Methods (500) of a network node (111) for creating and joining secure sessions for members (111-114) of a group of network nodes are provided. The methods comprise receiving an identity certificate and an assertion for the network node as well as a secret group key for the group. The method for creating a session further comprises creating (501) a session identifier and a secret session key for the session, and sending (502) an encrypted and authenticated broadcast message comprising the session identifier. The method for joining a session further comprises sending an encrypted and authenticated discovery message comprising the identity certificate and the assertion, and receiving an encrypted and authenticated discovery response message from another network node which is a member of the group. The disclosed combined symmetric key and public key scheme is based on the availability of three credentials at each node, i.e., the identity certificate, the assertion, and the secret group key, which are received from a trusted entity. Further, a computer program, a computer program product, and a network node are provided.