Verifiable Encryption in Anonymous Ad Hoc Groups

In a verifiable encryption, an asymmetrically encrypted ciphertext can be publicly verified to be decypherable by a designated receiver without revealing the plaintext. In this paper, we introduce publicly verifiable encryption that is intended for a single anonymous decypherer within an ad hoc group of n receivers. The verifier can ascertain that the ciphertext can be decrypted by one receiver, but it cannot find out the identity of the targeted decypherer among the n receivers in the ad hoc group. Our scheme is spontaneous: the prover does not need the collaboration from any exterior party such as TTP, group manager, or any member of the receiver group when forming this ad hoc group of n receivers. We also introduce two extensions. In the first extension a targeted subset of t receivers jointly recover the message. In the second extension, any member of a targeted subset of t receivers can recover the encrypted message. Both extensions preserve the anonymity of the targeted subset.