A method, by a data system, of anonymizing personalized event data, comprising the steps of: - receiving the personalized event data, the personalized event data containing an encrypted personal identifier and event data, - determining, whether an entry exists for said encrypted personal identifier in a key database of the data system, in which the encrypted personal identifier is associated with a system identifier which uniquely identifies the encrypted personal identifier in the data system, wherein each encrypted personal identifier is uniquely associated to one system identifier, wherein, if no entry is detected in the key database for the received encrypted personal identifier, - generating a new entry for the key database in which the received encrypted personal identifier is uniquely associated to a new system identifier, - storing the received encrypted personal identifier associated to the new system identifier as the new entry in the key database, - storing the new system identifier associated to the event data contained in the personalized event data in an event database, the event database storing system identifiers in association with the corresponding event data, characterised in that the step of determining, whether an entry exists for said encrypted personal identifier in the key database, is carried out on a basis of a ciphertext of the encrypted personal identifier, wherein the ciphertext of received encrypted personal identifier is further encrypted by the data system using asymmetric encryption before it is determined whether an entry exists in the key database for said encrypted personal identifier, wherein the encrypted personal identifiers contained in the entries of the key database were encrypted by the data system using the same asymmetric encryption before storing, wherein a key used for asymmetric encryption of the ciphertext is automatically deleted when a request for a data output other than the new system identifier or event data is detected. of anonymizing personalized event data, comprising the steps of: - receiving the personalized event data, the personalized event data containing an encrypted personal identifier and event data, - determining, whether an entry exists for said encrypted personal identifier in a key database of the data system, in which the encrypted personal identifier is associated with a system identifier which uniquely identifies the encrypted personal identifier in the data system, wherein each encrypted personal identifier is uniquely associated to one system identifier, - receiving the personalized event data, the personalized event data containing an encrypted personal identifier and event data, - determining, whether an entry exists for said encrypted personal identifier in a key database of the data system, in which the encrypted personal identifier is associated with a system identifier which uniquely identifies the encrypted personal identifier in the data system, wherein each encrypted personal identifier is uniquely associated to one system identifier, wherein, if no entry is detected in the key database for the received encrypted personal identifier, - generating a new entry for the key database in which the received encrypted personal identifier is uniquely associated to a new system identifier, - storing the received encrypted personal identifier associated to the new system identifier as the new entry in the key database, - storing the new system identifier associated to the event data contained in the personalized event data in an event database, the event database storing system identifiers in association with the corresponding event data, characterised in that the step of determining, whether an entry exists for said encrypted personal identifier in the key database, is carried out on a basis of a ciphertext of the encrypted personal identifier, - generating a new entry for the key database in which the received encrypted personal identifier is uniquely associated to a new system identifier, - storing the received encrypted personal identifier associated to the new system identifier as the new entry in the key database, - storing the new system identifier associated to the event data contained in the personalized event data in an event database, the event database storing system identifiers in association with the corresponding event data, characterised in that the step of determining, whether an entry exists for said encrypted personal identifier in the key database, is carried out on a basis of a ciphertext of the encrypted personal identifier, wherein the ciphertext of received encrypted personal identifier is further encrypted by the data system using asymmetric encryption before it is determined whether an entry exists in the key database for said encrypted personal identifier, wherein the encrypted personal identifiers contained in the entries of the key database were encrypted by the data system using the same asymmetric encryption before storing, wherein a key used for asymmetric encryption of the ciphertext is automatically deleted when a request for a data output other than the new system identifier or event data is detected.