A study on encryption method in group communication using RSA accumulator

This paper proposes an efficient encryption method for a group communication in which parts of a ciphertext are decrypted selectively by a recipient´s private key. Sensitive information, e.g., personal information, in a message is browsed by several users should be disclosed only to authorized users even if any user can read the other parts of the message. The simplest way to do this is by encrypting a message for each recipient independently. This simple approach, however, is not efficient because the increase of the ciphertext size depends on the number of recipients. The proposed method uses an RSA accumulator to calculate shared keys and uses a randomness re-using encryption scheme (RR-MRES) to encrypt a message. Thanks to the quasi-commutativity of RSA accumulators, and the efficiency of RR-MRESs, the proposed method reduces the ciphertext size needed for selective disclosure.