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Cryptanalysis and design of block ciphers. Jorge
NAKAHARA. (формат pdf, 1.5 М)
Cryptanalysis and design of block ciphers. Jorge NAKAHARA.
(zip архив
pdf файла, 1.3 М)
This thesis focuses on cryptanalysis techniques and design of block ciphers.
In
particular, modern analysis methods such as square, boomerang, impossible
differential and linear attacks are described and applied to real block
ciphers.
The first part of this thesis concentrates on the two most relevant modern
cryptanalysis techniques: linear and differential cryptanalysis. These
and related
techniques have been applied to SAFER K/SK/+/++, IDEA, Hierocrypt-
3, Hierocrypt-L1, Skipjack and the PES ciphers. In many attacks, the interaction
between the block cipher and its key schedule algorithm was exploited, so
that the complexity of key-recovery attacks could be reduced. These analyses
often led to the discovery of weak keys, namely, key values for which the
attack
complexity was comparatively lower than for a random key. In some cases,
the
existence of weak keys, derived from the original key schedule, and holding
for
the whole cipher, might suggest a need for a redesign of the key setup algorithm.
The second part of this thesis describes and analyzes new block ciphers,
called MESH, which were designed with the same group operations as the IDEA
block cipher. Three designs are presented: MESH-64, MESH-96, and MESH-
128. Their novel features include: (i) flexible block sizes (in increments
of 32
bits), (ii) larger round functions (MA-boxes) compared to IDEA, (iii) distinct
key-mixing layers for odd and even rounds, and (iv) new key schedule algorithms
that achieve fast key avalanche. Estimates for the software performance of
MESH ciphers indicate better or comparable speed to that of triple-DES. A
preliminary security evaluation of these three ciphers included truncated
and
impossible differentials, linear, square, slide and advanced-slide, multiplicative
differentials (on simplified versions), and Demirci’s attacks, among others.
The
initial results of these attacks seem to indicate that the MESH ciphers present
a relatively large margin of security against modern cryptanalysis techniques.
Other cipher designs are further suggested, based on the flexible MA-boxes,
and
on the alternative AM-boxes.