טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentDunkelman Orr
SubjectTechniques for Cryptanalysis of Block Ciphers
DepartmentDepartment of Computer Science
Supervisor Professor Eli Biham


Abstract

Block ciphers play an important role in many cryptographic and security protocols. They are used to conceal confidential information, not only during its broadcast over a public channel, but also when it is stored (e.g., on the hard disk of a personal computer). The security of these protocols and storing procedures relies on the security of the underlying block ciphers against various attacks.


The field of cryptanalysis of block ciphers focuses on assessing the security of block ciphers. The process of assessment of the security of a given block cipher is mostly concerned with applying various cryptanalytic techniques to the block cipher. Each of these techniques has a different attack model and aims at exploiting different design flaws.


This thesis presents new cryptanalytic techniques that can be used to assess the security of block ciphers. The techniques are: the rectangle attack, the enhanced differential-linear attack, the differential-bilinear attack, the higher-order differential-linear attack, and the differential-bilinear boomerang attack (the differential-bilinear attack is actually four different attacks, whose most general version is the differential-bilinear attack). We also introduce the related-key boomerang and the related-key rectangle attacks.


We apply these newly developed techniques to various block ciphers. For example, our related-key rectangle attack on AES-192 is the first theoretical attack on 9-round AES-192 faster than exhaustive key search. We also present the first attack on the full 3GPP block cipher KASUMI. We apply our proposed techniques to other ciphers as well, such as DES, Serpent, IDEA, and COCONUT98.