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Untraceable communication is about hiding the identity of the sender or the recipient of a message. Currently most systems used in practice (e.g., TOR) rely on the principle that a message is routed via several relays to obfuscate its path through the network. However, as this increases the end-to-end latency it is not ideal for applications like Voice-over-IP (VoIP) communication, where participants will notice annoying delays if the data does not arrive fast enough. We propose an approach based on the paradigm of Dining Cryptographer networks (DC-nets) that can be used to realize untraceable communication within small groups. The main features of our approach are low latency and resilience to packet-loss and fault packets sent by malicious players. We consider the special case of VoIP communication and propose techniques for a P2P implementation. We expose existing problems and sketch possible future large-scale systems composed of multiple groups.

The dining cryptographers protocol provides information-theoretically secure sender and recipient untraceability. However, the protocol is considered to be impractical because a malicious participant may disrupt the communication. We propose an implementation which provides information-theoretical security for senders and recipients, and in which a disruptor with limited computational capabilities can easily be detected.

The dining cryptographers protocol implements a multiple access channel in which senders and recipients are anonymous. A problem is that a malicious participant can disrupt communication by deliberately creating collisions. We propose a computationally secure dining cryptographers protocol with collision resolution that achieves a maximum stable throughput of 0.924 messages per round and which allows to easily detect disruptors.

We introduce a symmetry reduction technique for model checking temporal-epistemic properties of multi-agent systems defined in the mainstream interpreted systems framework. The technique, based on counterpart semantics, aims to reduce the set of initial states that need to be considered in a model. We present theoretical results establishing that there are neither false positives nor false negatives in the reduced model. We evaluate the technique by presenting the results of an implementation tested against two well known applications of epistemic logic, the muddy children and the dining cryptographers. The experimental results obtained confirm that the reduction in model checking time can be dramatic, thereby allowing for the verification of hitherto intractable systems., QC 20120207

Práce je zaměrena na kryptografické protokoly a jejich implementaci do programu CrypTool. Cílem práce je určit, které kryptografické protokoly by bylo vhodné do programu CrypTool implementovat a následně jejich implementaci provést takovým způsobem, aby jejich demonstrace byla co nejnázornější a nejvhodnější pro pochopení jejich konceptu. V souvislosti se zadáním této práce byly vybrány protokoly Dining Cryptographers, Coin Flipping a Zero-Knowledge. Jako další vhodné protokoly byly přidány Oblivious Transfer a Yao´s Millionaire Problem. V práci je podrobně popsán koncept vybraných protokolů a postup při jejich implementaci., This work is focused on cryptographic protocols and their implementation in the CrypTool. Goal of this work is to settle which cryptographics protocols are appropriate to implement and to find the way how to implement them most illustratively and usably for educational purposes. These protocols were chosen according to the work assigment: Dining Cryptograhpers, Coin Flippin and Zero-Knowledge. As other appropriate protocols were added: Yao´s Millionaire Problem and Oblivious Transfer. Detailed concept of chosen protocols and their implementation process are described in this work.

Práce je zaměrena na kryptografické protokoly a jejich implementaci do programu CrypTool. Cílem práce je určit, které kryptografické protokoly by bylo vhodné do programu CrypTool implementovat a následně jejich implementaci provést takovým způsobem, aby jejich demonstrace byla co nejnázornější a nejvhodnější pro pochopení jejich konceptu. V souvislosti se zadáním této práce byly vybrány protokoly Dining Cryptographers, Coin Flipping a Zero-Knowledge. Jako další vhodné protokoly byly přidány Oblivious Transfer a Yao´s Millionaire Problem. V práci je podrobně popsán koncept vybraných protokolů a postup při jejich implementaci., This work is focused on cryptographic protocols and their implementation in the CrypTool. Goal of this work is to settle which cryptographics protocols are appropriate to implement and to find the way how to implement them most illustratively and usably for educational purposes. These protocols were chosen according to the work assigment: Dining Cryptograhpers, Coin Flippin and Zero-Knowledge. As other appropriate protocols were added: Yao´s Millionaire Problem and Oblivious Transfer. Detailed concept of chosen protocols and their implementation process are described in this work.

Práce je zaměrena na kryptografické protokoly a jejich implementaci do programu CrypTool. Cílem práce je určit, které kryptografické protokoly by bylo vhodné do programu CrypTool implementovat a následně jejich implementaci provést takovým způsobem, aby jejich demonstrace byla co nejnázornější a nejvhodnější pro pochopení jejich konceptu. V souvislosti se zadáním této práce byly vybrány protokoly Dining Cryptographers, Coin Flipping a Zero-Knowledge. Jako další vhodné protokoly byly přidány Oblivious Transfer a Yao´s Millionaire Problem. V práci je podrobně popsán koncept vybraných protokolů a postup při jejich implementaci., This work is focused on cryptographic protocols and their implementation in the CrypTool. Goal of this work is to settle which cryptographics protocols are appropriate to implement and to find the way how to implement them most illustratively and usably for educational purposes. These protocols were chosen according to the work assigment: Dining Cryptograhpers, Coin Flippin and Zero-Knowledge. As other appropriate protocols were added: Yao´s Millionaire Problem and Oblivious Transfer. Detailed concept of chosen protocols and their implementation process are described in this work.

Práce je zaměrena na kryptografické protokoly a jejich implementaci do programu CrypTool. Cílem práce je určit, které kryptografické protokoly by bylo vhodné do programu CrypTool implementovat a následně jejich implementaci provést takovým způsobem, aby jejich demonstrace byla co nejnázornější a nejvhodnější pro pochopení jejich konceptu. V souvislosti se zadáním této práce byly vybrány protokoly Dining Cryptographers, Coin Flipping a Zero-Knowledge. Jako další vhodné protokoly byly přidány Oblivious Transfer a Yao´s Millionaire Problem. V práci je podrobně popsán koncept vybraných protokolů a postup při jejich implementaci., This work is focused on cryptographic protocols and their implementation in the CrypTool. Goal of this work is to settle which cryptographics protocols are appropriate to implement and to find the way how to implement them most illustratively and usably for educational purposes. These protocols were chosen according to the work assigment: Dining Cryptograhpers, Coin Flippin and Zero-Knowledge. As other appropriate protocols were added: Yao´s Millionaire Problem and Oblivious Transfer. Detailed concept of chosen protocols and their implementation process are described in this work.