Method for transmitting a message from a transmitter to a receiver, wherein - a telegram generated by the transmitter that satisfies the safety level, which telegram comprises the message and the check data formed with the message, is transferred to a transmitter-side access protection device, - the transmitter-side access protection device modifies the telegram and then transmits it over a connection, in particular a potentially insecure or potentially vulnerable transmission medium, to a receiver-side access protection device, - the receiver-side access protection device processes the modified telegram (T') and forwards the processed telegram (T'') to the receiver that satisfies the safety level of the transmitter and - the receiver verifies the processed telegram (T'') on the basis of the message contained therein and the check data contained therein and rejects the message if the check data does not correlate with the message, characterised in that - the transmitter-side access protection device is used in addition to the transmitter and a connection between the transmitter and the transmitter-side access protection device is implemented as more secure, with regard to manipulations, than the first-mentioned connection, wherein the transmitter-side access protection device modifies the telegram by - encrypting the check data as a whole or at least a part of the check data, which contains a security code formed with the message by the transmitter, using a secret key forming coded data (P', SC', (Y;SC)'), and - in the telegram replacing the check data, which has been encrypted, with the coded data (P', SC', (Y;SC)'), wherein the message remains unencrypted in the telegram, - the receiver-side access protection device is used in addition to the receiver and a connection between the receiver-side access protection device and the receiver is implemented as more secure, with regard to manipulations, than the first-mentioned connection, wherein the receiver-side access protection device forms the processed telegram (T'') by - encrypting the coded data (P', SC, (Y;SC)') and - replacing the coded data (P', SC', (Y;SC)') in the telegram (T') with the encrypted coded data. - a telegram generated by the transmitter that satisfies the safety level, which telegram comprises the message and the check data formed with the message, is transferred to a transmitter-side access protection device, - the transmitter-side access protection device modifies the telegram and then transmits it over a connection, in particular a potentially insecure or potentially vulnerable transmission medium, to a receiver-side access protection device, - the receiver-side access protection device processes the modified telegram (T') and forwards the processed telegram (T'') to the receiver that satisfies the safety level of the transmitter and - the receiver verifies the processed telegram (T'') on the basis of the message contained therein and the check data contained therein and rejects the message if the check data does not correlate with the message, - the transmitter-side access protection device is used in addition to the transmitter and a connection between the transmitter and the transmitter-side access protection device is implemented as more secure, with regard to manipulations, than the first-mentioned connection, wherein the transmitter-side access protection device modifies the telegram by - encrypting the check data as a whole or at least a part of the check data, which contains a security code formed with the message by the transmitter, using a secret key forming coded data (P', SC', (Y;SC)'), and - in the telegram replacing the check data, which has been encrypted, with the coded data (P', SC', (Y;SC)'), wherein the message remains unencrypted in the telegram, - encrypting the check data as a whole or at least a part of the check data, which contains a security code formed with the message by the transmitter, using a secret key forming coded data (P', SC', (Y;SC)'), and - in the telegram replacing the check data, which has been encrypted, with the coded data (P', SC', (Y;SC)'), wherein the message remains unencrypted in the telegram, - the receiver-side access protection device is used in addition to the receiver and a connection between the receiver-side access protection device and the receiver is implemented as more secure, with regard to manipulations, than the first-mentioned connection, wherein the receiver-side access protection device forms the processed telegram (T'') by - encrypting the coded data (P', SC, (Y;SC)') and - replacing the coded data (P', SC', (Y;SC)') in the telegram (T') with the encrypted coded data. - encrypting the coded data (P', SC, (Y;SC)') and - replacing the coded data (P', SC', (Y;SC)') in the telegram (T') with the encrypted coded data.