Design, Implementation and Cryptanalysis of Modern Symmetric Ciphers

The main objective of this thesis is to examine the trade-offs between security and efficiency within symmetric ciphers. This includes the influence that block ciphers have on the new generation of word-based stream ciphers. By incorporating block-cipher like components into their designs, word-based stream ciphers have experienced hundreds-fold improvement in speed over bit-based stream ciphers, without any observable security degradation. The thesis also emphasizes the importance of keying issues in block and stream ciphers, showing that by reusing components of the principal cipher algorithm in the keying algorithm, security can be enhanced without loss of key-agility or expanding footprint in software memory. Firstly, modern block ciphers from four recent cipher competitions are surveyed and categorized according to criteria that includes the high-level structure of the block cipher, the method in which non-linearity is instilled into each round, and the strength of the key schedule. In assessing the last criterion, a classification by Carter [45] is adopted and modified to improve its consistency. The classification is used to demonstrate that the key schedule of the Advanced Encryption Standard (AES) [62] is surprisingly flimsy for a national standard. The claim is supported with statistical evidence that shows the key schedule suffers from bit leakage and lacks sufficient diffusion. The thesis contains a replacement key schedule that reuses components from the cipher algorithm, leveraging existing analysis to improve security, and reducing the cipher's implementation footprint while maintaining key agility. The key schedule is analyzed from the perspective of an efficiency-security tradeoff, showing that the new schedule rectifies an imbalance towards e±ciency present in the original. The thesis contains a discussion of the evolution of stream ciphers, focusing on the migration from bit-based to word-based stream ciphers, from which follows a commensurate improvement in design flexibility and software performance. It examines the influence that block ciphers, and in particular the AES, have had upon the development of word-based stream ciphers. The thesis includes a concise literature review of recent styles of cryptanalytic attack upon stream ciphers. Also, claims are refuted that one prominent word-based stream cipher, RC4, suffers from a bias in the first byte of each keystream. The thesis presents a divide and conquer attack against Alpha1, an irregularly clocked bit-based stream cipher with a 128-bit state. The dominating aspect of the divide and conquer attack is a correlation attack on the longest register. The internal state of the remaining registers is determined by utilizing biases in the clocking taps and launching a guess and determine attack. The overall complexity of the attack is 261 operations with text requirements of 35,000 bits and memory requirements of 2 29.8 bits. MUGI is a 64-bit word-based cipher with a large Non-linear Feedback Shift Register (NLFSR) and an additional non-linear state. In standard benchmarks, MUGI appears to su®er from poor key agility because it is implemented on an architecture for which it is not designed, and because its NLFSR is too large relative to the size of its master key. An unusual feature of its key initialization algorithm is described. A variant of MUGI, entitled MUGI-M, is proposed to enhance key agility, ostensibly without any loss of security. The thesis presents a new word-based stream cipher called Dragon. This cipher uses a large internal NLFSR in conjunction with a non-linear filter to produce 64 bits of keystream in one round. The non-linear filter looks very much like the round function of a typical modern block cipher. Dragon has a native word size of 32 bits, and uses very simple operations, including addition, exclusive-or and s-boxes. Together these ensure high performance on modern day processors such as the Intel Pentium family. Finally, a set of guidelines is provided for designing and implementing symmetric ciphers on modern processors, using the Intel Pentium 4 as a case study. Particular attention is given to understanding the architecture of the processor, including features such as its register set and size, the throughput and latencies of its instruction set, and the memory layouts and speeds. General optimization rules are given, including how to choose fast primitives for use within the cipher. The thesis describes design decisions that were made for the Dragon cipher with respect to implementation on the Intel Pentium 4. Block Ciphers, Word-based Stream Ciphers, Cipher Design, Cipher Implementa- tion,

Securing Grid Data Using Mandatory Access Controls

The main contribution of this paper is to investigate issues in using Mandatory Access Controls (MACs), namely those provided by SELinux, to secure application-level data. Particular emphasis is placed on health-care records located on the grid. The paper disccuses the importance of a trusted computing base in providing application security. It de- scribes a secure three-tiered architecture, incorporating trusted hardware, SELinux, and application security mechanisms that are appropriate for securing sensitive application data

Utilizing SELinux to Mandate Ultra-secure Access Control of Medical Records

Ongoing concerns have been raised over the effectiveness of information technology products and systems in maintaining privacy protection for sensitive data. The aim is to ensure that sensitive health information can be adequately protected yet still be accessible only to those that "need-to-know". To achieve this and ensure sustainability over the longer term, it is advocated that an alternative, stable and secure system architecture is required. This paper considers the adoption of a model targeted at health information that provides much higher degrees of protection. A purpose built demonstrator that was developed based on enterprise-level systems software products is detailed. The long term aim is to provide a viable solution by utilizing contemporary, commercially supported operating system and allied software. The advantages and limitations in its application with a medical database are discussed. The future needs in terms of research, software development and changes in organizational policy for healthcare providers, are outlined

Current Approaches to Secure Health Information Systems are NOT Sustainable: An Analysis

This paper proposes a viable IT-based solution for ensuring the privacy and security of sensitive information in contemporary Health Information Systems (HIS)

Algebraic analysis of LEX

LEX is a stream cipher that progressed to Phase 3 of the eSTREAM stream cipher project. In this paper, we show that the security of LEX against algebraic attacks relies on a small equation system not being solvable faster than exhaustive search. We use the byte leakage in LEX to construct a system of 21 equa- tions in 17 variables. This is very close to the require- ment for an efficient attack, i.e. a system containing 16 variables. The system requires only 36 bytes of keystream, which is very low

Improved cryptanalysis of the common scrambling algorithm stream cipher

This paper provides a fresh analysis of the widely-used Common Scrambling Algorithm Stream Cipher (CSA-SC). Firstly, a new representation of CSA-SC with a state size of only 89 bits is given, a significant reduction from the 103 bit state of a previous CSA-SC representation. Analysis of this 89-bit representation demonstrates that the basis of a previous guess-and-determine attack is flawed. Correcting this flaw increases the complexity of that attack so that it is worse than exhaustive key search. Although that attack is not feasible, the reduced state size of our representation makes it obvious that CSA-SC is vulnerable to several generic attacks, for which feasible parameters are given

A Novel Use of RBAC to Protect Privacy in Distributed Health Care Information Systems

This paper examines the access control requirements of distributed health care information networks. Since the electronic sharing of an individual's personal health information requires their informed consent, health care information networks need an access control framework that can capture and enforce individual access policies tailored to the specific circumstances of each consumer. Role Based Access Control (RBAC) is examined as a candidate access control framework. While it is well suited to the task in many regards, we identify number of shortcomings, particularly in the range of access policy expression types that it can support. For efficiency and comprehensibility, access policies that grant access to a broad range of entities whilst explicitly denying it to subgroups of those entities need to be supported in health information networks. We argue that RBAC does not support policies of this type with sufficient flexibility and propose a novel adaptation of RBAC principles to address this shortcoming. We also describe a prototype distributed medical information system that embodies the improved RBAC model