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Fully homomorphic encryption using ideal lattices
 In Proc. STOC
, 2009
"... We propose a fully homomorphic encryption scheme – i.e., a scheme that allows one to evaluate circuits over encrypted data without being able to decrypt. Our solution comes in three steps. First, we provide a general result – that, to construct an encryption scheme that permits evaluation of arbitra ..."
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We propose a fully homomorphic encryption scheme – i.e., a scheme that allows one to evaluate circuits over encrypted data without being able to decrypt. Our solution comes in three steps. First, we provide a general result – that, to construct an encryption scheme that permits evaluation of arbitrary circuits, it suffices to construct an encryption scheme that can evaluate (slightly augmented versions of) its own decryption circuit; we call a scheme that can evaluate its (augmented) decryption circuit bootstrappable. Next, we describe a public key encryption scheme using ideal lattices that is almost bootstrappable. Latticebased cryptosystems typically have decryption algorithms with low circuit complexity, often dominated by an inner product computation that is in NC1. Also, ideal lattices provide both additive and multiplicative homomorphisms (modulo a publickey ideal in a polynomial ring that is represented as a lattice), as needed to evaluate general circuits. Unfortunately, our initial scheme is not quite bootstrappable – i.e., the depth that the scheme can correctly evaluate can be logarithmic in the lattice dimension, just like the depth of the decryption circuit, but the latter is greater than the former. In the final step, we show how to modify the scheme to reduce the depth of the decryption circuit, and thereby obtain a bootstrappable encryption scheme, without reducing the depth that the scheme can evaluate. Abstractly, we accomplish this by enabling the encrypter to start the decryption process, leaving less work for the decrypter, much like the server leaves less work for the decrypter in a serveraided cryptosystem.
On the Analysis of Cryptographic Assumptions in the Generic Ring Model ∗
, 2009
"... At Eurocrypt 2009 Aggarwal and Maurer proved that breaking RSA is equivalent to factoring in the generic ring model. This model captures algorithms that may exploit the full algebraic structure of the ring of integers modulo n, but no properties of the given representation of ring elements. This int ..."
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At Eurocrypt 2009 Aggarwal and Maurer proved that breaking RSA is equivalent to factoring in the generic ring model. This model captures algorithms that may exploit the full algebraic structure of the ring of integers modulo n, but no properties of the given representation of ring elements. This interesting result raises the question how to interpret proofs in the generic ring model. For instance, one may be tempted to deduce that a proof in the generic model gives some evidence that solving the considered problem is also hard in a general model of computation. But is this reasonable? We prove that computing the Jacobi symbol is equivalent to factoring in the generic ring model. Since there are simple and efficient nongeneric algorithms computing the Jacobi symbol, we show that the generic model cannot give any evidence towards the hardness of a computational problem. Despite this negative result, we also argue why proofs in the generic ring model are still interesting, and show that solving the quadratic residuosity and subgroup decision problems is generically equivalent to factoring. 1
The Generic Hardness of Subset Membership Problems under the Factoring Assumption
, 2009
"... Abstract. We analyze a large class of subset membership problems related to integer factorization. We show that there is no algorithm solving these problems efficiently without exploiting properties of the given representation of ring elements, unless factoring integers is easy. Our results imply th ..."
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Abstract. We analyze a large class of subset membership problems related to integer factorization. We show that there is no algorithm solving these problems efficiently without exploiting properties of the given representation of ring elements, unless factoring integers is easy. Our results imply that problems with high relevance for a large number of cryptographic applications, such as the quadratic residuosity and the subgroup decision problems, are generically equivalent to factoring.
Lower Bounds on BlackBox Ring Extraction
"... Abstract. The blackbox ring extraction problem is the problem of extracting a secret ring element from a blackbox by performing only the ring operations and testing for equality of elements. An efficient algorithm for the blackbox ring extraction problem implies the equivalence of the discrete lo ..."
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Abstract. The blackbox ring extraction problem is the problem of extracting a secret ring element from a blackbox by performing only the ring operations and testing for equality of elements. An efficient algorithm for the blackbox ring extraction problem implies the equivalence of the discrete logarithm and the DiffieHellman problem. At the same time this implies the inexistence of secure ringhomomorphic encryption schemes. It is unknown whether the known algorithms for the blackbox ring extraction problem are optimal. In this paper we derive exponentialtime lower complexity bounds for a large class of rings satisfying certain conditions. The existence of these bounds is surprising, having in mind that there are subexponentialtime algorithms for certain rings which are very similar to the rings considered in this work. In addition, we introduce a novel technique to reduce the problem of factoring integers to the blackbox ring extraction problem, extending previous work to a more general class of algorithms and obtaining a much tighter reduction. 1
Compressing and Disguising Elements in Discrete Logarithm Cryptography
, 2008
"... In the modern world, the ubiquity of digital communication is driven by the constantly evolving world of cryptography. Consequently one must efficiently implement asymmetric cryptography in environments which have limited resources at their disposal, such as smart–cards, ID cards, vehicular microchi ..."
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In the modern world, the ubiquity of digital communication is driven by the constantly evolving world of cryptography. Consequently one must efficiently implement asymmetric cryptography in environments which have limited resources at their disposal, such as smart–cards, ID cards, vehicular microchips and many more. It is the primary purpose of this thesis to investigate methods for reducing the bandwidth required by these devices. Part I of this thesis considers compression techniques for elliptic curve cryptography (ECC). We begin this by analysing how much data is actually required to establish domain parameters for ECC. Following the widely used cryptographic standards (for example: SEC 1), we show that naïvely implemented systems use extensively more data than is actually required and suggest a flexible and compact way to better implement these. This is especially of use in a multi–curve environment. We then investigate methods for reducing the inherent redundancy in the point representation of Koblitz systems; a by–product of the best known Pollard–ρ based attacks by Wiener & Zuccherato and Gallant, Lambert & Vanstone. We present methods which allow such systems to operate (with a high confidence) as efficiently as generic ones whilst maintaining all of their com
BLACK BOX, WHITE ARROW
"... Abstract. The present paper proposes a new and systematic approach to the socalled black box group methods in computational group theory. Instead of a single black box, we consider categories of black boxes and their morphisms. This makes new classes of black box problems accessible. For example, w ..."
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Abstract. The present paper proposes a new and systematic approach to the socalled black box group methods in computational group theory. Instead of a single black box, we consider categories of black boxes and their morphisms. This makes new classes of black box problems accessible. For example, we can enrich black box groups by actions of outer automorphisms. As an example of application of this technique, we construct Frobenius maps on black box groups of untwisted Lie type in odd characteristic (Section 6) and inversetranspose automorphisms on black box groups encrypting (P)SLn(Fq). One of the advantages of our approach is that it allows us to work in black box groups over finite fields of big characteristic. Another advantage is the explanatory power of our methods; as an example, we explain Kantor’s and Kassabov’s construction of an involution in black box groups encrypting SL2(2n). Due to the nature of our work we also have to discuss a few methodological issues of the black box group theory. 1.