Limiting service provision to group members

A system for limiting a service to members of a group who are registered with a membership authority, the system comprising:

• a first computer entity associated with a provider of said service and arranged to encrypt data based on encryption parameters comprising public data provided by the membership authority and an encryption key string, and to provide the encrypted data to a party;
• a second computer entity associated with said party and arranged to decrypt the encrypted data using a decryption key obtained from the membership authority;
• and
• a third computing entity associated with the membership authority and comprising:
• a membership-checking arrangement for checking whether said party is registered with the authority as a member of said group,
• a key-generation arrangement for generating the decryption key in dependence on said encryption key string and private data related to said public data, and
• a control arrangement for enabling the generation of the decryption key by the key-generation arrangement and/or the provision of the decryption key to the second computer entity, only if said party is a group member as checked by the membership-checking arrangement.

A computing entity comprising:

• a first data store for holding private data;
• a second data store for holding membership data indicative of members of a group,
• a membership-checking arrangement for checking whether a particular party is a member of said group,
• a communications interface for receiving an encryption key string from a party requesting the corresponding decryption key, and for outputting the requested decryption key to the requesting party;
• a decryption-key generation unit for using the private data and a received encryption key string to generate a corresponding decryption key for decrypting data encrypted using the encryption key string and public data derived using said private data;
• a control arrangement for enabling the generation of the decryption key by the decryption-key generation arrangement and/or the provision of the decryption key to a said requesting party via said communications interface, only if that party is a group member as checked by the membership-checking arrangement.