International Association for Cryptologic Research

International Association
for Cryptologic Research


Jun Furukawa


Universally Composable Undeniable Signature
Kaoru Kurosawa Jun Furukawa
How to define the security of undeniable signature schemes is a challenging task. This paper presents two security definitions of undeniable signature schemes which are more useful or natural than the existing definition. It then proves their equivalence. We first define the UC-security, where UC means universal composability. We next show that there exists a UC-secure undeniable signature scheme which does not satisfy the standard definition of security that has been believed to be adequate so far. More precisely, it does not satisfy the invisibility defined by \cite{DP96}. We then show a more adequate definition of invisibility which captures a wider class of (naturally secure) undeniable signature schemes. We finally prove that the UC-security against non-adaptive adversaries is equivalent to this definition of invisibility and the strong unforgeability in $\cF_{ZK}$-hybrid model, where $\cF_{ZK}$ is the ideal ZK functionality. Our result of equivalence implies that all the known proven secure undeniable signature schemes (including Chaum's scheme) are UC-secure if the confirmation/disavowal protocols are both UC zero-knowledge.
Identity-Based Broadcast Encryption
Ryuichi Sakai Jun Furukawa
Broadcast encryption schemes enable senders to efficiently broadcast ciphertexts to a large set of receivers in a way that only non-revoked receivers can decrypt them. Identity-based encryption schemes are public key encryption schemes that can use arbitrary strings as public keys. We propose the first public key broadcast encryption scheme that can use any string as a public key of each receiver. That is, identity-based broadcast encryption scheme. Our scheme has many desirable properties. The scheme is fully collusion resistant, and the size of ciphertexts and that of private key are small constants. The size of public key is proportional to only the maximum number of receiver sets to each of which the ciphertext is sent. Note that its size remains to be so although the number of potential receivers is super-polynomial size. Besides these properties, the achieving the first practical identity-based broadcast encryption scheme itself is the most interesting point of this paper. The security of our scheme is proved in the generic bilinear group model.
Efficient Identity-Based Encryption with Tight Security Reduction
In a famous paper of Crypto'01, Boneh and Franklin proposed the first identity-based encryption scheme (IBE), around fifteen years after the concept was introduced by Shamir. Their scheme security (more precisely, the notion of resistance against an IND-ID-CCA attacker) relies in the random oracle model. However, the reduction is far from being tight, and notably depends on the number of extractions queries. In this paper, we present an efficient modification to the Boneh-Franklin scheme that provides a tight reduction. Our scheme is basically an IBE under two keys, one of which is (randomly) detained by the recipient. It can be viewed as a continuation of an idea introduced by Katz and Wang; we will however show how our construction improves this last scheme. Our scheme features a tight reduction to the list bilinear Diffie-Hellman (LBDH) problem, which can be itself reduced tightly either to the gap bilinear Diffie-Hellman (GBDH) or the decisional bilinear Diffie-Hellman (DBDH) problems. Furthermore, for a relaxed notion of tightness (called weak-tightness) that we introduce and discuss in our paper, we show that there is a weakly tight reduction from our scheme to the computational bilinear Diffie-Hellman (CBDH) problem. Our scheme is very efficient, as one can precompute most of the quantity involved in the encryption process. Furthermore, the ciphertext size is very short: for proposed parameters, they are |M|+330 bits long.

Program Committees

Asiacrypt 2020
Asiacrypt 2019
Asiacrypt 2009