Kalendariumarkiv

2013-06-17 Disputation: Paul Stankovski

Monday June 17th, 2013, at 10.15 Paul Stankovski defends his doctoral dissertation “Cryptanalysis of Selected Stream Ciphers” in lecture hall E:1406, Ole Römers väg 3, S-223 63 Lund, Sweden.

Faculty opponent is Prof. Anne Canteaut, INRIA Paris-Rocquencourt, France.

Abstract:
The aim of this dissertation is to show some cryptanalytical results on a selection of stream ciphers. We have grouped theory and results into three main parts.

The first part focuses on the FCSR-based constructions X-FCSR and F-FCSR-H v3. For the X-FCSR family of stream ciphers we perform a severe state recovery attack. This attack works for both X-FCSR-128 and X-FCSR-256.

We then develop a generalized birthday algorithm for finding linear relations in FCSRs. This algorithm applies to the most recent and general FCSR architecture, the ring FCSR, so it can be used for analyzing the FCSR of any FCSR-based design. We apply the algorithm to produce an efficient distinguisher for F-FCSR-H v3, which was previously unbroken.

The second part of the dissertation covers topics related to the HC family of stream ciphers. First, a very general treatment of sampling methods is presented. Surprisingly, perhaps, a positive result is given. We prove that an efficient sampling method based on sampling vector weights is optimal in a given context. This sampling technique is employed to produce the best known distinguisher for HC-128.

We go on to show a few theoretical results on functions that use word rotation and xor. These results are applied to a modified variant of HC-128, and this application shows how the theory could be used in a cryptanalytical scenario. It also shows the important role of the addition operator in HC-128, without which the cipher would be much less secure.

In the third part of the dissertation we analyze stream ciphers, and block ciphers to a lesser extent, using algebraic methods. We develop a simple and intuitive greedy algorithm for automatic security testing of cryptographic primitives. This is done in a black box fashion, without using any information on the internal structure of the primitives. Despite this, it is shown how structural information is revealed very clearly under certain circumstances. The main features here are some nice results for the well-known stream ciphers Trivium, Grain-128 and Grain v1.

List of publications:
[1] P. Stankovski and M. Hell. An Optimal Sampling Technique for Distinguishing Random S-boxes. ISIT, July 2012, pp. 846-850, http://dx.doi.org/10.1109/ISIT.2012.6284680.
[2] P. Stankovski, M. Hell and T. Johansson. An Efficient State Recovery Attack on X-FCSR-256. FSE 2009, ed. by O. Dunkelman, vol. 5665, Lecture Notes in Computer Science, Springer-Verlag, 2009, pp. 23-37, ISBN: 978-3-642-03316-2, http://dx.doi.org/10.1007/978-3-642-03317-9_2.
[3] P. Stankovski, M. Hell and T. Johansson. An Efficient State Recovery Attack on the X-FCSR Family of Stream Ciphers, Journal of Cryptology, Online First TM (November 2012), pp. 1-22, ISSN: 0933-2790, http://dx.doi.org/10.1007/s00145-012-9130-9. Extended version of [2].
[4] P. Stankovski, M. Hell and T. Johansson. Analysis of Xorrotation with Application to an HC-128 Variant. ACISP12, ed. by W. Susilo, Y. Mu and J. Seberry, vol. 7372, LNCS, Springer Berlin Heidelberg, 2012, pp. 419-425, ISBN: 978-3-642-31447-6, http://dx.doi.org/10.1007/978-3-642-31448-3_31.
[5] P. Stankovski, S. Ruj, M. Hell and T. Johansson. Improved Distinguishers for HC-128. Designs, Codes and Cryptography 63 (Feb 2012), pp. 225-240, ISSN: 0925-1022, http://dx.doi.org/10.1007/s10623-011-9550-9.
[6] P. Stankovski. Greedy Distinguishers and Nonrandomness Detectors. Progress in Cryptology - INDOCRYPT 2010, ed. G. Gong and K. C. Gupta, vol. 6498, Lecture Notes in Computer Science, Springer-Verlag, 2010, pp. 210-226, http://dx.doi.org/10.1007/978-3-642-17401-8_16.
[7] H. Wang, P. Stankovski and T. Johansson. A Generalized Birthday Approach for Efficiently Finding Linear Relations in l-sequences. Accepted for publication in Designs, Codes and Cryptography on March 23rd, 2013.

2013-05-23
 

2013-06-14 Disputation: Kristin Persson

Friday 14th June, 2013, at 10.15 a.m. Kristin Persson defends her doctoral thesis "Radome Diagnostics: utilizing Source Reconstruction based on Surface Integral Representations" in lecture hall E:1406, Ole Römers väg 3, S-223 63 Lund, Sweden.

Faculty opponent is Prof. Margaret Cheney, Department of Electrical and Computer Engineering, Colorado State University, USA.

Abstract
In this thesis, an inverse source reconstruction method with great potential in radome diagnostics is presented. A radome is a cover that encloses an antenna in order to protect it from environmental influences. Radome diagnostics are acquired in the design process, the delivery control, and in performance verification of repaired and newly developed radomes. A measured near or far field may indicate deviations, e.g., beam deflection, but the origins of the flaws are not uncovered. In this thesis, radome diagnostics is performed by imaging the tangential electromagnetic fields on radome surfaces, disclosing the radome influence on the electromagnetic fields as well as the positions and influences of defects.

The source reconstruction is based on a surface integral representation together with the extinction theorem. The extinction theorem and its associated surface integral equation ensure that the reconstructed tangential electromagnetic fields have their sources within the radome. The presence of axial symmetry in the measurement set-up enables usage of the fast Fourier transform to reduce the computational complexity. Furthermore, the problem is solved by an in-house body of revolution method of moments (MoM) code utilizing a singular value decomposition (SVD) for regularization. The reconstruction is performed on a fictitious surface in free space, located precisely outside the physical surface of the radome, i.e., no a priori information of the material of the radome is requested. Moreover, both synthetic and measured data are used to verify the method.

In Papers [1-3], the measurement set-up is a reflector antenna covered by a monolithic radome, and the near field is measured on a cylindrical surface. The height of the radome corresponds to 29-43 wavelengths in the frequency interval 8.0-12.0 GHz. The amplitude and phase of the tangential electromagnetic fields are reconstructed on the radome surface and the influence of the radome is investigated. Moreover, the alteration of the phase due to the transmission of the radome, the insertion phase delay (IPD), is imaged. Defects in the form of square copper patches, with an edge length corresponding to 1.6-2.4 wavelengths in the considered frequency interval, are attached to the radome wall. These might serve as a model for e.g., a lightning conductor or a Pitot tube. The attached patches alter the near field, and by applying source reconstruction, the disturbances of the patches are focused and detectable.

In Paper [4], the field is measured on a spherical sector in the far-field region at 10.0 GHz. Two set-ups with dielectric defects attached to the radome surface, are investigated. The aim is to investigate if variations in the electrical thickness of the radome wall can be detected. It is concluded that it is possible to discover dielectric patches of various edge sizes (0.5-2.0 wavelengths), and with the smallest thickness corresponding to a phase shift of a couple of degrees.

In Paper [5], a frequency selective (FSS) radome corresponding to a height of 51 wavelengths at the frequency 9.35 GHz is investigated. The electrical performance of an FSS radome depends on the periodic structure of the elements in the radome frame. The periodic structure of the investigated radome is disrupted by horizontal defects (vertical displacements of elements) and vertical defects (a column of missing elements). The far-field data is measured on a spherical sector, and the far-field data reveals that the radome changes the radiation properties. The tangential electromagnetic fields on the radome surface are reconstructed for several antenna illuminations to image the cause of these alterations. Furthermore, it is shown that the different components of the electromagnetic fields are affected differently by the defects, implying that both co- and cross-components of the electric and magnetic fields need to be considered. Moreover, the Poynting's vector is employed to visualize how the defects block the field from the antenna.

References
[1] K. Persson, M. Gustafsson, and G. Kristensson. Reconstruction and visualization of equivalent currents on a radome using an integral representation formulation. Progress In Electromagnetics Research, 20, 65-90, 2010.
[2] K. Persson and M. Gustafsson. Reconstruction of equivalent currents using a near-field data transformation - with radome applications. Progress in Electromagnetics Research, 54, 179-198, 2005.
[3] K. Persson and M. Gustafsson. Reconstruction of equivalent currents using the scalar surface integral representation. Technical Report LUTEDX/(TEAT- 7131)/1-25/(2005), Lund University, Department of Electrical and Information Technology, P.O. Box 118, S-221 00 Lund, Sweden, 2005.
http://www.eit.lth.se.
[4] K. Persson, M. Gustafsson, G. Kristensson, and B. Widenberg. Radome diagnostics - source reconstruction of phase objects with an equivalent currents approach. Technical Report LUTEDX/(TEAT-7223)/1-22/(2012), Lund University, Department of Electrical and Information Technology, P.O. Box 118, S-221 00 Lund, Sweden, 2012. http://www.eit.lth.se.
[5] K. Persson, M. Gustafsson, G. Kristensson, and B. Widenberg. Source reconstruction by far-field data for imaging of defects in frequency selective radomes. IEEE Antennas and Wireless Propagation Letters, 12, 480-483, 2013.

2013-05-21
 

2013-06-03--04: EIT hosts 9th Swedish National Computer Networking Workshop

SNCNW, the Swedish National Computer Networking Workshop, will be held at the Dept. of Electrical and Information Technology (EIT).

The workshop is a creative and informal forum for researchers and practitioners in computer communication and networking.
The main objective of SNCNW is to give all research and development groups an opportunity to present their work.
The workshop also provides an excellent opportunity to discuss ideas among colleagues and establish contact between computer network researchers, practitioners and industry, and thus to help strengthen this field in Sweden.

Location: Room E:1406 in E-building, on the LTH campus.

Mer information

2013-05-14