Replay Attacks Network Systems Security

Mort Anvari

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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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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

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

2 3 • • •sequencenumbers

not yet received

received before

assumed received

• • •• • •

r-w+1

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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 rs

(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

1 w

srr

window before shift

11 ww

window after window after shiftshift

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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 >> r Window shifts to right Many good messages that arrive later

will be discarded

discarded good msgs

1 w

r

window before shift

s

11 wwwindow after window after shiftshift

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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 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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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

49 48 3 2 1 0

p q

seq# : 50• • •

seq# : 50

fresh yet discarded by q

seq# : 0

reset

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Scenario of Receiver Reset If q is reset, it can accept unbounded

number of replayed messages

49 48 3 2 1 0

p q

inserted by

adversary

seq# : 50• • •

seq# : 50

replayed yet accepted by q

seq# : 0

reset

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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 s+Kp 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(s+2Kp) and waits

After SAVE(s+2Kp) completes, p can send next message using seq# s+2Kp

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Convergence of Sender

Assume when p resets, SAVE(s) has not yet completed, and the last sent seq# is s+t, t < 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