Method for exchanging messages between security-relevant apparatuses in accordance with the following steps: a) provision of a plurality of security-relevant apparatuses (V1, V2, V3) linked to one another, wherein each security-relevant apparatus (V1, V2, V3) is assigned to a particular security class (S1, S2, S3, S4) and wherein the higher the security class, the higher is the level of security and reliability and therefore the security requirements fulfilled; b) assignment of pairs of keys, comprising a private key (PR1, PR2, PR3, PR4) and a public key (PU1, PU2, PU3, PU4) matching it, to each security class (S1, S2, S3, S4), wherein the keys and pairs of keys of each respective security class (S1, S2, S3, S4) differ from one another; c) assignment of the public keys (PU1, PU2, PU3, PU4) of each security class (S1, S2, S3, S4) to each security-relevant apparatus (V1, V2, V3); d) assignment of those private keys (P1, P2, P3, P4) that are lower than or equal to the security class of the respective security-relevant apparatus (V1, V2, V3) to each security-relevant apparatus (V1, V2, V3), according to a security class (S1, S2, S3, S4); e) sending and encrypting of a message or a check value by a security-relevant apparatus (V1, V2, V3) with a private key (PR1, PR2, PR3, PR4) of a particular security class (S1, S2, S3, S4) that is assigned to the one security-relevant apparatus (V1, V2, V3); f) receiving of the message or the check value and identification of the security class of the message by decrypting the message by means of one or more security-relevant apparatuses (V1, V2, V3) with the assigned public key (PU1, PU2, PU3, PU4) of the same security class (S1, S2, S3, S4) as the private key (PR1, PR2, PR3, PR4) used for encrypting in step e). a) provision of a plurality of security-relevant apparatuses (V1, V2, V3) linked to one another, wherein each security-relevant apparatus (V1, V2, V3) is assigned to a particular security class (S1, S2, S3, S4) and wherein the higher the security class, the higher is the level of security and reliability and therefore the security requirements fulfilled; b) assignment of pairs of keys, comprising a private key (PR1, PR2, PR3, PR4) and a public key (PU1, PU2, PU3, PU4) matching it, to each security class (S1, S2, S3, S4), wherein the keys and pairs of keys of each respective security class (S1, S2, S3, S4) differ from one another; c) assignment of the public keys (PU1, PU2, PU3, PU4) of each security class (S1, S2, S3, S4) to each security-relevant apparatus (V1, V2, V3); d) assignment of those private keys (P1, P2, P3, P4) that are lower than or equal to the security class of the respective security-relevant apparatus (V1, V2, V3) to each security-relevant apparatus (V1, V2, V3), according to a security class (S1, S2, S3, S4); e) sending and encrypting of a message or a check value by a security-relevant apparatus (V1, V2, V3) with a private key (PR1, PR2, PR3, PR4) of a particular security class (S1, S2, S3, S4) that is assigned to the one security-relevant apparatus (V1, V2, V3); f) receiving of the message or the check value and identification of the security class of the message by decrypting the message by means of one or more security-relevant apparatuses (V1, V2, V3) with the assigned public key (PU1, PU2, PU3, PU4) of the same security class (S1, S2, S3, S4) as the private key (PR1, PR2, PR3, PR4) used for encrypting in step e). <EOS>