1. CSCE 715: Network Systems Security
Chin-Tser Huang
University of South Carolina

2. Next Topic in Cryptographic Tools
Symmetric key encryption
Asymmetric key encryption
Hash functions and message digest
Nonce
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3. 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
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4. 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
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5. 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
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6. 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
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7. 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
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8. 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
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9. 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
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10. 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
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11. 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
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12. Comparison with TCP Sliding Window
Purpose: TCP sliding window is used for flow control, while anti-replay window for countering replay attack
Size: TCP sliding window is of dynamic size, while anti-replay window is of static size (64 recommended by IPsec)
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13. Comparison with TCP Sliding Window
Unit: TCP sliding window is byte-oriented, while anti-replay window is packet-oriented
Retransmission: same sequence number used in TCP sliding window, while new sequence number used in anti-replay window
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14. (advertised by receiver)
TCP Sliding Window
offered window
(advertised by receiver)
usable window 1 2 3 4 5 6 7 8 9 10 11 …
can’t send until
sent, not ACKed
window moves
sent and
acknowledged
can send ASAP
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15. Anti-Replay Window w is window size r is right edge of window1 2 3
• • • w
sequence
numbers
• • •
• • •
received before
r-w+1
right edge r
not yet received
assumed received
w is window size
r is right edge of window
Assume s is sequence number of next received message
Three cases to consider
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16. Cases of Anti-Replay Window
Case i: if s is smaller than sequence numbers in window, discard message s 1 w s r
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17. 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 1 w s s r
(discard)
(deliver)
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18. 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
window before shift 1 1 w w r s
window after shift
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19. Properties of Anti-Replay Window 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
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20. Problem with Anti-Replay Window
Receiver gets s, where s >> r
Window shifts to right
Many good messages that arrive later will be discarded
window before shift
window after shift 1 w 1 w s r
discarded good msgs
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21. 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
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22. 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
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23. 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
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24. Another Problem with Anti-Replay Window
Computer may reset due to transient fault or power loss
If either sender or receiver is reset and restarts from 0, then synchronization on sequence numbers is lost
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25. Scenario of Sender Reset
If p is reset, unbounded number of fresh messages are discarded by q p q
seq# : 50
seq# : 50 49 48 3 2 1
• • •
reset
seq# : 0
fresh messages yet discarded by q
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26. Scenario of Receiver Reset
If q is reset, it can accept unbounded number of replayed messages
inserted by
adversary p q
seq# : 50
seq# : 50 49 48 3 2 1
• • •
reset
seq# : 0
replayed yet accepted by q
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27. Overcome Reset Problems
IPsec Working Group: if reset, the Security Association (SA) is deleted and a new one is established -- very expensive
Our solution: periodically push current state of SA into persistent memory (e.g. hard drive); if reset, restore state of SA from this memory
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28. 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
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29. SAVE at Sender s is sequence number at p
Every Kp messages, p executes SAVE(s) to store current s in persistent memory
Choose appropriate Kp such that in spite of execution delay, SAVE(s) is guaranteed to complete before message numbered s+Kp is sent
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30. 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+2Kp) and waits
After SAVE(s+2Kp) completes, p can send next message using seq# s+2Kp
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31. Convergence of Sender
Assume when p resets, SAVE(s) has not yet completed, and the last sent seq# is s+t
t < Kp otherwise SAVE(S) should have completed
When p wakes up, s-Kp will be fetched
Therefore, adding 2Kp to fetched seq# guarantees that next sent seq# is fresh
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32. Convergence of Sender
Assume when p resets, SAVE(s) has completed, and the last sent seq# is s+u
u < Kp otherwise SAVE(S+Kp) should have started
When p wakes up, s will be fetched
Therefore, adding 2Kp to fetched seq# guarantees that next sent seq# is fresh
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33. Convergence of Sender
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34. 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 2Kq
When p or q is reset, no replayed message will be accepted by q
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35. Next Class Address Resolution Protocol (ARP) and its security problems
Secure ARP
Read paper on website
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