A replay attack is a form of network attack in which a valid data transmission is maliciously or fraudulently repeated, delayed, or retransmitted. This is done either by the originator or by an adversary who intercepts the data and retransmits it. Replay attacks are a subset of man-in-the-middle attacks where the attacker can’t decrypt or modify the data, but reuses it for fraudulent purposes.
In the context of cryptography and secure communications, a replay attack could be used to undermine the security of an encrypted communication channel or authentication protocol. For example, if an attacker can capture a network packet representing a successful login, they might replay that packet to gain access without knowing the user’s actual credentials.
Various methods exist to prevent replay attacks. One common approach is to use a nonce, a random or non-repeating value that is included in each message. If a server receives a message with a nonce it has seen before, it can assume the message is a replay and ignore it. Similarly, timestamps can be used to ensure that messages are only valid for a short period of time.