toistohyökkäyksiä

Toistohyökkäys, or replay attack in English, is a type of cyberattack where an attacker intercepts and possibly relays a valid data transmission. The attacker then retransmits the intercepted data, either to gain unauthorized access to a system or to disrupt its normal operation. This can be achieved by capturing a message and then replaying it later, pretending to be a legitimate user.

In a simple replay attack, the attacker simply captures a communication and sends it again. This can be effective against systems that do not implement mechanisms to detect repeated transmissions. For example, if a system authenticates a user based on a password or a token, and that token is captured, the attacker could replay the captured token to gain access. More sophisticated replay attacks might involve modifying the intercepted data before replaying it, or using it in conjunction with other attacks.

To counter replay attacks, various security measures can be employed. Timestamping is a common method, where each message is assigned a unique timestamp. The receiving system can then discard any message with an outdated timestamp, assuming it is a replayed message. Sequence numbers are another effective technique. Each message is assigned a sequence number, and the receiver keeps track of the expected sequence number. Any message arriving with a number out of sequence is considered suspect. Cryptographic techniques like nonces (numbers used once) are also used, where a unique, unpredictable number is generated for each communication session and included in messages. The receiver can then verify that the nonce has not been used before. These methods help ensure the integrity and authenticity of communications and protect against unauthorized retransmission of data.