “Voice Spam Control with Gray Leveling,” Proceedings of 2nd VoIP Security Workshop, Washington DC, June 1-2 2005. • F. Hammer et al. “Elements of Interactivity in Telephone Conversations,” Proceedings of 8th International Conference on Spoken Language Processing (ICSLP/INTERSPEECH 2004), Vol3, pp.1741-1744, Jeju Island, Korea, Oct. 2004.