An Incomplete List of Theses

• Jianhong Xu, "Design and Implementation of an Advanced Substitution-Permutation Encryption Network", M.Eng. Thesis, Memorial University of Newfoundland, St. John's, Newfoundland, Aug. 1997.
  
  To solve the problems of data security in modern electronic communication environments and applications, researchers have been placing much effort on the design of efficient and secure ciphers. Substitution-permutation encryption networks (SPNs) are an important class of private-key block ciphers. The objective of this thesis is to develop an advanced substitution-permutation encryption network that not only is efficiently secure but also can be simply implemented in hardware and software.
  Two of the most powerful attacks are linear cryptanalysis and differential cryptanalysis. After investigating the application of linear cryptanalysis to an SPN, a new nonlinearity criterion for the design of S-boxes is presented. S-boxes satisfying this criterion strengthen the ability of an SPN to frustrate linear cryptanalysis. As well, we propose a novel linear transformation as the method of interconnection between rounds of S-boxes. The use of the linear transformation increases the resistance of an SPN to both linear cryptanalysis and differential cryptanalysis.
  Finally, we implement an SPN which consists of our new linear transformation and 4x4 S-boxes satisfying our new design criterion by using a Field Programmable Gate Array (FPGA). The simulation results confirm that the digital hardware implementation of the SPN is practical and not complicated.
  [Postscript not available.]
  
  
• Xia Zhu, "A New Class of Unbalanced CAST Ciphers and Its Security Analysis",M.Eng. Thesis, Memorial University of Newfoundland, St. John's, Newfoundland, May 1997.
  
  The original CAST cipher is an efficient and secure private-key block cipher designed to be an alternative to the Data Encryption Standard (DES). In this thesis, we present a new class of unbalanced CAST ciphers which employs the same structure of S-box and round function of the original CAST cipher but has a lower memory requirement. Unbalanced CAST ciphers with one or two 8x32 S-boxes in the round function require only 1/4 to 1/2 the memory of the original CAST cipher, respectively.
  This thesis examines the application of differential and linear cryptanalysis, two of the most powerful methodologies for attacking private-key block ciphers, to the unbalanced CAST ciphers. The results of analysis show that a 48-round unbalanced CAST cipher with one 8x32 S-box and a 24-round unbalanced CAST cipher with two 8x32 S-boxes, both of which are equivalent to a 12-round original CAST cipher in efficiency, are resistant to both differential and linear cryptanalysis.
  We also investigate the unbalanced CAST ciphers from the perspective of information theory. The results suggest that the maximum static and dynamic input-output bit information leakages for the unbalanced CAST ciphers constructed by 8x32 S-boxes are much smaller than for DES.
  The conclusion reached by the thesis is that unbalanced CAST ciphers can be considered to be efficient, secure ciphers which require less memory than the original CAST cipher.
  [Postscript not available.]
  
  
• H.M. Heys, "The Design of Substitution-Permutation Network Ciphers Resistant to Cryptanalysis", Ph.D. Thesis, Queen's University, Kingston, Ontario, Aug. 1994.
  
  In this thesis, we examine a fundamental class of private-key block ciphers referred to as substitution-permutation networks (SPNs). In particular, we study design principles which demonstrably aid in providing resistance to cryptanalysis.
  The objective of the analysis is to develop a flexible architecture than can be efficiently implemented in as few rounds as possible to provide an adequate level of security. Toward this end, we present new forms of cryptanalysis of a fundamental class of SPNs called tree-structured SPNs and develop a novel, effective attack which exploits key-dependent degeneracies and that is applicable to all classes of SPNs. As well, we consider the application of two established, powerful attacks: differential cryptanalysis and linear cryptanalysis. We find that the appropriate selection of S-boxes and S-box interconnections greatly reduces the effectiveness of the attacks. In particular, we establish the following general design principles: (i) large S-boxes are cryptographically stronger than smaller S-boxes, (ii) the diffusion of bit changes is an important S-box property, and (iii) the permutation between rounds of S-boxes may be effectively replaced by a diffusive linear transformation.
  We also examine the avalanche characteristics of SPNs in response to both plaintext and key changes. We develop an analytical model of the avalanche criterion and the relate the key avalanche property to the application of a key clustering attack. The results of the analysis further confirm the general design principles suggested above.
  [Postscript not available.]
  
  

[HomePage] [Research] [Courses] [Links]