A method exchanges messages with different security classes between security-relevant devices. Key pairs containing a private key and a public key corresponding to the private key are assigned to each security class, wherein the keys and key pairs of each security class differ from each other. Each security-relevant device has all the public keys for decrypting messages for each security class and the relevant private keys for encrypting messages corresponding to a security class that is lower than or equal to the security class of the security-relevant device. Upon receipt, the security class for the message is identified by decryption by use of the public key.