A method and system for transmitting a message in real time between users in a closed network of a vehicle (1), in particular a rail-mounted vehicle, are provided, wherein safety-relevant real-time data (D) of a message (N), including a transmitter ID of a transmitter (3) within the closed network, is encrypted (S1) using a private key (KprivA) of the transmitter (3) in order to generate an encrypted message (N′); the encrypted message (N′), together with the unencrypted transmitter ID of the transmitter (3), is transmitted (S2) from the transmitter (3) to a receiver (4) within the closed network via a vehicle bus (2), and the encrypted message (N′) is decrypted by the receiver (4) using a public key (KpublicA) of the transmitter (3) identified by the received, unencrypted transmitter ID in order to retrieve the unencrypted message (N). The received transmitter ID is compared with the transmitter ID contained in the retrieved message (N) in order to determine whether the message (N) has been transmitted correctly.