In a typical broadcast encryption scenario, a sender wishes to securely transmit messages to a subset of receivers, the intended set, using a broadcast channel. Several schemes for broadcast encryption exist and they allow the sender to reach a privileged set of receivers and by the use of encryption block all others from receiving the message. Most of the existing broadcast encryption literature assumes that the intended set and the privileged set are equal but this is not always necessary. In some applications a slight difference between the intended and the privileged set may be tolerated if the cost of transmitting the message decreases sufficiently. It has been suggested that a few free-riders, users not in the intended set but in the privileged set, may be allowed in some scenarios. In rare cases the opposite could also be possible, that is some users are in the intended set but not in the privileged set. Our approach is to use the information theoretic concept of distortion to measure the discrepancy between the intended and the privileged sets. As a cost measure we use the average number of transmissions required to send one message. As an example of the use for these measures we have developed three simple algorithms that aim to lower the cost by adding some distortion; one greedy algorithm and two versions of an algorithm based on randomness. By simulations we have compared them using our cost and distortion measures. The subset difference (SD) scheme has been used as the underlying broadcast encryption scheme. The greedy algorithm is not tightly bound to the SD scheme while the two randomness-based algorithms take some use of the properties of the SD scheme.

This report considers the subset difference scheme for broadcast encryption and the number of broadcast transmissions required when using this scheme. For cases where the privileged users are gathered in a few groups we derive the worst case number of transmissions. We also present an upper bound for the number of transmissions based on the number of transitions between privileged and nonprivileged users in the user set.

The need for broadcast encryption arises when a sender wishes to securely distribute messages to varying subsets of receivers, using a broadcast channel, for instance in a pay-TV scenario. This is done by selecting subsets of users and giving all users in the same subset a common decryption key. The subsets will in general be overlapping so that each user belongs to many subsets and has several different decryption keys. When the sender wants to send a message to some users, the message is encrypted using keys that those users have. In this thesis we describe some broadcast encryption schemes that have been proposed in the literature. We focus on stateless schemes which do not require receivers to update their decryption keys after the initial keys have been received; particularly we concentrate on the Subset Difference (SD) scheme.

We consider the effects that the logical placement of the receivers in the tree structure used by the SD scheme has on the number of required transmissions for each message. Bounds for the number of required transmissions are derived based on the adjacency of receivers in the tree structure. The tree structure itself is also studied, also resulting in bounds on the number of required transmissions based on the placement of the users in the tree structure.

By allowing a slight discrepancy between the set of receivers that the sender intends to send to and the set of receivers that actually can decrypt the message, we can reduce the cost in number of transmissions per message. We use the concept of distortion to quantify the discrepancy and develop three simple algorithms to illustrate how the cost and distortion are related.

We consider the subset difference scheme for broadcast encryption and count the number of required transmissions when using this scheme. The subset scheme organizes receivers in a tree structure and we note that isomorphic trees yield the same number of required transmissions. We then study the group properties of isomorphism classes of trees. Finally we formulate some research questions for further study of the performance of the subset difference scheme.

We consider the broadcast encryption problem where one sender wishes to transmit messages securely to a selected set of receivers using a broadcast channel, as is the case in digital television for example. Specifically, we study the subset difference scheme for broadcast encryption and the number of broadcast transmissions required when using this scheme. The effects of adjacency in the user set are considered and we introduce the notion of transitions in the user set as a means to quantify the adjacency. We present upper and lower bounds for the number of transmissions based on the number of transitions between privileged and nonprivileged users in the user set. For cases where the privileged users are gathered in a few groups we derive the maximum number of transmissions.

In this paper we consider the subset difference scheme for broadcast encryption and count the number of required broadcast transmissions when using this scheme. The subset difference scheme organizes receivers in a tree structure and we note that isomorphic trees yield the same number of required broadcast transmissions. Based on the isomorphism the trees can be partitioned into classes. We suggest to use the vast amount of tools available from the theory of groups to analyze the subset difference scheme and therefore we formulate the mappings between isomorphic trees using concepts from group theory. Finally we identify some research issues for further study of the performance of the subset difference scheme using group theory.

A digital fingerprint is a unique pattern embedded in a digital document to be able to identify a specific copy when it is used illegally. We have looked at two specific code structures for fingerprinting purpose. Binary linear codes, often used as error correcting codes, and what we call a binary sorted code.

In a fingerprinting system, the tracing properties are not properties only of the code, they are also dependent of how descendant words can be created. In this correspondence a simple characterization of descendant set models is presented, and relations between different tracing properties in these descendant set models are derived.

In this paper we present a simplified model for deep sub-micron, on-chip, parallel data buses. Using this model a coding technique similar to Bus Invert Coding is presented, but with a better performance in the proposed model. The coding technique can be realized using low-complexity encoding and decoding circuitry, and with a complexity that scales linearly with the bus width. Simulation results show that the energy dissipation decreases with approximately 20% for buses with up to 16 wires.

In this paper we present a simplified model of parallel, on-chip buses, motivated by the movement toward CMOS technologies where the ratio between inter-wire capacitance and wire-to-ground capacitance is very large. We also introduce a ternary bus state representation, suitable for the bus model. Using this representation we propose a coding scheme without memory which reduces energy dissipation in the bus model by approximately 20-30% compared to an uncoded system. At the same time the proposed coding scheme is easy to realize, in terms of standard cells needed, compared to several previously proposed solutions.

Traceability codes are identifiable parent property (IPP) codes with the additional requirement that Hamming distance can be used to trace a parent of a word. Traceability codes can be used for constructing digital fingerprints in order to deter users from illegally copying digital data. We construct a class of traceability codes and determine the exact parameters of some of the codes in this class.

Three binary fingerprinting code classes with properties similar to codes with the identifiable parent property are proposed. In order to compare such codes a new combinatorial quality measure is introduced. In the case of two cooperating pirates the measure is derived for the proposed codes, upper and lower bounds are constructed and the results of computer searches for good codes in the sense of the quality measure are presented. Some properties of the quality measure are also derived.

Using a newly introduced alternative to a conventional SRAM cell a binary zero can be written with a much lower power consumption than a binary one. Such a solution reduces power consumption, especially if there are few ones in the data, that is, if the data has a low Hamming weight. If the data is not inherently of low weight, this can be achieved by encoding the data. In the paper such coding is discussed and in small cases energy efficient encoding and decoding realizations are presented.

We discuss coding for deep sub-micron buses with highly sequential data, the typical application being address buses, and we note that coding techniques specifically targetted at this application are considerably better than general techniques. Previously proposed coding schemes are described and a new, non-redundant coding technique with a very small realization and more than 50% reduction of power dissipation is presented.

A coding technique for deep sub-micron address buses with inter-wire capacitances dominating the wire-to-ground capacitances is presented. This code is similar to Gray codes, in the sense that it defines an ordering of the binary space, such that adjacent codewords dissipate little energy when sent consecutively. The ordering is shown to be close to optimal, as to the energy dissipation, when sending the whole sequence in order. A circuit diagram realizing the coder is presented, using only n-1 two-input gates, where n is the bus width. Simulations show an improvement in energy dissipation of more than 50% over an uncoded bus in several cases, depending on the data being coded.

We present two coding techniques for reducing the power dissipation in deep sub-micron, parallel data buses. The techniques differ in their parameter values and are suitable in different scenarios. In both cases typical reduction in power dissipation is 20%.

We introduce the concept of mobility-based communication in ad hoc networks, meaning that the packet transport is performed mainly by the nodes' movement. We outline a model for such networks, utilizing a stochastic model for the geographical location of the nodes. A test case is defined in which three strategies for packet forwarding are presented and evaluated.

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A new approach to low redundancy coding for reducing power dissipation in parallel on-chip, deep sub-micron buses is presented. It is shown that the new approach allows lower power dissipation than previous solutions in the given model, yielding reductions of 24% to 41% compared to uncoded transmission for the considered bus widths. Finally some important open problems are given.

The widespread use of computer technology for information handling resulted in the need for higher data protection.The usage of high profile cryptographic protocols and algorithms do not always necessarily guarantee high security. They are needed to be used according to the needs of the organization depending upon certain characteristics and available resources.The communication system in a cryptographic environment may become vulnerable to attacks if the cryptographic packages don’t meet their intended goals.

This master’s thesis is targeted towards the goal of evaluating contemporary cryptographic algorithms and protocols collectively named as cryptographic packages as per security needs of the organization with the available resources.

The results have shown that there certainly is a need for careful evaluations of cryptographic packages given with available resources otherwise it could turn into creating more severe problems such as network bottlenecks, information and identity loss, non trustable environment and computational infeasibilities resulting in huge response times. In contrast, choosing the right package with right security parameters can lead to a secure and best performance communication environment.