International Association for Cryptologic Research

International Association
for Cryptologic Research

CryptoDB

Zhengan Huang

Publications

Year
Venue
Title
2021
ASIACRYPT
Simulation-Based Bi-Selective Opening Security for Public Key Encryption
Selective opening attacks (SOA) (for public-key encryption, PKE) concern such a multi-user scenario, where an adversary adaptively corrupts some fraction of the users to break into a subset of honestly created ciphertexts, and tries to learn the information on the messages of some unopened (but potentially related) ciphertexts. Until now, the notion of selective opening attacks is only considered in two settings: sender selective opening (SSO), where part of senders are corrupted and messages together with randomness for encryption are revealed; and receiver selective opening (RSO), where part of receivers are corrupted and messages together with secret keys for decryption are revealed. In this paper, we consider a more natural and general setting for selective opening security. In the setting, the adversary may adaptively corrupt part of senders and receivers \emph{simultaneously}, and get the plaintext messages together with internal randomness for encryption and secret keys for decryption, while it is hoped that messages of uncorrupted parties remain protected. We denote it as Bi-SO security since it is reminiscent of Bi-Deniability for PKE. We first formalize the requirement of Bi-SO security by the simulation-based (SIM) style, and prove that some practical PKE schemes achieve SIM-Bi-$\text{SO}$-CCA security in the random oracle model. Then, we suggest a weak model of Bi-SO security, denoted as SIM-wBi-$\text{SO}$-CCA security, and argue that it is still meaningful and useful. We propose a generic construction of PKE schemes that achieve SIM-wBi-$\text{SO}$-CCA security in the standard model and instantiate them from various standard assumptions. Our generic construction is built on a newly presented primitive, namely, universal$_{\kappa}$ hash proof system with key equivocability, which may be of independent interest.
2020
ASIACRYPT
Possibility and Impossibility Results for Receiver Selective Opening Secure PKE in the Multi-Challenge Setting 📺
Public key encryption (PKE) schemes are usually deployed in an open system with numerous users. In practice, it is common that some users are corrupted. A PKE scheme is said to be receiver selective opening (RSO) secure if it can still protect messages transmitted to uncorrupted receivers after the adversary corrupts some receivers and learns their secret keys. This is usually defined by requiring the existence of a simulator that can simulate the view of the adversary given only the opened messages. Existing works construct RSO secure PKE schemes in a single-challenge setting, where the adversary can only obtain one challenge ciphertext for each public key. However, in practice, it is preferable to have a PKE scheme with RSO security in the multi-challenge setting, where public keys can be used to encrypt multiple messages. In this work, we explore the possibility for achieving PKE schemes with receiver selective opening security in the multi-challenge setting. Our contributions are threefold. First, we demonstrate that PKE schemes with RSO security in the single-challenge setting are not necessarily RSO secure in the multi-challenge setting. Then, we show that it is impossible to achieve RSO security for PKE schemes if the number of challenge ciphertexts under each public key is a priori unbounded. In particular, we prove that no PKE scheme can be RSO secure in the $k$-challenge setting (i.e., the adversary can obtain $k$ challenge ciphertexts for each public key) if its secret key contains less than $k$ bits. On the positive side, we give a concrete construction of PKE scheme with RSO security in the $k$-challenge setting, where the ratio of the secret key length to $k$ approaches the lower bound 1.
2018
PKC
Hedged Nonce-Based Public-Key Encryption: Adaptive Security Under Randomness Failures
Nowadays it is well known that randomness may fail due to bugs or deliberate randomness subversion. As a result, the security of traditional public-key encryption (PKE) cannot be guaranteed any more. Currently there are mainly three approaches dealing with the problem of randomness failures: deterministic PKE, hedged PKE, and nonce-based PKE. However, these three approaches only apply to different application scenarios respectively. Since the situations in practice are dynamic and very complex, it’s almost impossible to predict the situation in which a scheme is deployed, and determine which approach should be used beforehand.In this paper, we initiate the study of hedged security for nonce-based PKE, which adaptively applies to the situations whenever randomness fails, and achieves the best-possible security. Specifically, we lift the hedged security to the setting of nonce-based PKE, and formalize the notion of chosen-ciphertext security against chosen-distribution attacks (IND-CDA2) for nonce-based PKE. By presenting two counterexamples, we show a separation between our IND-CDA2 security for nonce-based PKE and the original NBP1/NBP2 security defined by Bellare and Tackmann (EUROCRYPT 2016). We show two nonce-based PKE constructions meeting IND-CDA2, NBP1 and NBP2 security simultaneously. The first one is a concrete construction in the random oracle model, and the second one is a generic construction based on a nonce-based PKE scheme and a deterministic PKE scheme.
2015
EPRINT
2013
PKC