Semantically Secure Authenticated Encryption Scheme and the Same Scheme for Ad-hoc Group Called a Ring

In 1994, Horster et al. first proposed an authenticated encryption scheme that can have the signature only be verified by the specified recipient while keeping the message secret from the public. Since then, several researchers proposed authenticated encryption schemes in which the recipient can convert the signature into an ordinary one for public verifiability. However, all of these schemes do not consider that once the attacker knows the message then he can also easily convert that signature into an ordinary signature. In this paper, at first we propose a new authenticated encryption scheme which prevents such an attack. We then propose the same scheme in ad-hoc groups based on the proposed authenticated encryption scheme