1. CSCE 790: Computer Network Security Chin-Tser Huang huangct@cse.sc.edu University of South Carolina

2. 9/18/20032 A Scenario of Replay Attack Alice authorizes a transfer of funds from her account to Bob’s account An eavesdropping adversary makes a copy of this message Adversary replays this message at some later time

3. 9/18/20033 Replay Attacks Adversary takes past messages and plays them again whole or part of message to same or different receiver Encryption algorithms not enough to counter replay attacks

4. 9/18/20034 Freshness Identifiers Sender attaches a freshness identifier to message to help receiver determine whether message is fresh Three types of freshness identifiers nonces timestamps sequence numbers

5. 9/18/20035 Nonces A random number generated for a special occasion Need to be unpredictable and not used before Disadvantage is not suitable for sending a stream of messages Mostly used in challenge-response protocols

6. 9/18/20036 Timestamps Sender attaches an encrypted real-time timestamp to every message Receiver decrypts timestamp and compares it with current reading if difference is sufficiently small, accept message otherwise discard message Problem is synchronization between sender and receiver

7. 9/18/20037 Sequence Numbers Sender attaches a monotonically increasing counter value to every message Sender needs to remember last used number and receiver needs to remember largest received number

8. 9/18/20038 Operation of Sequence Numbers Sender increments sequence number by 1 after sending a message Receiver compares sequence number of received message with largest received number If larger than largest received number, accept message and update largest received number If less than largest received number, discard message

9. 9/18/20039 Problem with Sequence Numbers IPsec uses sequence number to counter replay attacks However reorder can occur in IP Messages with larger sequence number may arrive before messages with smaller sequence numbers When reordered messages with smaller sequence numbers arrive later, they will be discarded

10. 9/18/200310 Anti-Replay Window Protocol in IPsec Protect IPsec messages against replay attacks and counter the problem of reorder Sender puts a sequence number in every message Receiver uses a sliding window to keep track of the received sequence numbers

11. 9/18/200311 Anti-Replay Window w is window size r is right edge of window Assume s is sequence number of next received message Three cases to consider 1 w right edge r 23 sequence numbers not yet received received before assumed received r-w+1

12. 9/18/200312 Cases of Anti-Replay Window Case i: if s is smaller than sequence numbers in window, discard message s 1w sr

13. 9/18/200313 Cases of Anti-Replay Window Case ii: s is in window if s has not been received yet, then deliver message s if s has been received, then discard message s 1w srs (deliver)(discard)

14. 9/18/200314 Cases of Anti-Replay Window Case iii: if s is larger than sequence numbers in window, then deliver message s and slide the window so that s becomes its new right edge 1w sr window before shift 1w window after shift

15. 9/18/200315 Properties of Protocol Discrimination: receiver delivers at most one copy of every message sent by sender w-Delivery: receiver delivers at least one copy of each message that is neither lost nor suffered a reorder of degree w or more, where w is window size

16. 9/18/200316 Problem with Anti-Replay Window Receiver gets s, where s >> r Window shifts to right Many good messages that arrive later will be discarded discarded good msgs 1w r window before shift s 1w window after shift

17. 9/18/200317 Automatic Shift vs. Controlled Shift Automatic shift: window automatically shifts to the right to cover the newly received sequence number without any consideration of how far the newly received sequence number is ahead Controlled shift: if the newly received sequence number is far ahead, discard it without shifting window in the hope that those skipped sequence numbers may arrive later

18. 9/18/200318 Three Properties of Controlled Shift Adaptability receiver determines whether to sacrifice a newly received message according to the current characteristics of the environment Rationality receiver sacrifices only when messages that could be saved are more than messages that are sacrificed Sensibility receiver stops sacrificing if it senses that the messages it means to save are not likely to come

19. 9/18/200319 Additional Case with Controlled Shift Case iv: s is more than w positions to the right of window receiver estimates number of good messages it is going to lose if it shifts the window to s if the estimate is larger than d+1, where d is the counter of discarded messages, and d+1 is less than dmax, then receiver discards this message and increments d by 1 otherwise, receiver delivers the message, shifts the window to the right, and resets d to 0

20. 9/18/200320 Another Problem with Anti-Replay Window Computer may reset due to transient fault If either sender or receiver is reset and restarts from 0, then synchronization on sequence numbers is lost

21. 9/18/200321 Scenario of Sender Reset If p is reset, unbounded number of fresh messages are discarded by q 49483210 pq seq# : 50 seq# : 50 fresh yet discarded by q seq# : 0 reset

22. 9/18/200322 Scenario of Receiver Reset If q is reset, it can accept unbounded number of replayed messages 49483210 pq inserted by adversary seq# : 50 seq# : 50 replayed yet accepted by q seq# : 0 reset

23. 9/18/200323 Overcome Reset Problems IPsec Working Group: if reset, the SA is deleted and a new one is established -- very expensive Our solution: periodically push current state of SA into persistent memory; if reset, restore state of SA from this memory

24. 9/18/200324 SAVE and FETCH When SAVE is executed, the last sequence number or right edge of window will be stored in persistent memory When FETCH is executed, the last stored sequence number or right edge of window will be loaded from persistent memory into memory

25. 9/18/200325 SAVE at Sender s is sequence number at p Every K p messages, p executes SAVE(s) to store current s in persistent memory In spite of execution delay, SAVE(s) is guaranteed to complete before message numbered s+K p is sent

26. 9/18/200326 FETCH at Sender When p wakes up after reset, p executes FETCH(s) to fetch s stored in persistent memory After FETCH(s) completes, p executes SAVE(s+2K p ) and waits After SAVE(s+2K p ) completes, p can send next message using seq# s+2K p

27. 9/18/200327 Convergence of Sender Assume when p resets, SAVE(s) has not yet completed, and the last sent seq# is s+t, t < K p When p wakes up, s-K p will be fetched Therefore, adding 2K p to fetched seq# guarantees that next sent seq# is fresh

28. 9/18/200328 Results of SAVE and FETCH When p is reset, some sequence numbers will be abandoned by p, but no message sent from p to q will be discarded provided no message reorder occurs When q is reset, the number of discarded messages is bounded by K q When p or q is reset, no replayed message will be accepted by q

29. 9/18/200329 Next Class Address Resolution Protocol (ARP) and its security problems Secure ARP Read paper on website