Quantum-resistant Anonymous IBE with Traceable Identities.

Peter D. Pešić

Ziyuan Liu, Yi‐Fan Tseng, Raylin Tso +2 more

Abstract Identity‐based encryption (IBE), introduced by Shamir, eliminates the need for public‐key infrastructure. The sender can simply encrypt a message by using the recipient's identity (such as email or IP address) without needing to look up the public key. In particular, when ciphertexts of an IBE do not reveal recipient's identity, this scheme is known as an anonymous IBE scheme. Recently, Blazy et al. (ARES '19) analysed the trade‐off between public safety and unconditional privacy in anonymous IBE and introduced a new notion that incorporates traceability into anonymous IBE, called anonymous IBE with traceable identities (AIBET). However, their construction is based on the discrete logarithm assumption, which is insecure in the quantum era. In this paper, we first formalize the consistency of tracing key of the AIBET scheme to ensure that a ciphertext cannot be traced with the use of wrong tracing keys. Subsequently, we present a generic formulation concept that can be used to transform structure‐specific lattice‐based anonymous IBE schemes into an AIBET. Finally, we apply this concept to Katsumata and Yamada's compact anonymous IBE scheme (Asiacrypt '16) to obtain the first quantum‐resistant AIBET scheme that is adaptively secure under the ring learning with errors assumption without random oracle.

Atsushi Takayasu

Revocable identity-based encryption (RIBE) is an extension of IBE that satisfies a key revocation mechanism to manage a number of users dynamically and efficiently. To resist quantum attacks, two adaptively secure lattice-based RIBE schemes are known in the (quantum) random oracle model ((Q)ROM). Wang et al.’s scheme that is secure in the ROM has large secret keys depending on the depth of a binary tree and its security reduction is not tight. Ma and Lin’s scheme that is secure in the QROM has large ciphertexts depending on the length of identities and is not anonymous. In this paper, we propose an adaptively secure lattice-based RIBE scheme that is secure in the QROM. Our scheme has compact parameters, where the ciphertext-size is smaller than Wang et al.’s scheme and the secret key size is the same as Ma and Lin’s scheme. Moreover, our scheme is anonymous and its security reduction is completely tight. We design the proposed scheme by modifying Ma–Lin’s scheme instantiated by the Gentry–Peikert–Vaikuntanathan (GPV) IBE. We can obtain the advantages of our scheme by making use of Katsumata et al.’s proof technique of the GPV IBE in the QROM.

Peter D. Pešić

Peter D. Pešić

C. LIN, Yong Li, Qingqing Zhang +1 more

An anonymous identity based encryption (anonymous IBE) scheme requires that an adversary can not determine the identity of the recipient from a ciphertext encrypted by the corresponding public key. The anonymity was formalized in previous works [1], [13], and this can be considered under chosen plaintext attack and adaptive chosen ciphertext attack, yielding two notions of security, ID-II-CPA and ID-II-CCA, where II denotes “indistinguishability of identities.” However, how to obtain an ID-II-CCA secure anonymous IBE in the random oracle model is still a challenging problem. We firstly propose a new notion of plaintext awareness in the two identities setting, called PATI. Secondly, we prove that the IBE scheme is ID-II-CCA secure if it is PATI secure. Finally, we propose the first generic conversion for anonymous IBE from ID-II-CPA to ID-II-CCA in the random oracle model.