A security system prevents a commonly shared encryption key from being
deciphered by an unwelcome party, while providing easier administration of
encryption keys. The security system includes a memorizer 2, an
encrypter/decrypter 8, a user administrator 3, a key obtainer 4, and a key
distributor 5. Once the chat client joins a channel, the user
administrator 3 obtains and stores user information from the chat server.
The user information includes a nickname list. The key obtainer 4 selects
one from other user terminals to request an encryption key therefrom. Once
the key obtainer 4 receives the encryption key sent by the selected user
terminal, the key obtainer 4 stores the encryption key in the memorizer 2.
When the user terminal receives a request for an encryption key from
another user, the key distributor retrieves and sends the encryption key
from the memorizer 2 of the requesting user terminal. The security system
1 should have a key updater 6, which updates an encryption key whenever a
predetermined trigger occurs. Thus, the encryption key is less likely to
be deciphered. Only user terminals with the key distribution and/or update
properties can distribute and/or update an encryption key. The key
distribution and update properties can be granted as part of the user
information.
