Different levels of security are provided in a security system so that users can decide the security level of their own communications. Users can choose a low level of security and maintain the security overhead as low as possible. Alternatively, they can choose higher levels of security with attendant increases in security overhead. The different levels of security are created by the use of one or more of two keys: an encryption key is used to encrypt plaintext data in a delta and a message authentication key is used to authenticate and insure integrity of the data. Two keys are used to avoid re-encrypting the encrypted data for each member of the telespace. In one embodiment, the security level is determined when a telespace is created and remains fixed through out the life of the telespace. For a telespace, the security level may range from no security at all to security between the members of the telespace and outsiders to security between pairs of members of the telespace. In another embodiment, subgroups called "tribes" can be formed within a telespace and each tribe adopts the security level of the telespace in which it resides.