Provides encryption methods, and systems and apparatus corresponding decryption method systems and apparatus in which terminals belonging to a subset selected as a recipient group can collaborate to decrypt encrypted information. In an example embodiment, a sender and recipients communicate information over a network. The sender sends information encrypted by using a group key that can be decrypted by collaboration among a given number of recipients to the recipients in a predetermined recipient group. On the other hand, the recipients receive encrypted data from the sender, exchange partial information concerning the encrypted data among a plurality of recipients in the recipient group to obtain decryption information used for decrypting the encrypted data, and decrypt the sent information by using the decryption information.