Abstract. We propose a polynomial time quantum algorithm for solving the discrete logarithm problem in matrices over finite group rings. The hardness of this problem was recently employed in the design of a key-exchange protocol proposed by D. Kahrobaei, C. Koupparis, and V. Shpilrain [4]. Our result implies that the Kahrobaei et al. protocol does not belong to the realm of post-quantum cryptography. Keywords and phrases: Group-based cryptography, semidirect product, matrix monoids, group-rings, Diffie-Hellman, key-exchange, discrete logarithm problem, quantum algorithms, post-quantum cryptography.
Abstract. We propose a polynomial time quantum algorithm for solving the discrete logarithm problem in matrices over finite group rings. The hardness of this problem was recently employed in the design of a key-exchange protocol proposed by D. Kahrobaei, C. Koupparis, and V. Shpilrain [4]. Our result implies that the Kahrobaei et al. protocol does not belong to the realm of post-quantum cryptography. Keywords and phrases: Group-based cryptography, semidirect product, matrix monoids, group-rings, Diffie-Hellman, key-exchange, discrete logarithm problem, quantum algorithms, post-quantum cryptography.
Abstract The semidirect discrete logarithm problem (SDLP) is the following analogue of the standard discrete logarithm problem in the semidirect product semigroup G⋊End(G) G⋊End(G) for a finite semigroupG. Given g∈G,σ∈End(G) g∈G,σ∈End(G) , and h=∏i=0t−1σi(g) h=∏i=0t-1σi(g) for some integert, the SDLP (G,σ) (G,σ) , forgandh, asks to determinet. As Shor’s algorithm crucially depends on commutativity, it is believed not to be applicable to the SDLP. For generic semigroups, the best known algorithm for the SDLP is based on Kuperberg’s subexponential time quantum algorithm. Still, the problem plays a central role in the security of certain proposed cryptosystems in the family ofsemidirect product key exchange. This includes a recently proposed signature protocol called SPDH-Sign. In this paper, we show that the SDLP is even easier in some important special cases. Specifically, for a finite groupG, we describe quantum algorithms for the SDLP in G⋊Aut(G) G⋊Aut(G) for the following two classes of instances: the first one is whenGis solvable and the second is whenGis a matrix group and a power of σ σ with a polynomially small exponent is an inner automorphism ofG. We further extend the results to groups composed of factors from these classes. A consequence is that SPDH-Sign and similar cryptosystems whose security assumption is based on the presumed hardness of the SDLP in the cases described above are insecure against quantum attacks. The quantum ingredients we rely on are not new: these are Shor’s factoring and discrete logarithm algorithms and well-known generalizations.
Quantum cryptography is a promising field for securing information against quantum computers. Shor's algorithm, a quantum algorithm, can factor large prime numbers in polynomial time and solve the discrete logarithm problem, which is the basis for classical cryptographic algorithms like Rivest-Shamir-Adleman algorithm (RSA), Diffie-Hellman key exchange (DHKE), and Elliptic Curve Diffie-Hellman algorithm(ECDH). These algorithms pose a significant threat to their security and reliability. Post-quantum cryptography techniques are being implemented by all industries to address the threat posed by quantum supremacy. The chapter discusses the potential threat of Shor's algorithm on classical cryptographic algorithms and discusses post-quantum cryptographic algorithms like lattice-based, multivariate, code-based, and Hash-based cryptography. It also discusses recent advancements that have improved the security and usefulness of these algorithms.
We investigate the computational complexity of the discrete logarithm, the computational Diffie-Hellman and the decisional Diffie-Hellman problems in some identity black-box groups G_{p,t}, where p is a prime number and t is a positive integer. These are defined as quotient groups of vector space Z_p^{t+1} by a hyperplane H given through an identity oracle. While in general black-box groups with unique encoding these computational problems are classically all hard and quantumly all easy, we find that in the groups G_{p,t} the situation is more contrasted. We prove that while there is a polynomial time probabilistic algorithm to solve the decisional Diffie-Hellman problem in Gp,1, the probabilistic query complexity of all the other problems is Omega(p), and their quantum query complexity is Omega(sqrt(p)). Our results therefore provide a new example of a group where the computational and the decisional Diffie-Hellman problems have widely different complexity.
This thesis contains a collection of efficient quantum algorithms and classical pre- and post-processing algorithms for various number- and group-theoretical problems with concrete cryptanalytical applications. More specifically, the focus is on the integer factoring problem (IFP), the discrete logarithm problem (DLP), and the order-finding problem (OFP), and on certain important variations of these problems. It is for example explained how the OFP and complete IFP may be solved efficiently in a single run of a reduced version of Shor's algorithm for the OFP with very high success probability, as evidenced by a lower bound that is 1 − o(1), how the short DLP in groups of unknown order may be solved efficiently and with an advantage over Shor's algorithm for the DLP (that furthermore requires the order to be known), as evidenced by a lower bound on the single-run success probability that is 1 − o(1), how the RSA IFP may be solved efficiently and with an advantage over Shor's algorithm for the IFP, by reducing the RSA IFP to a short DLP, how the DLP may heuristically be solved efficiently, and with high success probability that is 1 − o(1), in a single run of a reduced version of Shor’s algorithm for the DLP that admits efficient implementation, how the DLP may be solved in groups of unknown order, such as Schnorr groups, by jointly solving an OFP and a DLP at lower overall cost but higher per-run cost compared to first solving the OFP and then the DLP, how tradeoffs — between the number of group operations that are evaluated quantumly and the number of runs that are required — may be achieved in all of the aforementioned algorithms, and how all of the aforementioned algorithms may be simulated efficiently classically, for arbitrary large cryptographically relevant problem instances, when the solution to the instance is known, enabling the number of runs required when performing tradeoffs to be tightly estimated. It is furthermore for example explained how Regev's recent quantum algorithm for the IFP may be extended to the DLP and OFP, yielding i) a reduction in the circuit size and depth by a factor compared to Shor for these problems when working in , for N an n-bit integer, at the expense of performing runs, and ii) slight improvements of Regev's orginal algorithm for the IFP, how the above algorithms may be simulated efficiently classically, for large special-form problem instances that are classically tractable, and that Regev’s post-processing, and the post-processing for our extensions, are robust to an arbitrarily large fraction < 1 of the runs being bad. The above has implications for cryptography based on the intractability of the IFP and DLP — including but not limited to for widely used schemes such as RSA, DSA and Diffie–Hellman. The algorithms in this thesis are all derived from, and make notable improvements to, the seminal works of Shor. Some of the contributions are joint works with other authors.
We present our perspectives on the current status of, integer factoring and discrete logarithm, algorithms underlying the strength of classical public-key cryptography protocols used in key-exchange, encryption and digital signatures. We present an expository overview of the algebraic, number-theoretic and algorithmic principles behind the canonical, powerful algorithms used in the factoring of large, composite RSA modulii. These cover the two main ideas of group order based methods, and quadratic congruence based methods. We provide the resource estimates for the record breaking factoring based on the general number field sieve. Next we present an overview of the algorithmic theory of quantum computation behind, the polynomial time Shor's quantum algorithm for, period finding in hidden subgroup structure problems. This algorithm has played a key role in advancing post-quantum cryptography, and also in the exciting, contemporary developments in quantum computing, networking, sensing, and materials science as a whole. Latest advances in quantum computing rest on a variety of physics, engineering, algorithms, compilation, hardware, software techniques. There are several tradeoffs based on number of qubits, coherence times, control issues, errors and fault tolerance. In this environment, nuances of mathematical ideas, behind every step of the intricate arithmetical and algebraic computations under quantum models plays a significant role. We provide an overview of some of the recent ideas behind such algorithms. We conclude with a recent deep question on the ultimate feasibility of ultra large-scale, error-free, discrete, quantum computations required typically in cryptanalysis.