A satellite system comprises first and second devices, one of the first and second devices being a satellite and the other one of the first and second devices being a device arranged to send signals to the satellite. A session key can be distributed in the satellite system by obtaining the session key at the first device, protecting the session key at the first device, based on a private key of the first device and a public key of the second device, and transmitting the protected session key from the first device to the second device. Replay detection information can be transmitted with the protected session key, for determining whether the session key has been transmitted previously.