Replay Attacks and Possible Countermeasures

replay attackReplay attacks are a variation on the man-in-the-middle theme. In a replay attack an agent is once again placed within the client/server line of communication. In the case of a replay attack, however, the transaction data is recorded for the express purpose of allowing the data to be modified and replayed to the server at a later time for nefarious purposes.

An example of a replay attack is an instance where one party wants to prove their identity to a another party. If a third party eavesdrops on the conversation, they can intercept the password. Once the exchange is over, the eavesdropper can send the password and impersonate the party to whom the password belongs to gain unauthorized access to the other party.

Defenses Against Replay Attacks

As with other man-in-the-middle attacks, replay attacks can be countered using encryption, timestamps, serial numbers and packet sequences so that the server can detect that the data is being replayed from a previous session. One effective method of avoiding replay attacks which uses encryption is to use session tokens. Let us assume that A is required to send a password to B. If session tokens are used, B will send a one-time token to A, which A will then use to transform the password and send the result to B. On the other side, B performs the same transformation, and if both values match, the login will be successful. If C eavesdrops on A and B and captures the transformed password, C can try to use it to authenticate with B. But B will send a session token, and if C sends the transformed password captured earlier, the transformations will not match, and authentication will fail.


If C knows that B is using session tokens, C might be able to pose as B, presenting some predicted future token, and convince A to use that token in A’s transformation. C can then replay A’s reply at a later time, when the previously predicted token is presented by B, and B will accept the authentication. For that reason, session tokens should be chosen by a pseudo-random process.

One-time passwords are similar to session tokens in that the password expires after it has been used or after a very short period of time. They can be used to authenticate individual transactions in addition to sessions.

Replay attacks can also be thwarted by the use of message authentication codes (MACs), short pieces of information used to authenticate a message and to provide integrity and authenticity assurances on the message. MAC algorithms accept as input a secret key and an arbitrary-length message to be authentication, and outputs a MAC. This value protects both a message’s data integrity and its authenticity by virtue of the fact that the verifiers possessing the secret key to detect any changes to the message content.

Timestamping is another means of preventing a replay attack. Synchronization is achieved using a secure protocol. As an example, B can broadcast the time on their clock along with a message authentication code (MAC). If A wants to send B a message, they can include an estimate of the time on B’s clock in their message (which also sends a MAC). B only accepts messages for which the timestamp is within a reasonable tolerance.


External Links:

Replay attack on Wikipedia

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