Method for establishing a secure data communication between a plurality of controllers in each of a plurality of elevators of an elevator group, wherein the plurality of elevators has been initially installed without any encryption being established in communicating data between their controllers, wherein before commissioning the elevator group, an encryption establishing step is executed for establishing data encryption in future data communication between the plurality of controllers, the encryption establishing step comprising: - initiating, upon receiving a "prepare security" command, generation of a random encryption key pair in each of the controllers of the elevator group, the encryption key pair comprising a private key and a public key; - each of the controllers storing its generated private key in a protected area in a file system and advertising its identity and its public key to the controllers of all other elevators of the elevator group; - each of the controllers storing the public keys obtained upon advertising from the other controllers in the elevator group; - finalizing the encryption establishing step upon one of lapsing of a predetermined time period since starting the encryption establishing step and occurrence of a finalising event; wherein, after finalizing the encryption establishing step, no obtaining and storing of further public keys is accepted in each of the controllers and future data communication between the plurality of controllers is established by encrypting a data communication package by a sending controller using the public encryption key of a receiving controller and decrypting the data communication package upon receiving it in the receiving controller using the private key stored in the receiving controller. wherein the plurality of elevators has been initially installed without any encryption being established in communicating data between their controllers, wherein before commissioning the elevator group, an encryption establishing step is executed for establishing data encryption in future data communication between the plurality of controllers, the encryption establishing step comprising: - initiating, upon receiving a "prepare security" command, generation of a random encryption key pair in each of the controllers of the elevator group, the encryption key pair comprising a private key and a public key; - each of the controllers storing its generated private key in a protected area in a file system and advertising its identity and its public key to the controllers of all other elevators of the elevator group; - each of the controllers storing the public keys obtained upon advertising from the other controllers in the elevator group; - finalizing the encryption establishing step upon one of lapsing of a predetermined time period since starting the encryption establishing step and occurrence of a finalising event; - initiating, upon receiving a "prepare security" command, generation of a random encryption key pair in each of the controllers of the elevator group, the encryption key pair comprising a private key and a public key; - each of the controllers storing its generated private key in a protected area in a file system and advertising its identity and its public key to the controllers of all other elevators of the elevator group; - each of the controllers storing the public keys obtained upon advertising from the other controllers in the elevator group; - finalizing the encryption establishing step upon one of lapsing of a predetermined time period since starting the encryption establishing step and occurrence of a finalising event; wherein, after finalizing the encryption establishing step, no obtaining and storing of further public keys is accepted in each of the controllers and future data communication between the plurality of controllers is established by encrypting a data communication package by a sending controller using the public encryption key of a receiving controller and decrypting the data communication package upon receiving it in the receiving controller using the private key stored in the receiving controller.