Till Stegers

e-mail: *my last name* at cs.ucdavis.edu
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Short Bio

I am a PhD student in the Department of Computer Science at the University of California, Davis. From 2001 to 2005, I studied Mathematics with Computer Science at TU Darmstadt in Germany. In 2002/2003, I was fortunate to attend the graduate program in Mathematics at Tulane University in New Orleans. I joined UC Davis in Fall 2005, and I am excited to work on topics in cryptography and security. My advisor is Phil Rogaway. You can find my curriculum vitae here.

 

Papers

 Source Code Review of the Sequoia Voting System
Report to the Secretary of State of California, July 20, 2007
as part of the "Top-to-bottom Review" led by Matt Bishop and David Wagner
Authors: Matt Blaze, Arel Cordero, Sophie Engle, Chris Karlof, Naveen Sastry, Micah Sherr, Till Stegers, and Kai-Ping Yee
Download PDF file of our report
Decertification/recertification decision by Secretary of State Debra Bowen

Very Brief Abstract: We found significant security weaknesses throughout the Sequoia system. The nature of these weaknesses raises serious questions as to whether the Sequoia software can be relied upon to protect the integrity of elections. Every software mechanism for transmitting election results and every software mechanism for updating software lacks reliable measures to detect or prevent tampering. In certain cases, audit mechanisms may be able to detect and recover from some attacks, depending on county-specific procedures; other attacks may be more difficult to detect after-the-fact even with very rigorous audits.

Computational Soundness of Formal Indistinguishability and Static Equivalence
ASIAN 2006 and Cryptology ePrint Archive Report 2006/323
Gergei Bana, Payman Mohassel, and Till Stegers
Download full version: http://eprint.iacr.org/2006/323.pdf

Abstract: In the research of the relationship between the formal and the computational view of cryptography, a recent approach uses static equivalence from cryptographic pi calculi as a notion of formal indistinguishability. Previous work has shown that this yields the soundness of natural interpretations of some interesting equational theories, such as certain cryptographic operations and a theory of XOR. In this paper however, we argue that static equivalence is too coarse for sound interpretations of equational theories in general. We show some explicit examples how static equivalence fails to work in interesting cases. To fix this problem, we propose a notion of formal indistinguishability that is more flexible than static equivalence. We provide a general framework along with general theorems, and then discuss how this new notion works for the explicit examples where static equivalence failed to ensure soundness. We also improve the treatment by using ordered sorts in the formal view, and by allowing arbitrary probability distributions of the interpretations.
 
Security Analysis of the eVACS Open-Source Voting System
Manuscript, 2005
Ananya Das, Yuan Niu, Till Stegers
Download PDF file: eVACS-final-report.pdf

Abstract: The electronic Voting and Counting System (eVACS) is an open-source software used in an electronic voting trial in the Australian Capital Territory, and has been recommended for use in future elections. In this paper, we report results from a review of the eVACS code and design, supported by static analysis tools. While no "hot exploits" have been found, several bad practices were identified.

 

Theses

Faugère's F5 Algorithm Revisited
Diplom thesis
Technische Universität Darmstadt, 2005 (last revised 2007-04-27)
Preprint: http://eprint.iacr.org/2006/404
Advisor: Johannes Buchmann
Download PDF file: diplom_stegers.pdf
Download Magma source code for F5: f5_magma.tar.gz

Abstract: We present and analyse the F5 algorithm for computing Gröbner bases. On the practical side, we correct minor errors in Faugère's pseudo code, and report our experiences implementing the -- to our knowledge -- first working public version of F5. While not designed for efficiency, it will doubtless be useful to anybody implementing or experimenting with F5. In addition, we list some experimental results, hinting that the version of F5 presented in Faugère's original paper can be considered as more or less naive, and that Faugère`s actual implementations are a lot more sophisticated. We also suggest further improvements to the F5 algorithm and point out some problems we encountered when attempting to merge F4 and F5 to an "F4.5" algorithm. On the theoretical side, we slightly refine Faugère's theorem that it suffices to consider all normalized critical pairs, and give the first full proof, completing his sketches. We strive to present a more accessible account of the termination and correctness proofs of F5. Unfortunately, we still rely on a conjecture about the correctness of certain optimizations. Finally, we suggest directions of future research on F5.

A Survey of Concrete Categories Where All Reflexive Objects Are Degenerate
Master of Science thesis
Tulane University, 2003
Advisor: Michael Mislove
Download PostScript file: thesis-refl-objects.ps.gz

Abstract: This thesis provides a survey of different degeneracy results for models of the untyped lambda calculus. The first chapter gives a motivation and introduces the basic syntactical notions. It also describes the set of lambda-terms using universal algebra. In the second chapter, we discuss cartesian closed categories, the definition of an environment model and briefly compare it to other notions of a model. The last chapter gives proofs for the degeneracy of models in the categories of partially ordered sets, complete-ultrametric spaces and (under the assumption of compactness) compactly generated Hausdorff spaces.

Aspects of the Discrete Logarithm
Bachelor of Science thesis
Technische Universität Darmstadt, 2002
Advisor: Michael Wüstner
Download PostScript file: dl.ps.gz

Abstract: We introduce the problem of discrete logarithms in groups and present the explicit formula for the group of units of a finite field. The Silver-Pohlig-Hellman algorithm for finite groups and the general outline of index-calculus methods are discussed, as well as some specialized index-calculus algorithms for finite fields.


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