CSCE 715: Network Systems Security

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CSCE 715Network Systems Security

• Chin-Tser Huang
• huangct_at_cse.sc.edu
• University of South Carolina

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A Scenario of Replay Attack

• Alice authorizes a transfer of funds from her
  account to Bobs account
• An eavesdropping adversary makes a copy of this
  message
• Adversary replays this message at some later time

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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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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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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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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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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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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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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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Anti-Replay Window Protocolin 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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Anti-Replay Window
1
w
2
3

sequence numbers


received before
right edge r
r-w1
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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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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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
r
s
(deliver)
(discard)
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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
w
1
w
window after shift
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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

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Problem with Anti-Replay Window

• Receiver gets s, where s gtgt r
• Window shifts to right
• Many good messages that arrive later will be
  discarded

window before shift
window after shift
1
w
1
w
r
discarded good msgs
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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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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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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 d1, where d is
  the counter of discarded messages, and d1 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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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

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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
0

reset
seq 0
fresh yet discarded by q
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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
0

reset
seq 0
replayed yet accepted by q
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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

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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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SAVE at Sender

• s is sequence number at p
• Every Kp 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
  sKp is sent

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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(s2Kp)
  and waits
• After SAVE(s2Kp) completes, p can send next
  message using seq s2Kp

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Convergence of Sender

• Assume when p resets, SAVE(s) has not yet
  completed, and the last sent seq is st, t lt Kp
• 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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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 Kq
• When p or q is reset, no replayed message will be
  accepted by q

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Next Class

• Address Resolution Protocol (ARP) and its
  security problems
• Secure ARP
• Read paper on website