CryptoDB

Steven D. Galbraith

Publications

Year
Venue
Title
2019
EUROCRYPT
We give a new signature scheme for isogenies that combines the class group actions of CSIDH with the notion of Fiat-Shamir with aborts. Our techniques allow to have signatures of size less than one kilobyte at the 128-bit security level, even with tight security reduction (to a non-standard problem) in the quantum random oracle model. Hence our signatures are potentially shorter than lattice signatures, but signing and verification are currently very expensive.
2019
PKC
We consider the problem of constructing Diffie-Hellman (DH) parameters which pass standard approaches to parameter validation but for which the Discrete Logarithm Problem (DLP) is relatively easy to solve. We consider both the finite field setting and the elliptic curve setting.For finite fields, we show how to construct DH parameters (p, q, g) for the safe prime setting in which $p=2q+1$ is prime, q is relatively smooth but fools random-base Miller-Rabin primality testing with some reasonable probability, and g is of order q mod p. The construction involves modifying and combining known methods for obtaining Carmichael numbers. Concretely, we provide an example with 1024-bit p which passes OpenSSL’s Diffie-Hellman validation procedure with probability $2^{-24}$ (for versions of OpenSSL prior to 1.1.0i). Here, the largest factor of q has 121 bits, meaning that the DLP can be solved with about $2^{64}$ effort using the Pohlig-Hellman algorithm. We go on to explain how this parameter set can be used to mount offline dictionary attacks against PAKE protocols. In the elliptic curve case, we use an algorithm of Bröker and Stevenhagen to construct an elliptic curve E over a finite field ${\mathbb {F}}_p$ having a specified number of points n. We are able to select n of the form $h\cdot q$ such that h is a small co-factor, q is relatively smooth but fools random-base Miller-Rabin primality testing with some reasonable probability, and E has a point of order q. Concretely, we provide example curves at the 128-bit security level with $h=1$ , where q passes a single random-base Miller-Rabin primality test with probability 1/4 and where the elliptic curve DLP can be solved with about $2^{44}$ effort. Alternatively, we can pass the test with probability 1/8 and solve the elliptic curve DLP with about $2^{35.5}$ effort. These ECDH parameter sets lead to similar attacks on PAKE protocols relying on elliptic curves.Our work shows the importance of performing proper (EC)DH parameter validation in cryptographic implementations and/or the wisdom of relying on standardised parameter sets of known provenance.
2017
ASIACRYPT
2016
ASIACRYPT
2015
EPRINT
2015
EPRINT
2015
EPRINT
2015
EPRINT
2014
EPRINT
2011
JOFC
2010
PKC
2009
EPRINT
Elliptic curves over finite fields have applications in public key cryptography. A Koblitz curve is an elliptic curve $E$ over $\F_2$; the group $E( \Ftn )$ has convenient features for efficient implementation of elliptic curve cryptography. Wiener and Zuccherato and Gallant, Lambert and Vanstone showed that one can accelerate the Pollard rho algorithm for the discrete logarithm problem on Koblitz curves. This implies that when using Koblitz curves, one has a lower security per bit than when using general elliptic curves defined over the same field. Hence for a fixed security level, systems using Koblitz curves require slightly more bandwidth. We present a method to reduce this bandwidth. Our method is appropriate for applications such as Diffie-Hellman key exchange or Elgamal encryption. We show that, with a low probability of failure, our method gives the expected bandwidth for a given security level.
2009
EUROCRYPT
2008
PKC
2008
EPRINT
We present efficiently computable homomorphisms of the groups $G_2$ and $G_T$ for pairings $G_1 \times G_2 \rightarrow G_T$. This allows exponentiation in $G_2$ and $G_T$ to be accelerated using the Gallant-Lambert-Vanstone method.
2008
EPRINT
Efficiently computable homomorphisms allow elliptic curve point multiplication to be accelerated using the Gallant-Lambert-Vanstone (GLV) method. We extend results of Iijima, Matsuo, Chao and Tsujii which give such homomorphisms for a large class of elliptic curves by working over quadratic extensions and demonstrate that these results can be applied to the GLV method. Our implementation runs in between 0.70 and 0.84 the time of the previous best methods for elliptic curve point multiplication on curves without small class number complex multiplication. Further speedups are possible when using more special curves.
2008
EPRINT
From the viewpoint of x-coordinate-only arithmetic on elliptic curves, switching between the Edwards model and the Montgomery model is quasi cost-free. We use this observation to speed up Montgomery's algorithm, reducing the complexity of a doubling step from 2M + 2S to 1M + 3S for suitably chosen curve parameters.
2008
EPRINT
We analyse the efficiency of pairing computations on hyperelliptic curves given by a real model using a balanced divisor at infinity. Several optimisations are proposed and analysed. Genus two curves given by a real model arise when considering pairing friendly groups of order dividing $p^{2}-p+1$. We compare the performance of pairings on such groups in both elliptic and hyperelliptic versions. We conclude that pairings can be efficiently computable in real models of hyperelliptic curves.
2008
EPRINT
To reduce bandwidth in elliptic curve cryptography one can transmit only $x$-coordinates of points (or $x$-coordinates together with an extra bit). For further computation using the points one can either recover the $y$-coordinates by taking square roots or one can use point multiplication formulae which use $x$-coordinates only. We consider how to efficiently use point compression in pairing-based cryptography. We give a method to compute compressed Weil pairings using $x$-coordinates only. We also show how to compute the compressed Tate and ate pairings using only one $y$-coordinate. Our methods are more efficient than taking square roots when the embedding degree is small. We implemented the algorithms in the case of embedding degree 2 curves over $\F_p$ where $p \equiv 3 \pmod{4}$ and found that our methods are $10-15\%$ faster than the analogous methods using square roots.
2008
EPRINT
We discuss arithmetic in the Jacobian of a hyperelliptic curve $C$ of genus $g$. The traditional approach is to fix a point $P_\infty \in C$ and represent divisor classes in the form $E - d(P_\infty)$ where $E$ is effective and $0 \le d \le g$. We propose a different representation which is balanced at infinity. The resulting arithmetic is more efficient than previous approaches when there are 2 points at infinity. This is a corrected and extended version of the article presented in ANTS 2008. We include an appendix with explicit formulae to compute a very efficient baby step' in genus 2 hyperelliptic curves given by an imaginary model.
2007
EPRINT
We discuss some applications of the pairing inversion problem and outline some potential approaches for solving it. Our analysis of these approaches gives further evidence that pairing inversion is a hard problem.
2006
EPRINT
Many research papers in pairing based cryptography treat pairings as a black box''. These papers build cryptographic schemes making use of various properties of pairings. If this approach is taken, then it is easy for authors to make invalid assumptions concerning the properties of pairings. The cryptographic schemes developed may not be realizable in practice, or may not be as efficient as the authors assume. The aim of this paper is to outline, in as simple a fashion as possible, the basic choices that are available when using pairings in cryptography. For each choice, the main properties and efficiency issues are summarized. The paper is intended to be of use to non-specialists who are interested in using pairings to design cryptographic schemes.
2006
EPRINT
Recent progress on pairing implementation has made certain pairings extremely simple and fast to compute. Hence, it is natural to examine if there are consequences for the security of pairing-based cryptography. This paper gives a method to compute eta pairings in a way which avoids the requirement for a final exponentiation. The method does not lead to any improvement in the speed of pairing implementation. However, it seems appropriate to re-evaluate the security of pairing based cryptography in light of these new ideas. A multivariate attack on the pairing inversion problem is proposed and analysed. Our findings support the belief that pairing inversion is a hard computational problem.
2006
EPRINT
Frey proposed the idea of disguising' an elliptic curve. This is a method to obtain a black box' representation of a group. We adapt this notion to finite fields and tori and study the question of whether such systems are secure. Our main result is an algebraic attack which shows that it is not secure to disguise the torus $T_2$. We also show that some methods for disguising an elliptic curve are not secure. Finally, we present a method to disguise an elliptic curve which seems to resist our algebraic attack.
2006
EPRINT
D\'ech\ene has proposed generalized Jacobians as a source of groups for public-key cryptosystems based on the hardness of the Discrete Logarithm Problem (DLP). Her specific proposal gives rise to a group isomorphic to the semidirect product of an elliptic curve and a multiplicative group of a finite field. We explain why her proposal has no advantages over simply taking the direct product of groups. We then argue that generalized Jacobians offer poorer security and efficiency than standard Jacobians.
2006
EPRINT
Distortion maps are a useful tool for pairing based cryptography. Compared with elliptic curves, the case of hyperelliptic curves of genus $g > 1$ is more complicated since the full torsion subgroup has rank $2g$. In this paper we prove that distortion maps always exist for supersingular curves of genus $g>1$ and we give several examples in genus $2$.
2005
EPRINT
To help motivate the Weil pairing, we discuss it in the context of elliptic curves over the field of complex numbers.
2004
EPRINT
It is already known that the Weil and Tate pairings can be used to solve many decision-Diffie-Hellman (DDH) problems on elliptic curves. A natural question is whether all DDH problems are easy on supersingular curves. To answer this question it is necessary to have suitable distortion maps. Verheul states that such maps exist, and this paper gives methods to construct them. The paper therefore shows that all DDH problems on supersingular elliptic curves are easy. We also discuss the issue of which DDH problems on ordinary curves are easy. A related contribution is a discussion of distortion maps which are not isomorphisms. We give explicit distortion maps for elliptic curves with complex multiplication of discriminants $D=-7$ and $D=-8$.
2004
EPRINT
Miyaji, Nakabayashi and Takano (MNT) gave families of group orders of ordinary elliptic curves with embedding degree suitable for pairing applications. In this paper we generalise their results by giving families corresponding to non-prime group orders. We also consider the case of ordinary abelian varieties of dimension 2. We give families of group orders with embedding degrees 5, 10 and 12.
2004
EPRINT
We present a general technique for the efficient computation of pairings on supersingular Abelian varieties. This formulation, which we call the eta pairing, generalises results of Duursma and Lee for computing the Tate pairing on supersingular elliptic curves in characteristic three. We then show how our general technique leads to a new algorithm which is about twice as fast as the Duursma-Lee method. These ideas are then used for elliptic and hyperelliptic curves in characteristic 2 with very efficient results. In particular, the hyperelliptic case is faster than all previously known pairing algorithms.
2003
EPRINT
A public key cryptosystem based on Drinfeld modules has been proposed by Gillard, Leprevost, Panchishkin and Roblot. The paper shows how an adversary can directly recover a private key using only the public key, and so the cryptosystem is insecure.
2002
EUROCRYPT
2002
EPRINT
The Weil and Tate pairings are a popular new gadget in cryptography and have found many applications, including identity-based cryptography. In particular, the pairings have been used for key exchange protocols. This paper studies the bit security of keys obtained using protocols based on pairings (that is, we show that obtaining certain bits of the common key is as hard as computing the entire key). These results are valuable as they give insight into how many hard-core'' bits can be obtained from key exchange using pairings.
2002
JOFC
2001
ASIACRYPT
2001
EPRINT
Two public key encryption schemes based on anomalous elliptic curves over rings are studied. It is argued that these schemes are not secure.
2001
EPRINT
This paper is concerned with generalisations of Paillier's probabilistic encryption scheme from the integers modulo a square to elliptic curves over rings. Paillier himself described two public key encryption schemes based on anomalous elliptic curves over rings. It is argued that these schemes are not secure. A more natural generalisation of Paillier's scheme to elliptic curves is given.
2001
EPRINT
In this paper we extend the Weil descent attack due to Gaudry, Hess and Smart (GHS) to a much larger class of elliptic curves. This extended attack still only works for fields of composite degree over $\F_2$. The principle behind the extended attack is to use isogenies to find a new elliptic curve for which the GHS attack is effective. The discrete logarithm problem on the target curve can be transformed into a discrete logarithm problem on the new isogenous curve. One contribution of the paper is to give an improvement to an algorithm of Galbraith for constructing isogenies between elliptic curves, and this is of independent interest in elliptic curve cryptography. We conclude that fields of the form $\F_{q^7}$ should be considered weaker from a cryptographic standpoint than other fields. In addition we show that a larger proportion than previously thought of elliptic curves over $\F_{2^{155}}$ should be considered weak.
1999
ASIACRYPT

PKC 2020
TCC 2019
Asiacrypt 2019
Asiacrypt 2018
Crypto 2018
Asiacrypt 2017
Crypto 2016
Asiacrypt 2016
Crypto 2015
PKC 2015
Asiacrypt 2014
Eurocrypt 2013
PKC 2013
Crypto 2012
Eurocrypt 2012
Crypto 2011
Eurocrypt 2010
Crypto 2009
PKC 2008
Asiacrypt 2007
PKC 2007
Crypto 2007
Eurocrypt 2005