10-04-2017, 07:55 PM
Block Ciphers - Analysis, Design and Applications
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Abstract
In this thesis we study cryptanalysis, applications and design of secret key
block ciphers. In particular, the important class of Feistel ciphers is studied,
which has a number of rounds, where in each round one applies a cryptographically
weak function.
Applications
The main application of block ciphers is that of encryption. We study the
available modes of operation for encryption, introduce a new taxonomy for
attacks on block ciphers and derive a new theoretical upper bound for attacks
on block ciphers. Also another important application of block ciphers is
studied; as building blocks for cryptographic hash functions. Finally we
examine how to use block ciphers as building blocks in the design of digital
signature schemes. In particular we analyse Merkle s proposed scheme and
show that under suitable and reasonable conditions, Merkle s scheme is secure
and practical.
Cryptanalysis
We study the most important known attacks on block ciphers, linear
cryptanalysis
and differential cryptanalysis and introduce a new attack based on
simple relations. Differential cryptanalysis makes use of so-called differentials
(A,B), i.e., a pair of plaintexts with difference A, which after a certain
number of rounds result in a difference B with a non-negligible probability.
This fact can be used to derive (parts of) the secret key.
Design
We discuss principles for the design of secure block ciphers. For both linear
and differential cryptanalysis we establish lower bounds on the complexities
of success of attacks. It is furthermore shown that there exist functions,
which can be used to construct block ciphers provable secure against both
linear and differential attacks, the two most important attacks known to
date. Furthermore we define so-called strong key schedules. A block cipher
with a strong key schedule is shown to be secure against attacks based on
simple relations and the improved immunity to other attacks is discussed.
Also we give a simple design of a strong key schedule.