Method and arrangement for communications security

In a method for establishing a secure communication in a packet based network comprising an access network (13) having access points (10) for two or more mobile terminals (11) belonging to the access network, a first access point is contacted by one mobile terminal in the intention of initiating a session from the mobile terminal. A secret key is generated using a function f stored in the access points acting on the information from the mobile terminal at the first access point by a converter known by two or more access points. The secret key is sent from the first access point to the mobile terminal using encryption, which is decrypted at the mobile terminal. The secret key is then used as a shared security key in communication between the mobile terminal and any access point knowing the converter.