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Cryptoanalysis of a Post-quantum Cryptography Algorithm
Štumpf, Daniel ; Hojsík, Michal (advisor) ; Příhoda, Pavel (referee)
National Institute of Standards and Technology (NIST) is currently running a stan- dardization process for a post-quantum cryptography primitives. Depending on the al- gorithms building blocks these primitives can be divided into five categories. In the first part of this thesis we described all five categories and compared their characteristics. The most important aspect of the schemes for NIST is security against both classical and quantum adversaries. We chose one of the five categories (namely, we picked lattice- based cryptosystems) for further cryptanalysis. As we think that the security analysis of some of the second round candidates in the NIST standardization project is not suffi- ciently well described in their specification documents and some known attacks are not considered at all, we provide a unified security analysis of these schemes. We described two currently known attacks (primal and dual attacks) against lattice-based schemes, estimated cost of these attacks against the lattice-based candidates in the second round of the NIST standardization project and compared these values with the security claimed by these candidates. In most cases our estimations matches those published in the speci- fication documents and therefore we conclude that the security estimates claimed by the candidates are...
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Web-based application for cryptographic protocols visualization
Ryšavá, Pavla ; Dzurenda, Petr (referee) ; Ricci, Sara (advisor)
Práce se zabývá vytvořením interaktivní webové aplikace pro substituční šifry a jejich interaktivní kryptoanalýzu. V práci je implementováno šest šifer a zástupci monoalfabetických šifer jsou Caesarova šifra, Atbaš a substituce s klíčovým slovem. Dále zástupci polyalfabetických šifer jsou Vigenrova šifra, Kryptos a Vernamova šifra. Pro interaktivní analýzu je použita frekvenční analýza, index koincidence a n-gramová statistika jako fitness funkce. Výsledek byl dosažen za pomoci HTML5, CSS a skriptovacího jazyka ReactJS což je JavaScriptová knihovna s možností typové kontroly proměnných.
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Side-channel analysis
Novosadová, Tatiana ; Lieskovan, Tomáš (referee) ; Gerlich, Tomáš (advisor)
This thesis deals with an issue of side channels, elliptic curves and their implementation in algorithms. Theoretical part is aimed at describing the basic concepts and various possibilities of side-channel attacks, with a more detailed focus on power analysis. The second theoretical part is focused on describing methods of elliptic curves and different elliptic curves-based protocols. The practical part deals with implementation of such cryptosystem on smart card, using the experimental environment to measure power consumption and analysis of this consumption.
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Crytographic attacks on TLS protocol
Oupický, Jan ; Tůma, Jiří (advisor) ; Boháček, Milan (referee)
The aim of this work is to introduce the reader to the protocol TLS and a few selected attacks against the protocol. In the first part we will define the necessary cryptographic definitions used in the following chapters. In the second part we will briefly talk about the history of procotols TLS and SSL and then we will closely look into how they work. The last part is about the analysis of the chosen cryptographically interesting attacks (Padding oracle on CBC mode, POODLE, BEAST and CRIME) against protocols TLS and SSL. 1
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Lego Technic encryption demonstrator
Jančík, Jakub ; Malina, Lukáš (referee) ; Hajný, Jan (advisor)
The bachelor thesis is mostly about mechanical encryption. Primary task was to build demonstrator encryptor using only LEGO Technic building kit, which would allow polyalphabetical substitution encryption. The theoretical part includes information about origins of steganography, cryptography and cryptanalysis, it explains the process of the polyalphabetical substitution encryption and it will introduce Enigma – how it went on during development, on what principle it works and thanks to what it stopped being safe. Device was extended with LEGO MINDSTORMS which allowed to build the Enigma demonstrator.
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Key reconstruction from the inner state of RC4
Sladký, Lukáš ; Boháček, Milan (advisor) ; Drápal, Aleš (referee)
In the present work we analyse the key scheduling algorithm of the RC4 stream cipher. The internal permutation generated by the algorithm is biased towards the secret key which can be utilized in the key recovery attempts. Multiple sources were combined to provide proven formulae for these biases. We have completed missing proofs, experimentally verified resulting probabilities and created examples to illustrate usage of the biases in key recovery attempts. This together with methods for extracting information about the secret key gives analysis of the key scheduling algorithm. We have also selected an efficient key retrieval algorithm and implemented an improved version which results in better success probabilities of the key recovery. We have also provided a tool for extracting the key from a permutation obtained by the key scheduling algorithm. Powered by TCPDF (www.tcpdf.org)
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Cube Attacks
Bárta, Josef ; Hojsík, Michal (advisor) ; Příhoda, Pavel (referee)
Based on the Cube Attack by Itai Dinur and Adi Shamir and another, in the essence simi- lar, method we devised a new polynomial linearisation technique, which proved to be more powerful, than the Cube Attack alone. Moreover, we present detailed description with formal proof not only of our findings, but also of the Cube Attack. Finally, we demonstrate the results of our efforts on a Trivium variant that is reduced in key and initialisation vector bit count. We managed to linearise polynomials representing a keystream bit output after up to 621 initialisation rounds using purely techniques described in this thesis, compared to 581 initialisation rounds with original attack.
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Applications of Gröbner bases in cryptography
Fuchs, Aleš ; Šťovíček, Jan (advisor) ; Žemlička, Jan (referee)
Title: Applications of Gröbner bases in cryptography Author: Aleš Fuchs Department: Department of Algebra Supervisor: Mgr. Jan Št'ovíček Ph.D., Department of Algebra Abstract: In the present paper we study admissible orders and techniques of multivariate polynomial division in the setting of polynomial rings over finite fields. The Gröbner bases of some ideal play a key role here, as they allow to solve the ideal membership problem thanks to their properties. We also explore features of so called reduced Gröbner bases, which are unique for a particular ideal and in some way also minimal. Further we will discuss the main facts about Gröbner bases also in the setting of free algebras over finite fields, where the variables are non-commuting. Contrary to the first case, Gröbner bases can be infinite here, even for some finitely generated two- sided ideals. In the last chapter we introduce an asymmetric cryptosystem Polly Cracker, based on the ideal membership problem in both commutative and noncommutative theory. We analyze some known cryptanalytic methods applied to these systems and in several cases also precautions dealing with them. Finally we summarize these precautions and introduce a blueprint of Polly Cracker reliable construction. Keywords: noncommutative Gröbner bases, Polly Cracker, security,...
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Time-memory tradeoff útoky
Seidlová, Monika ; Hojsík, Michal (advisor) ; Holub, Štěpán (referee)
Martin Hellman proposed the first time-memory tradeoff attack on block ciphers. It is a chosen plaintext attack, in which the attacker precomputes a large amount of data for some block cipher and can then use it repeatedly in attacks on that block cipher. An improvement suggested by Ron Rivest speeds up the attack by reducing the number of memory accesses. Another modification of the original attack called rainbow tables speeds up the attack even more and brings other advantages. Time-memory tradeoff attacks can also be applied to stream ciphers as known plaintext attacks. This bachelor thesis describes in detail the original attack, its improvements and a modification to stream ciphers. As an example, we summarize an attack on A5/1, a stream cipher used in mobile phones. We also propose a new time-memory tradeoff attack on block ciphers called r-coloured rainbows. The new attack is a modification of Hellman's attack and shares similarities with the rainbow table attack. We give a comparison of the properties of the three attacks and conclude that, for certain block ciphers, our attack may be the most effective of the three.
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Analysis of voice over IP protocols
Boháček, Milan ; El Bashir, Robert (advisor) ; Hojsík, Michal (referee)
In the presented work we focus on both implementation and protocol of the voice over IP application Skype. We analyse several versions of Skype clients and deduce inner workings of the Skype protocol. We present details about the cryptographic primitives used by the Skype clients and their impact on the security of the communication. We emphasize several places of suspicious leaks of the internal states of random generators and deduce rules for the detection of the Skype traffic on the firewall. In the end, we mention a simple enhancement of the current Skype clients that, in practice, can detect an ongoing eavesdropping.
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