Most of current public-key cryptography is considered insecure against attacks from sufficiently powerful quantum computers. Post-quantum cryptography studies methods to secure information... Show moreMost of current public-key cryptography is considered insecure against attacks from sufficiently powerful quantum computers. Post-quantum cryptography studies methods to secure information resistant against such attacks. One proposal is isogeny-based cryptography, which bases its security on computational hardness assumptions related to maps between elliptic curves. We analyze the security of isogeny-based cryptographic schemes, in particular those based on class group actions. We find special cases in which the underlying computational hardness assumptions can be broken, sometimes even by classical computers. Furthermore, we study a method, known as radical isogenies, to accelerate the execution of isogeny-based protocols. Finally, we introduce multivariate generalizations of Hilbert class polynomials, which may yield computational benefits compared to their univariate counterparts. Show less
This thesis deals primarily with the study of Galois representations attached to torsion points on elliptic curves. In the first chapter we consider the problem of determining the image of the... Show moreThis thesis deals primarily with the study of Galois representations attached to torsion points on elliptic curves. In the first chapter we consider the problem of determining the image of the Galois representation flE attached to a non-CM elliptic curve over the rational number field Q. We give a deterministic algorithm that determines the image of flE as a subgroup of GL2(__Z), where the output is given as an integer m together with a finite subgroup G(m) _ GL2(Z/mZ). The image of flE is then the subgroup of all elements of GL2(__Z) whose reduction modulo m belongs to G(m). In the second part we develop a method using character sums that uses the image of flE to describe densities of sets of primes p for which _E (Fp) has certain prescribed properties. If E is an elliptic curve over Q, then it follows by work of Serre and Hooley that, under the assumption of the Generalized Riemann Hypothesis, the density of primes p such that the group of Fprational points of the reduced curve _E(Fp) is cyclic can be written as an infinite product r ___ of local factors ___ reflecting the degree of the _-torsion fields, multiplied by a factor that corrects for the entanglements between the various torsion fields. We show that this correction factor can be interpreted as a character sum, and the resulting description allows us to easily determine nonvanishing criteria for it. We apply our character sum method to a variety of other settings. Among these, we consider the aforementioned problem with the additional condition that the primes p lie in a given arithmetic progression. We also study the conjectural constants appearing in Koblitz__s conjecture, a conjecture which relates to the density of primes p for which the cardinality of the group of Fp-points of E is prime. The unifying theme in all these settings is that the constants we are interested in are completely determined by the image of flE. The final chapter deals with the classification of non-Serre curves. An elliptic curve over Q is a Serre curve if its attached Galois representation is as large as possible, and it is known that most elliptic curves over Q are of this type. We exhibit a modular curve of level 6 that completes a set of modular curves which parametrise non-Serre curves. This modular curve also gives an infinite family of elliptic curves with non-abelian "entanglement fields". Exhibiting such a family is naturally motivated by questions arising in the previous chapter regarding the classification of elliptic curves to which we can apply the character sum method described above. Show less
The 1st chapter is of an introductory nature. It discusses the basic invariants of algebraic number fields and asks whether or to which extent such invariants characterize the number field. It... Show moreThe 1st chapter is of an introductory nature. It discusses the basic invariants of algebraic number fields and asks whether or to which extent such invariants characterize the number field. It surveys some of the older results in the area before focusing on the case of absolute abelian Galois groups that occurs center stage in the next two chapters, and on a question for elliptic curves that can be attacked with the techniques from those two chapters. Chapters 2 & 3 include also the non-trivial proof of the fact that the key criterion to find imaginary quadratic fields with `minimal__ absolute abelian Galois groups can also be used to find Galois groups that are "provably" non-minimal. Chapter 4 moves in a different direction. It explicitly computes adelic point groups of elliptic curves over the field of rational numbers, and shows that the outcome can be made as explicit as in the case of the minimal absolute abelian Galois groups, and, in an even stronger sense than in that case, barely depends on the particular elliptic curve. The results obtained do generalize to arbitrary number fields, and it is this generalization that we plan to deal with in a forthcoming paper. Show less
Dit proefschrift gaat over algoritmen in de getaltheorie. Het woord algoritme is een verbastering van de naam van de Perzische wiskundige Muhammad ibn Musa al-Khwarizmi (790-850)
