## CryptoDB

### Keisuke Tanaka

#### Affiliation: Tokyo Institute of Technology

#### Publications

**Year**

**Venue**

**Title**

2019

CRYPTO

CCA Security and Trapdoor Functions via Key-Dependent-Message Security
Abstract

We study the relationship among public-key encryption (PKE) satisfying indistinguishability against chosen plaintext attacks (IND-CPA security), that against chosen ciphertext attacks (IND-CCA security), and trapdoor functions (TDF). Specifically, we aim at finding a unified approach and some additional requirement to realize IND-CCA secure PKE and TDF based on IND-CPA secure PKE, and show the following two main results.As the first main result, we show how to achieve IND-CCA security via a weak form of key-dependent-message (KDM) security. More specifically, we construct an IND-CCA secure PKE scheme based on an IND-CPA secure PKE scheme and a secret-key encryption (SKE) scheme satisfying one-time KDM security with respect to projection functions (projection-KDM security). Projection functions are very simple functions with respect to which KDM security has been widely studied. Since the existence of projection-KDM secure PKE implies that of the above two building blocks, as a corollary of this result, we see that the existence of IND-CCA secure PKE is implied by that of projection-KDM secure PKE.As the second main result, we extend the above construction of IND-CCA secure PKE into that of TDF by additionally requiring a mild requirement for each building block. Our TDF satisfies adaptive one-wayness. We can instantiate our TDF based on a wide variety of computational assumptions. Especially, we obtain the first TDF (with adaptive one-wayness) based on the sub-exponential hardness of the constant-noise learning-parity-with-noise (LPN) problem.

2019

CRYPTO

Adaptively Secure and Succinct Functional Encryption: Improving Security and Efficiency, Simultaneously
Abstract

Functional encryption (FE) is advanced encryption that enables us to issue functional decryption keys where functions are hardwired. When we decrypt a ciphertext of a message m by a functional decryption key where a function f is hardwired, we can obtain f(m) and nothing else. We say FE is selectively or adaptively secure when target messages are chosen at the beginning or after function queries are sent, respectively. In the weakly-selective setting, function queries are also chosen at the beginning. We say FE is single-key/collusion-resistant when it is secure against adversaries that are given only-one/polynomially-many functional decryption keys, respectively. We say FE is sublinearly-succinct/succinct when the running time of an encryption algorithm is sublinear/poly-logarithmic in the function description size, respectively.In this study, we propose a generic transformation from weakly-selectively secure, single-key, and sublinearly-succinct (we call “building block”) PKFE for circuits into adaptively secure, collusion-resistant, and succinct (we call “fully-equipped”) one for circuits. Our transformation relies on neither concrete assumptions such as learning with errors nor indistinguishability obfuscation (IO). This is the first generic construction of fully-equipped PKFE that does not rely on IO.As side-benefits of our results, we obtain the following primitives from the building block PKFE for circuits: (1) laconic oblivious transfer (2) succinct garbling scheme for Turing machines (3) selectively secure, collusion-resistant, and succinct PKFE for Turing machines (4) low-overhead adaptively secure traitor tracing (5) key-dependent message secure and leakage-resilient public-key encryption. We also obtain a generic transformation from simulation-based adaptively secure garbling schemes that satisfy a natural decomposability property into adaptively indistinguishable garbling schemes whose online complexity does not depend on the output length.

2018

PKC

Key Dependent Message Security and Receiver Selective Opening Security for Identity-Based Encryption
Abstract

We construct two identity-based encryption (IBE) schemes. The first one is IBE satisfying key dependent message (KDM) security for user secret keys. The second one is IBE satisfying simulation-based receiver selective opening (RSO) security. Both schemes are secure against adaptive-ID attacks and do not have any a-priori bound on the number of challenge identities queried by adversaries in the security games. They are the first constructions of IBE satisfying such levels of security.Our constructions of IBE are very simple. We construct KDM secure IBE by transforming KDM secure secret-key encryption using IBE satisfying only ordinary indistinguishability against adaptive-ID attacks (IND-ID-CPA security). Our simulation-based RSO secure IBE is based only on IND-ID-CPA secure IBE.We also demonstrate that our construction technique for KDM secure IBE is used to construct KDM secure public-key encryption. More precisely, we show how to construct KDM secure public-key encryption from KDM secure secret-key encryption and public-key encryption satisfying only ordinary indistinguishability against chosen plaintext attacks.

2018

PKC

Simple and Generic Constructions of Succinct Functional Encryption
Abstract

We propose simple and generic constructions of succinct functional encryption. Our key tool is exponentially-efficient indistinguishability obfuscator (XIO), which is the same as indistinguishability obfuscator (IO) except that the size of an obfuscated circuit (or the running-time of an obfuscator) is slightly smaller than that of a brute-force canonicalizer that outputs the entire truth table of a circuit to be obfuscated. A “compression factor” of XIO indicates how much XIO compresses the brute-force canonicalizer. In this study, we propose a significantly simple framework to construct succinct functional encryption via XIO and show that XIO is a powerful enough to achieve cutting-edge cryptography. In particular, we prove the followings:Single-key weakly succinct secret-key functional encryption (SKFE) is constructed from XIO (even with a bad compression factor) and one-way function.Single-key weakly succinct public-key functional encryption (PKFE) is constructed from XIO with a good compression factor and public-key encryption.Single-key weakly succinct PKFE is constructed from XIO (even with a bad compression factor) and identity-based encryption.
Our new framework has side benefits. Our constructions do not rely on any number theoretic or lattice assumptions such as decisional Diffie-Hellman and learning with errors assumptions. Moreover, all security reductions incur only polynomial security loss. Known constructions of weakly succinct SKFE or PKFE from XIO with polynomial security loss rely on number theoretic or lattice assumptions.

2018

ASIACRYPT

A Framework for Achieving KDM-CCA Secure Public-Key Encryption
Abstract

We propose a framework for achieving a public-key encryption (PKE) scheme that satisfies key dependent message security against chosen ciphertext attacks (KDM-CCA security) based on projective hash function. Our framework can be instantiated under the decisional diffie-hellman (DDH), quadratic residuosity (QR), and decisional composite residuosity (DCR) assumptions. The constructed schemes are KDM-CCA secure with respect to affine functions and compatible with the amplification method shown by Applebaum (EUROCRYPT 2011). Thus, they lead to PKE schemes satisfying KDM-CCA security for all functions computable by a-priori bounded size circuits. They are the first PKE schemes satisfying such a security notion in the standard model using neither non-interactive zero knowledge proof nor bilinear pairing. The above framework based on projective hash function captures only KDM-CCA security in the single user setting. However, we can prove the KDM-CCA security in the multi user setting of our concrete instantiations by using their algebraic structures explicitly. Especially, we prove that our DDH based scheme satisfies KDM-CCA security in the multi user setting with the same parameter setting as in the single user setting.

2016

ASIACRYPT

2010

EPRINT

Security of Encryption Schemes in Weakened Random Oracle Models
Abstract

Liskov proposed several weakened versions of the random oracle model, called {\em weakened random oracle models} (WROMs), to capture the vulnerability of ideal compression functions, which are expected to have the standard security of hash functions, i.e., collision resistance, second-preimage resistance, and one-wayness properties. The WROMs offer additional oracles to break such properties of the random oracle. In this paper, we investigate whether public-key encryption schemes in the random oracle model essentially require the standard security of hash functions by the WROMs. In particular, we deal with four WROMs associated with the standard security of hash functions; the standard, collision tractable, second-preimage tractable, first-preimage tractable ones (ROM, CT-ROM, SPT-ROM, and FPT-ROM, respectively), done by Numayama et al. for digital signature schemes in the WROMs. We obtain the following results: (1) The OAEP is secure in all the four models. (2) The encryption schemes obtained by the Fujisaki-Okamoto conversion (FO) are secure in the SPT-ROM. However, some encryption schemes with FO are insecure in the FPT-ROM. (3) We consider two artificial variants wFO and dFO of FO for separation of the WROMs in the context of encryption schemes. The encryption schemes with wFO (dFO, respectively) are secure in the CT-ROM (ROM, respectively). However, some encryption schemes obtained by wFO (dFO, respectively) are insecure in the SPT-ROM (CT-ROM, respectively). These results imply that standard encryption schemes such as the OAEP and FO-based one do not always require the standard security of hash functions. Moreover, in order to make our security proofs complete, we construct an efficient sampling algorithm for the binomial distribution with exponentially large parameters, which was left open in Numayama et al.'s paper.

2008

ASIACRYPT

#### Program Committees

- Crypto 2019
- PKC 2010

#### Coauthors

- Keita Emura (1)
- Goichiro Hanaoka (4)
- Ryotaro Hayashi (3)
- Ai Ishida (1)
- Toshiyuki Isshiki (1)
- Akinori Kawachi (4)
- Fuyuki Kitagawa (7)
- Toshihide Matsuda (1)
- Takahiro Matsuda (4)
- Ryo Nishimaki (4)
- Akira Numayama (3)
- Tatsuaki Okamoto (2)
- Yusuke Sakai (1)
- Damien Stehlé (1)
- Ron Steinfeld (1)
- Shigenori Uchiyama (1)
- Yuyu Wang (2)
- Keita Xagawa (5)
- Takashi Yamakawa (1)
- Zongyang Zhang (1)