PCMAE: A Proxy Convertible Multi-AE Scheme and Its Variant

This paper presents a novel proxy convertible multi-authenticated encryption (multi-AE) scheme and its variant with message linkages. The proposed scheme allows two or more original signers to cooperatively delegate their signing power to an authorized proxy signer, such that the proxy signer can generate a valid authenticated ciphertext on behalf of the original signing group and only a designated recipient is capable of decrypting the ciphertext and verifying its embedded proxy multi-signature. Its variant with message linkages further benefits the encryption of a large message by dividing it into many smaller message blocks. The proposed proxy convertible multi-AE scheme and its variant can simultaneously fulfill the security requirements of confidentiality and authenticity. Thus, they are applicable to those group-oriented confidential applications with proxy delegation, e.g., proxy on-line auction, proxy contract signing and so on. In case of a later dispute over repudiation, our proposed scheme also allows a designated recipient to convert the ciphertext into an original proxy multi-signature for public verification. In addition, the security of confidentiality against indistinguishability under adaptive chosen-ciphertext attacks (IND-CCA2) and that of unforgeability against existential forgery under adaptive chosen-message attacks (EF-CMA) are proved in the random oracle model.