This paper proposes a flexible tree-based key management framework for a terminal to connect with multiple content distribution systems (called as CDSs in this paper). In an existing tree-based key management scheme, a terminal keeps previously distributed node keys which are used for decrypting contents from a CDS. According to our proposal, the terminal can calculate its node keys of a selected CDS as the need arises, using the "public bulletin board" of the CDS. The public bulletin board is generated by a management center of the individual CDS, depending on a tree structure which it determines in its convenience. After the terminal calculates its node keys, it can get a content of the CDS using the calculated node keys.

In this paper, we propose a similarity searchable encryption in the symmetric key setting for the weighted Euclidean distance, by extending the functional encryption scheme for inner product proposed by Bishop et al. [4]. Our scheme performs predetermined encoding independently of vectors x and y, and it obtains the weighted Euclidean distance between the two vectors while they remain encrypted.

This paper proposes a group key distribution scheme with a user exclusion. The user exclusion is how to distribute an encryption key over a broadcast channel shared by n users so that all but d excluded users can get the group key. In the broadcast channel such as Pay-TV, Internet multicast and a mobile telecommunication for a group, a manager should exclude a dishonest user or an unauthorized terminal as soon as possible to protect the secrecy of the group communication. However, it takes a long time for the user exclusion on a large group, if the distributor distributes the group key to each user except the excluded one. We propose a scheme in which the amount of transmission and the key storage of each user do not depend on the number of users of the group. Moreover, our scheme does not require a fixed and privileged distributor.