This talk presents the key reinstallation attack. It abuses design or implementation flaws in cryptographic protocols to reinstall an already-in-use key. This resets the key’s associated parameters such as transmit nonces and receive replay counters. Several cryptographic Wi-Fi handshakes are affected by the attack.All protected Wi-Fi networks use the 4-way handshake to generate a fresh session key. So far, this 14-year-old handshake has remained free from attacks. However, we show that the 4-way handshake is vulnerable to a key reinstallation attack. Here, the adversary tricks a victim into reinstalling an already-in-use key. This is achieved by manipulating and replaying handshake messages. When reinstalling the key, associated parameters such as the incremental transmit packet number (nonce) and receive packet number (replay counter) are reset to their initial value.In contrast with previous talks, we will also discuss the vulnerability disclosure process that we followed. Since our discovery affected numerous vendors, coordinating the disclosure was far from trivial. We discuss several lessons that we learned from this experience.