The invention relates to an apparatus (1a) and a method for controlling a critical system (S), as well as to a device (3a,3b) and a method for the distribution of messages for controlling said critical system (S), wherein said apparatus (1a) is configured for encrypting a first control message by using the first private key, transmitting said first encrypted message to a second apparatus (1b), receiving a second encrypted message generated by a second apparatus (1b) and encrypted by said second apparatus (1b) by using a second private key, decrypting said second encrypted message by using a public key associated with said second private key, verifying the second decrypted message on the basis of said first message and, if the verification is successful, encrypting at least said second encrypted message with said first private key, thereby generating a third encrypted message, and transmitting said third encrypted message.