Methods, systems, and devices for wireless communications are described. A first wireless device may communicate, with a second wireless device, a first set of messages of a first message type that are acknowledged according to an acknowledgement procedure. The first wireless device, the second wireless device, or both may select a first subset of messages of the first plurality of messages and a first subset of bits from each message of the first subset of messages based on the first set of messages being acknowledged and according to a sampling configuration common to both the first wireless device and the second wireless device. The first wireless device, the second wireless device, or both may generate a key using bit values of the first subset of bits from each message of the first subset of messages, encoding a message using the key, and communicate the encoded message.