УЗАГАЛЬНЕННЯ НЕКОМУТАТИВНОГО ПРОТОКОЛУ УЗГОДЖЕННЯ КЛЮЧА

The concept of public key cryptography was first introduced by Diffie and Hellman in 1976 using discrete logarithm problem as base of difficulty. In 1985, T. ElGamal proposed public key cryptosystem scheme based on discrete logarithm problem. Elliptic curve cryptosystems were first proposed in 1985 independently by Neil Koblitz and Victor Miller. Elliptic curve cryptosystems are unique in using elliptic curve groups for arithmetic. This cryptosystem is based on discrete logarithm problem in the group of points of an elliptic curve defined over a finite field. The discrete logarithm problem in an elliptic curve group appears to be much harder than the discrete logarithm problem in other groups. Hence elliptic curves cryptosystem can match the security of other cryptosystems while using smaller key. In this paper we will discuss a VLSl implementation of elliptic curves cryptosystem for ElGamal encryption scheme.

We show that finding an efficiently computable injective homomorphism from the XTR subgroup into the group of points over GF(p2) of a particular type of supersingular elliptic curve is at least as hard as solving the Diffie–Hellman problem in the XTR subgroup. This provides strong evidence for a negative answer to the question posed by Vanstone and Menezes at the Crypto 2000 Rump Session on the possibility of efficiently inverting the MOV embedding into the XTR subgroup. As a side result we show that the Decision Diffie–Hellman problem in the group of points on this type of supersingular elliptic curves is efficiently computable, which provides an example of a group where the Decision Diffie–Hellman problem is simple, while the Diffie–Hellman and discrete logarithm problems are presumably not. So-called distortion maps on groups of points on elliptic curves that play an important role in our cryptanalysis also lead to cryptographic applications of independent interest. These applications are an improvement of Joux’s one round protocol for tripartite Diffie–Hellman key exchange and a non-refutable digital signature scheme that supports escrowable encryption. We also discuss the applicability of our methods to general elliptic curves defined over finite fields which includes a classification of elliptic curve groups where distortion maps exist.

There have been many digital signature schemes were developed based on the discrete logarithm problem on a finite field. In this study, we use the elliptic curve discrete logarithm problem to build new collective signature schemes. The cryptosystem on elliptic curve allows to generate digital signatures with the same level of security as other cryptosystems but with smaller keys. To extend practical applicability and enhance the security level of the group signature protocols, we propose two new types of collective digital signature schemes based on the discrete logarithm problem on the elliptic curve: i) the collective digital signature scheme shared by several signing groups and ii) the collective digital signature scheme shared by several signing groups and several individual signers. These two new types of collective signatures have combined the advantages of group digital signatures and collective digital signatures. These signatures have a fixed size and do not depend on the number of members participating in the creation of the final collective signature. One of the advantages of the proposed collective signature protocols is that they can be deployed on top of the available public key infrastructures.

The main topic of this thesis is related to the state of the art in designing cryptographic primitives from a hardware point of view. A special emphasis is dedicated to low-power/low-energy CMOS design. A set of solutions is proposed including an LFSR based stream cipher with self-synchronizing capabilities, a new memory-less Rijndael block cipher architecture and a public key scheme in the class of discrete logarithm. The former is based on arithmetic in large finite field, mainly Galois extension field GF(2‴). These solutions are droved using low-energy techniques, in order to decrease both the switching activity and the total delay. The fundamental motivation supporting this work, is to demonstrate that practical solutions can be obtained for implementing such complex primitives in large scaled circuits, that arc at once, high performance architectures (low-power, high-speed) and cryptographicaly strong, using the well known trade-off between area-speed or area-power. Security constraint has been duly considered, mainly by increasing the key-size. In this work, we explore the general aspects of designing the above mentioned cryptographic functions. We give an extensive survey of some cryptographic primitives from the hardware point of view and expose their security properties. The thesis favours stream cipher and public-key schemes, as currently the most promising advance to capture the notion of key generation and establishment and data bulk encryption. One contribution is the convenient notation for expressing cryptographic self-synchronizing stream ciphers SSSC schemes and our SSMG proposal, a scheme based on packet fingerprint identification, that relies on keyed cryptographic hash function to achieve the security requirements. We maintain an important distinction between hardware implementation and algorithm's security, because the security of cryptographic primitives cannot be based on mathematically strong functions only but requires an extensive cryptanalysis at different levels including the application. This causes a concern for a formalization of the security of an implemented cryptographic primitive. Nevertheless, while some schemes arc well known to be secure such as DL based public key schemes and enough cryptanalyzed such as the new standard Rijndael, some security aspects of the SSMG are discussed. A part of this work studies the specific aspects related to hardware implementation of Rijndael block cipher, the new standard designed to be a substitute for DES. An efficient architecture is developed targeting FPGA implementation, by simply avoiding memory blocks dedicated to the implementation of S-boxes and replacing them by on-chip forward computation using composite Galois field. This technique helps to reduce considerably the amount of hardware required at the cost of little increase of the switching activity. The main conclusion is that, while security constraint of cryptographic primitives increases the hardware complexity and reduces the performances, practical solutions exist for reducing such complexities while keeping or increasing the level of security. Nevertheless, major open questions remain both for a firm theoretical foundation and the proper cryptanalysis of certain solutions.

Multisignatures allow n signers to produce a short joint signature on a single message. Multisignatures were achieved in the plain model with a non-interactive protocol in groups with bilinear maps, by Boneh et al, and by a three-round protocol under the Discrete Logarithm (DL) assumption, by Bellare and Neven, with multisignature verification cost of, respectively, O(n) pairings or exponentiations. In addition, multisignatures with O(1) verification were shown in so-called Key Verification (KV) model, where each public key is accompanied by a short proof of well-formedness, again either with a non-interactive protocol using bilinear maps, by Ristenpart and Yilek, or with a three-round protocol under the Diffie-Hellman assumption, by Bagherzandi and Jarecki.

In this paper, we compare the decoding error rates in the error floors for non-binary low-density parity-check (LDPC) codes over the general linear group with those for non-binary LDPC codes over finite field transmitted over the q-ary memoryless symmetric channel under belief propagation decoding. To analyze non-binary LDPC codes defined over both general linear group GL(m, F 2 ) and finite field F 2m , we investigate non-binary LDPC codes defined over GL(m 3 , F 2m4 ). We propose a method to lower the error floors for non-binary LDPC codes. In this analysis, we see that the optimized non-binary LDPC codes defined over general linear group have the same decoding performance in the error floors as those defined over finite field. The non-binary LDPC codes defined over general linear group have more choices of the labels in the edges which satisfy the condition for the optimization.

At CRYPTO 2003, Rubin and Silverberg introduced the concept of torus-based cryptography over a finite field. We extend their setting to the ring of integers modulo N . We so obtain compact representations for cryptographic systems that base their security on the discrete logarithm problem and the factoring problem. This results in smaller key sizes and substantial savings in memory and bandwidth. But unlike the case of finite fields, analogous trace-based compression methods cannot be adapted to accommodate our extended setting when the underlying systems require more than a mere exponentiation. As an application, we present an improved, torus-based implementation of the ACJT group signature scheme.

Recently, several striking advances have taken place regarding the discrete logarithm problem (DLP) in finite fields of small characteristic, despite progress having remained essentially static for nearly thirty years, with the best known algorithms being of subexponential complexity. In this expository article we describe the key insights and constructions which culminated in two independent quasi-polynomial algorithms. To put these developments into both a historical and a mathematical context, as well as to provide a comparison with the cases of so-called large and medium characteristic fields, we give an overview of the state-of-the-art algorithms for computing discrete logarithms in all finite fields. Our presentation aims to guide the reader through the algorithms and their complexity analyses ab initio.

Computing discrete logarithms in finite fields is a main concern in cryptography. The best algorithms in large and medium characteristic fields (e.g., G F ( p 2 ) \rm {GF}(p^2) , G F ( p 12 ) \rm {GF}(p^{12}) ) are the Number Field Sieve and its variants (special, high-degree, tower). The best algorithms in small characteristic finite fields (e.g., G F ( 3 6 ⋅ 509 ) \rm {GF}(3^{6 \cdot 509}) ) are the Function Field Sieve, Joux’s algorithm, and the quasipolynomial-time algorithm. The last step of this family of algorithms is the individual logarithm computation. It computes a smooth decomposition of a given target in two phases: an initial splitting, then a descent tree. While new improvements have been made to reduce the complexity of the dominating relation collection and linear algebra steps, resulting in a smaller factor basis (database of known logarithms of small elements), the last step remains at the same level of difficulty. Indeed, we have to find a smooth decomposition of a typically large element in the finite field. This work improves the initial splitting phase and applies to any nonprime finite field. It is very efficient when the extension degree is composite. It exploits the proper subfields, resulting in a much more smooth decomposition of the target. This leads to a new trade-off between the initial splitting step and the descent step in small characteristic. Moreover it reduces the width and the height of the subsequent descent tree.

To ensure secure communication between two parties (Alice and Bob), Public Key Agreement (PKA) algorithms play a crucial role. The Diffie-Hellman (D-H) algorithm, introduced by W. Diffie and M. Hellman in 1976, is the most well-known PKA algorithm, based on exponentiation over a finite field. Over time, various PKA algorithms have been developed based on this original idea—these are referred to as D-H realizations or D-H algorithms. A common issue with D-H realizations is their computational inefficiency, particularly in environments with resource-constrained devices. This inefficiency arises from the high computational complexity of generating public and secret shared keys (SSKs), often resulting in communication delays. In this paper, we propose a modified D-H protocol designed to reduce the computational time required by one party, thereby decreasing the total time needed to establish the SSK in asymmetric environments where Alice and Bob have different computational capabilities. The core of this approach leverages the group homomorphism property of the underlying function to enable parallelization and reduce the complexity of Alice’s computations, together with an asymmetric key agreement structure in which both parties may follow distinct computational rules for public key and SSK generation. We demonstrate that the proposed protocol retains the security properties of the original D-H realizations, preserving the hardness of the discrete logarithm (DL), computational Diffie-Hellman (CDH), and decisional Diffie-Hellman (DDH) problems under an asymmetric setting. We also discuss its applicability to other cryptographic protocols. Experimental results show that the modified protocol significantly improves computational efficiency, particularly by reducing Alice's computational burden through parallel processing.

The concept of a hybrid scheme with connection of SIDH and ECDH is nowadays very popular. In hardware implementations it is convenient to use a classical key exchange algorithm, which is based on the same finite field as SIDH. Most frequently used hybrid scheme is SIDH-ECDH. On the other hand, using the same field as in SIDH, one can construct schemes over Fpn, like Diffie-Hellman or XTR scheme, whose security is based on the discrete logarithm problem. In this paper, idea of such schemes will be presented. The security of schemes, which are based on the discrete logarithm problem over fields Fp; Fp2 ; Fp4 ; Fp6 and Fp8 , for primes p used in SIDH, will be analyzed. At the end, the propositions of practical applications of these schemes will be presented.

Abstruct- Elliptic cuwe cryptosystems have the potential to provide relatively small block size, high-security public key schemes that can be efficiently implemented. As with other known public key schemes, such as RSA and discrete exponentiation in a finite field, some care must be exercised when selecting the parameters involved, in this case the elliptic curve and the underlying field. Specific classes of cuwes that give little or no advantage over previously known schemes are discussed. The main result of the paper is to demonstrate the reduction of the elliptic curve logarithm problem to the logarithm problem in the multiplicative group of an extension of the underlying finite field. For the class of supersingular elliptic curves, the reduction takes probabilistic polynomial time, thus providing a probabilistic subexponential time algorithm for the former problem. Index Tem- Discrete logarithms, elliptic curves, public key CryPtOSraPhY.

In 2004, Muzereau, Smart and Vercauteren [A. Muzereau, N. P. Smart and F. Vercauteren, The equivalence between the DHP and DLP for elliptic curves used in practical applications, LMS J. Comput. Math. 7 2004, 50–72] showed how to use a reduction algorithm of the discrete logarithm problem to Diffie–Hellman problem in order to estimate lower bound for the Diffie–Hellman problem on elliptic curves. They presented their estimates on various elliptic curves that are used in practical applications. In this paper, we show that a much tighter lower bound for the Diffie–Hellman problem on those curves can be achieved if one uses the multiplicative group of a finite field as auxiliary group. The improved lower bound estimates of the Diffie–Hellman problem on those recommended curves are also presented. Moreover, we have also extended our idea by presenting similar estimates of DHP on some more recommended curves which were not covered before. These estimates of DHP on these curves are currently the tightest which lead us towards the equivalence of the Diffie–Hellman problem and the discrete logarithm problem on these recommended elliptic curves.

We propose a new computational problem called the twin Diffie–Hellman problem. This problem is closely related to the usual (computational) Diffie–Hellman problem and can be used in many of the same cryptographic constructions that are based on the Diffie–Hellman problem. Moreover, the twin Diffie–Hellman problem is at least as hard as the ordinary Diffie–Hellman problem. However, we are able to show that the twin Diffie–Hellman problem remains hard, even in the presence of a decision oracle that recognizes solutions to the problem—this is a feature not enjoyed by the Diffie–Hellman problem, in general. Specifically, we show how to build a certain “trapdoor test” that allows us to effectively answer decision oracle queries for the twin Diffie–Hellman problem without knowing any of the corresponding discrete logarithms. Our new techniques have many applications. As one such application, we present a new variant of ElGamal encryption with very short ciphertexts, and with a very simple and tight security proof, in the random oracle model, under the assumption that the ordinary Diffie–Hellman problem is hard. We present several other applications as well, including a new variant of Diffie and Hellman’s non-interactive key exchange protocol; a new variant of Cramer–Shoup encryption, with a very simple proof in the standard model; a new variant of Boneh–Franklin identity-based encryption, with very short ciphertexts; a more robust version of a password-authenticated key exchange protocol of Abdalla and Pointcheval.

Implementing an improved ID‐based cryptographic mechanism is the main objective of the proposed work. In this article, an ID‐based cryptographic (IBC) technique using generalized discrete logarithm problem (GDLP) and the integer factorization problem (IFP) presented by Meshram et al have improved. Meshram et al have given IBC technique without using the bilinear pair and also, reveal that their technique can attain data protection and security objectives. Besides, their technique will deter the adversary from eavesdropping the encrypted information or the secret key of the user. However, it has found that their system carries a deadlock problem. Encryption process, as expected by the user, is not guaranteed to be secure. It is because the user may require private information about key authentication center (KAC), which has kept secret from users. Pang et al have proposed an improved technique that overcomes the deadlock problem. It has found that Pang et al have not discussed the analysis and proofs of security. In this article, generalized discrete logarithms in multiplicative group over finite fields and IFP have used to improve the technique and also a key distribution system has discussed. It has analyzed that the proposed strategy is safer than the technique described by Pang et al. Also, it has found that the proposed technique addresses the deadlock problem.