System for providing a user with a respective access to different services of at least one service provider in a network considering privacy and security via one sole user-related unique digital identifier (D-ID), the system comprising at least a D-ID middleware (100, 320, 400, 520, 620) and a D-ID-agent (310, 510, 610), wherein: - the D-ID agent (310, 510, 610) is at least partly run on any desired terminal device of the user and is configured to generate the D-ID, at least one pseudonym for the user and a user-defined and pseudonym-specific number of secrets, to compute, using the number of secrets and a cryptographic hash function, a root value of a pseudonym-specific Merkle-tree (200, 650) having the secrets as its leafs, to transmit the at least one pseudonym and the corresponding root value, both encrypted using an authentication and encryption method, to the D-ID middleware (100, 320, 400, 520, 620), and to use a secret of the number of secrets as needed to access one desired service of the different services of the at least one service provider (SP1, SP2, SP3, SP630), - the D-ID middleware (100, 320, 400, 520, 620) is run by an operator and constitutes the middleware between the terminal device of the user and the at least one service provider with respect to a verification of the D-ID of the user, the D-ID middleware (100, 320, 400, 520, 620) is configured to receive and maintain the D-ID of the user, a Merkle-tree root element for each of the at least one pseudonym for the user, the root element comprising the respective pseudonym and the corresponding root value, and a trust level for each root element, wherein, whenever the user intends to access the one desired service of the different services of the at least one service provider (SP1, SP2, SP3, SP630), the user selects one of the at least one pseudonym and the D-ID agent (310, 510, 610) reveals to the service provider (SP1, SP2, SP3, SP630) a secret of the pseudonym-specific number of secrets and a corresponding authentication path as derived from the pseudonym-specific Merkle-tree (200, 650), the service provider (SP1, SP2, SP3, SP630) forwards the pseudonym to the D-ID middleware (100, 320, 400, 520, 620) in order to receive the corresponding root value and trust level, and computes based on the secret and the authentication path a root value and verifies if it is matching with the root value it has received from the D-ID middleware (100, 320, 400, 520, 620) for the pseudonym, thus verifying that the user is the owner of the pseudonym and if yes, allowing to the user access to the one desired service of the different services in the case that the trust level of the selected pseudonym corresponds to that one required by the at least one service provider (SP1, SP2, SP3, SP630). - the D-ID agent (310, 510, 610) is at least partly run on any desired terminal device of the user and is configured to generate the D-ID, at least one pseudonym for the user and a user-defined and pseudonym-specific number of secrets, to compute, using the number of secrets and a cryptographic hash function, a root value of a pseudonym-specific Merkle-tree (200, 650) having the secrets as its leafs, to transmit the at least one pseudonym and the corresponding root value, both encrypted using an authentication and encryption method, to the D-ID middleware (100, 320, 400, 520, 620), and to use a secret of the number of secrets as needed to access one desired service of the different services of the at least one service provider (SP1, SP2, SP3, SP630), - the D-ID middleware (100, 320, 400, 520, 620) is run by an operator and constitutes the middleware between the terminal device of the user and the at least one service provider with respect to a verification of the D-ID of the user, the D-ID middleware (100, 320, 400, 520, 620) is configured to receive and maintain the D-ID of the user, a Merkle-tree root element for each of the at least one pseudonym for the user, the root element comprising the respective pseudonym and the corresponding root value, and a trust level for each root element,