Browsing by Author "Chu, Cheng-Kang"

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Conditional oblivious cast
(2006)
Chu, Cheng-Kang
;
Tzeng, Wen-Guey
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資訊工程學系
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Department of Computer Science
We introduce a new notion of conditional oblivious cast (COC), which involves three parties: a sender S and two receivers A and B. Receivers A and B own their secrets x and y, respectively, and the sender S holds the message m. In a COC scheme for the predicate Q (Q-COC), A and B send x and y in a masked form to S, and then S sends m to A and B such that they get m if and only if Q(x, y) = 1. Besides, the secrets x and y can not be revealed to another receiver nor the sender. We also extend COC to 1-out-of-2 COC (COC(2)/(1)) in which S holds two messages m(0) and m(1), and A and B get m(1) if Q(x, y) = 1 and m(0) otherwise. We give the definitions for COC and COC(2)/(1), and propose several COC and COC(2)/(1) schemes for "equality", "inequality", and "greater than" predicates. These are fundamental schemes that are useful in constructing more complex secure interactive protocols. Our schemes are efficiently constructed via homomorphic encryption schemes and proved secure under the security of these encryption schemes.
Efficient Identity-Committable Signature and Group-Oriented Ring Signature Schemes
(2009-09-01)
Chu, Cheng-Kang
;
Tzeng, Wen-Guey
;
資訊工程學系
;
Department of Computer Science
The identity of "Deep Throat", a pseudonym of the information source in the Watergate scandal, remained mysterious for more than three decades. In 2005, an ex-FBI official claimed that he was the anonymous source. Nevertheless, some are still inconvinced. In this paper, we introduce a new notion of identity-committable signatures (ICS) to ensure the anonymity of "Deep Throat" inside a group. A member of an organization can sign a message on behalf of himself (regular signature) or the organization (identity-committed signature). In the latter case, the signer's identity is hidden from anyone, and can be opened by himself only. We describe the requirements of ICS and give the formal definition of it. Then we extend the notion of ICS to group-oriented ring signatures (GRS) which further allow the signer to hide his identity behind multiple groups. Since the signer can include the whole members of a group at a time, our GRS scheme is more efficient and practical than general ring signature schemes. Finally, we provide concrete constructions of ICS and GRS with information-theoretic anonymity, that is, the identity of the signer is fully-protected.
Efficient k-out-of-n oblivious transfer schemes
(2008)
Chu, Cheng-Kang
;
Tzeng, Wen-Guey
;
交大名義發表
;
National Chiao Tung University
Oblivious transfer is an important cryptographic protocol in various security applications. For example, in on-line transactions, a k-out-of-n oblivious transfer scheme allows a buyer to privately choose k-out of n digital goods from a merchant without learning information about other n-k goods. In this paper, we propose several efficient two-round k-out-of-n oblivious transfer schemes, in which the receiver R sends O(k) messages to the sender S, and S sends O( n) messages back to R. The schemes provide unconditional security for either sender or receiver. The computational security for the other side is based on the Decisional Diffie-Hellman (DDH) or Chosen-Target Computational Diffie-Hellman (CT-CDH) problems. Our schemes have the nice property of universal parameters, that is, each pair of R and S need not hold any secret before performing the protocol. The system parameters can be used by all senders and receivers without any trapdoor specification. In some cases, our OT(n)(k) schemes are the most efficient ones in terms of the communication cost, either in rounds or the number of messages. Moreover, one of our schemes is extended to an adaptive oblivious transfer scheme. In that scheme, S sends O( n) messages to R in one round in the commitment phase. For each query of R, only O( 1) messages are exchanged and O( 1) operations are performed. The preliminary version of this paper was published at PKC ' 05 [ Chu and Tzeng 2005].
Identity-based proxy re-encryption without random oracles
(2007)
Chu, Cheng-Kang
;
Tzeng, Wen-Guey
;
交大名義發表
;
資訊工程學系
;
National Chiao Tung University
A proxy re-encryption scheme allows Alice to temporarily delegate the decryption rights to Bob via a proxy. Alice gives the proxy a re-encryption key so that the proxy can convert a ciphertext for Alice into the ciphertext for Bob. In this paper, we propose two identity-based proxy re-encryption schemes, which are both proved secure in the standard model. The first one is efficient in both computation and ciphertext length, and the other one achieves chosen-ciphertext security. Our solutions answer the open problems left in the previous work.
Identity-committable signatures and their extension to group-oriented ring signatures
(2007)
Chu, Cheng-Kang
;
Tzeng, Wen-Guey
;
資訊工程學系
;
Department of Computer Science
The identity of "Deep Throat", a pseudonym of the information source in the Watergate scandal, remained mysterious for more than three decades. In 2005, an ex-FBI official claimed that he was the anonymous source. Nevertheless, some are still inconvinced. In this paper, we introduce a new notion of identity-committable signatures (ICS) to ensure the anonymity of "Deep Throat" inside a group. A member of an organization can sign a message on behalf of himself (regular signature) or the organization (identity-committed signature). In the latter case, the signer's identity is hidden from anyone, and can be opened by himself only. We describe the requirements of ICS and give the formal definition of it. Then we extend the notion of ICS to group-oriented ring signatures (GRS) which further allow the signer to hide his identity behind multiple groups. We believe a GRS scheme is more efficient and practical than a ring signature scheme for leaking secrets. Finally, we provide concrete constructions of ICS and GRS with information-theoretic anonymity, that is, the identity of the signer is fully-protected.
Key-Aggregate Cryptosystem for Scalable Data Sharing in Cloud Storage
(2014-02-01)
Chu, Cheng-Kang
;
Chow, Sherman S. M.
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Tzeng, Wen-Guey
;
Zhou, Jianying
;
Deng, Robert H.
Data sharing is an important functionality in cloud storage. In this paper, we show how to securely, efficiently, and flexibly share data with others in cloud storage. We describe new public-key cryptosystems that produce constant-size ciphertexts such that efficient delegation of decryption rights for any set of ciphertexts are possible. The novelty is that one can aggregate any set of secret keys and make them as compact as a single key, but encompassing the power of all the keys being aggregated. In other words, the secret key holder can release a constant-size aggregate key for flexible choices of ciphertext set in cloud storage, but the other encrypted files outside the set remain confidential. This compact aggregate key can be conveniently sent to others or be stored in a smart card with very limited secure storage. We provide formal security analysis of our schemes in the standard model. We also describe other application of our schemes. In particular, our schemes give the first public-key patient-controlled encryption for flexible hierarchy, which was yet to be known.