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ChosenCiphertext Security from IdentityBased Encryption. Adv
 in Cryptology — Eurocrypt 2004, LNCS
, 2004
"... We propose simple and efficient CCAsecure publickey encryption schemes (i.e., schemes secure against adaptive chosenciphertext attacks) based on any identitybased encryption (IBE) scheme. Our constructions have ramifications of both theoretical and practical interest. First, our schemes give a n ..."
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Cited by 279 (14 self)
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We propose simple and efficient CCAsecure publickey encryption schemes (i.e., schemes secure against adaptive chosenciphertext attacks) based on any identitybased encryption (IBE) scheme. Our constructions have ramifications of both theoretical and practical interest. First, our schemes give a new paradigm for achieving CCAsecurity; this paradigm avoids “proofs of wellformedness ” that have been shown to underlie previous constructions. Second, instantiating our construction using known IBE constructions we obtain CCAsecure encryption schemes whose performance is competitive with the most efficient CCAsecure schemes to date. Our techniques extend naturally to give an efficient method for securing also IBE schemes (even hierarchical ones) against adaptive chosenciphertext attacks. Coupled with previous work, this gives the first efficient constructions of CCAsecure IBE schemes. 1
On the security of joint signature and encryption
, 2002
"... We formally study the notion of a joint signature and encryption in the publickey setting. We refer to this primitive as signcryption, adapting the terminology of [35]. We present two definitions for the security of signcryption depending on whether the adversary is an outsider or a legal user of t ..."
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Cited by 154 (6 self)
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We formally study the notion of a joint signature and encryption in the publickey setting. We refer to this primitive as signcryption, adapting the terminology of [35]. We present two definitions for the security of signcryption depending on whether the adversary is an outsider or a legal user of the system. We then examine generic sequential composition methods of building signcryption from a signature and encryption scheme. Contrary to what recent results in the symmetric setting [5, 22] might lead one to expect, we show that classical “encryptthensign” (EtS) and “signthenencrypt” (StE) methods are both secure composition methods in the publickey setting. We also present a new composition method which we call “committhenencryptandsign” (CtE&S). Unlike the generic sequential composition methods, CtE&S applies the expensive signature and encryption operations in parallel, which could imply a gain in efficiency over the StE and EtS schemes. We also show that the new CtE&S method elegantly combines with the recent “hashsignswitch” technique of [30], leading to efficient online/offline signcryption. Finally and of independent interest, we discuss the definitional inadequacy of the standard notion of chosen ciphertext (CCA2) security. We suggest a natural and very slight relaxation of CCA2security, which we call generalized CCA2ecurity (gCCA2). We show that gCCA2security suffices for all known uses of CCA2secure encryption, while no longer suffering from the definitional shortcomings of the latter.
Improved Efficiency for CCASecure Cryptosystems Built Using IdentityBased Encryption
, 2004
"... Recently, Canetti, Halevi, and Katz showed a general method for constructing CCAsecure encryption schemes from identitybased encryption schemes in the standard model. We improve the efficiency of their construction, and show two specific instantiations of our resulting scheme which offer the most ..."
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Cited by 88 (9 self)
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Recently, Canetti, Halevi, and Katz showed a general method for constructing CCAsecure encryption schemes from identitybased encryption schemes in the standard model. We improve the efficiency of their construction, and show two specific instantiations of our resulting scheme which offer the most efficient encryption (and, in one case, key generation) of any CCAsecure encryption scheme to date.
Perfectly OneWay Probabilistic Hash Functions
"... Probabilistic hash functions that hide all partial information on their input were recently introduced. This new cryptographic primitive can be regarded as a function that offers "perfect onewayness", in the following sense: Having access to the function value on some input is equ ..."
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Cited by 88 (10 self)
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Probabilistic hash functions that hide all partial information on their input were recently introduced. This new cryptographic primitive can be regarded as a function that offers &quot;perfect onewayness&quot;, in the following sense: Having access to the function value on some input is equivalent to having access only to an oracle that answers &quot;yes &quot; if the correct input is queried, and answers &quot;no &quot; otherwise. Constructions of this primitive (originally called oracle hashing and here renamed perfectly oneway functions) were given based on certain strong variants of the DiffieHellman assumption. In this work we present several constructions of perfectly oneway functions; some constructions are based on clawfree permutation, and others are based on any oneway permutation. One of our constructions is simple and efficient to the point of being attractive from a practical point of view.
ZeroKnowledge Sets
, 2003
"... We show how a polynomialtime prover can commit to an arbitrary finite set S of strings so that, later on, he can, for any string x, reveal with a proof whetherÜËorÜ�Ë, without revealing any knowledge beyond the verity of these membership assertions. Our method is non interactive. Given a public ran ..."
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Cited by 58 (0 self)
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We show how a polynomialtime prover can commit to an arbitrary finite set S of strings so that, later on, he can, for any string x, reveal with a proof whetherÜËorÜ�Ë, without revealing any knowledge beyond the verity of these membership assertions. Our method is non interactive. Given a public random string, the prover commits to a set by simply posting a short and easily computable message. After that, each time it wants to prove whether a given element is in the set, it simply posts another short and easily computable proof, whose correctness can be verified by any one against the public random string. Our scheme is very efficient; no reasonable prior way to achieve our desiderata existed. Our new primitive immediately extends to providing zeroknowledge “databases.”
How to Securely Outsource Cryptographic Computations
 In Theory of Cryptography (2005
"... Abstract. We address the problem of using untrusted (potentially malicious) cryptographic helpers. We provide a formal security definition for securely outsourcing computations from a computationally limited device to an untrusted helper. In our model, the adversarial environment writes the software ..."
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Cited by 47 (0 self)
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Abstract. We address the problem of using untrusted (potentially malicious) cryptographic helpers. We provide a formal security definition for securely outsourcing computations from a computationally limited device to an untrusted helper. In our model, the adversarial environment writes the software for the helper, but then does not have direct communication with it once the device starts relying on it. In addition to security, we also provide a framework for quantifying the efficiency and checkability of an outsourcing implementation. We present two practical outsourcesecure schemes. Specifically, we show how to securely outsource modular exponentiation, which presents the computational bottleneck in most publickey cryptography on computationally limited devices. Without outsourcing, a device would need O(n) modular multiplications to carry out modular exponentiation for nbit exponents. The load reduces to O(log 2 n) for any exponentiationbased scheme where the honest device may use two untrusted exponentiation programs; we highlight the CramerShoup cryptosystem [13] and Schnorr signatures [28] as examples. With a relaxed notion of security, we achieve the same load reduction for a new CCA2secure encryption scheme using only one untrusted CramerShoup encryption program. 1
Cryptography in NC0
, 2006
"... We study the parallel timecomplexity of basic cryptographic primitives such as oneway functions (OWFs) and pseudorandom generators (PRGs). Specifically, we study the possibility of implementing instances of these primitives by NC 0 functions, namely by functions in which each output bit depends on ..."
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Cited by 47 (11 self)
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We study the parallel timecomplexity of basic cryptographic primitives such as oneway functions (OWFs) and pseudorandom generators (PRGs). Specifically, we study the possibility of implementing instances of these primitives by NC 0 functions, namely by functions in which each output bit depends on a constant number of input bits. Despite previous efforts in this direction, there has been no convincing theoretical evidence supporting this possibility, which was posed as an open question in several previous works. We essentially settle this question by providing strong positive evidence for the possibility of cryptography in NC 0. Our main result is that every “moderately easy ” OWF (resp., PRG), say computable in NC 1, can be compiled into a corresponding OWF (resp., “lowstretch ” PRG) in which each output bit depends on at most 4 input bits. The existence of OWF and PRG in NC 1 is a relatively mild assumption, implied by most numbertheoretic or algebraic intractability assumptions commonly used in cryptography. A similar compiler can also be obtained for other cryptographic primitives such as oneway permutations, encryption, signatures, commitment, and collisionresistant hashing. Our techniques can also be applied to obtain (unconditional) constructions of “noncryptographic ” PRGs. In particular, we obtain ɛbiased generators and a PRG for spacebounded computation in which each output bit depends on only 3 input bits. Our results make use of the machinery of randomizing polynomials (Ishai and Kushilevitz, 41st FOCS, 2000), which was originally motivated by questions in the domain of informationtheoretic secure multiparty computation. 1
Concurrent nonmalleable commitments
 In FOCS
, 2005
"... We present a nonmalleable commitment scheme that retains its security properties even when concurrently executed a polynomial number of times. That is, a maninthemiddle adversary who is simultaneously participating in multiple concurrent commitment phases of our scheme, both as a sender and as a ..."
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Cited by 42 (14 self)
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We present a nonmalleable commitment scheme that retains its security properties even when concurrently executed a polynomial number of times. That is, a maninthemiddle adversary who is simultaneously participating in multiple concurrent commitment phases of our scheme, both as a sender and as a receiver, cannot make the values he commits to depend on the values he receives commitments to. Our result is achieved without assuming an apriori bound on the number of executions and without relying on any setup assumptions. Our construction relies on the existence of standard clawfree permutations and only requires a constant number of communication rounds. 1
Unconditionally Secure Commitment and Oblivious Transfer Schemes Using Private Channels and a Trusted Initializer
, 1999
"... We present a new and very simple commitment scheme that does not depend on any assumptions about computational complexity; the Sender and Receiver may both be computationally unbounded. Instead, the scheme utilizes a "trusted initializer " who participates only in an initial setup ..."
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Cited by 42 (0 self)
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We present a new and very simple commitment scheme that does not depend on any assumptions about computational complexity; the Sender and Receiver may both be computationally unbounded. Instead, the scheme utilizes a &quot;trusted initializer &quot; who participates only in an initial setup phase. The scheme also utilizes private channels between each pair of parties. The Sender is able to easily commit to a large value; the scheme is not just a &quot;bitcommitment &quot; scheme. We also observe that 1outofn oblivious transfer is easily handled in the same model, using a simple OT protocol due to Bennett et al.[2].
Reducing complexity assumptions for statisticallyhiding commitment
 In EUROCRYPT
, 2005
"... We revisit the following question: what are the minimal assumptions needed to construct statisticallyhiding commitment schemes? Naor et al. show how to construct such schemes based on any oneway permutation. We improve upon this by showing a construction based on any approximable preimagesize one ..."
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Cited by 36 (8 self)
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We revisit the following question: what are the minimal assumptions needed to construct statisticallyhiding commitment schemes? Naor et al. show how to construct such schemes based on any oneway permutation. We improve upon this by showing a construction based on any approximable preimagesize oneway function. These are oneway functions for which it is possible to efficiently approximate the number of preimages of a given output. A special case is the class of regular oneway functions where all points in the image of the function have the same number of preimages. We also prove two additional results related to statisticallyhiding commitment. First, we prove a (folklore) parallel composition theorem showing, roughly speaking, that the statistical hiding property of any such commitment scheme is amplified exponentially when multiple independent parallel executions of the scheme are carried out. Second, we show a compiler which transforms any commitment scheme which is statistically hiding against an honestbutcurious receiver into one which is statistically hiding even against a malicious receiver. 1