1. Replay Attacks

2. Replay Attack
First, attacker intercepts a message
Not difficult to do

3. Replay Attack
Later, attacker retransmits (replays) the message to the original destination host
Does not have to be able to read a message to replay it

4. Replay Attack Why replay attacks?
To gain access to resources by replaying an authentication message
In a denial-of-service attack, to confuse the destination host

5. Thwarting Replay Attacks
Put a time stamp in each message to ensure that the message is “fresh”
Do not accept a message that is too old
Place a sequence number in each message
Do not accept a duplicated message
Message
Time
Stamp
Sequence
Number

6. Thwarting Replay Attacks
In request-response applications,
Sender of request generates a nonce (random number)
Places the nonce in the request
Server places the nonce in the response
Neither party accepts duplicate nonces
Request
Response
Nonce
Nonce

7. Thwarting Replay Attacks
To prevent changes in the message being replayed
Message integrity is needed
Requires a digital signature or equivalent
See HMAC under IPsec
Message
Digital Signature
Or HMAC