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ETH Series in
Information Security and Cryptography
edited by Ueli Maurer
Volume 8
Johan Sjödin
Weak Pseudorandomness
and Unpredictability
1st edition / 1. Auflage
2007, XII, 108 pages / Seiten, € 65,00.
ISBN 3-86628-145-5
Basing the security of practical cryptographic schemes
on weakened assumptions, which are hence more likely to hold, and improving
their efficiency are central research goals in cryptography. This thesis
continues this quest.
We study the Feistel-network which is a popular
structure underlying many block-ciphers – e.g. DES – where the cipher is
constructed from many simpler rounds, each defined by some function. Our main
result shows that in the information-theoretic setting four rounds with
functions which are secure against non-adaptive chosen-plaintext attacks are
enough and necessary to get a permutation which is secure against
chosen-plaintext attacks. We also prove that this statement unfortunately does
not translate to the practically more relevant pseudorandom setting.
This thesis also comprises a study on weak
pseudorandom functions (WPRFs) and, in particular, shows how to transform a
WPRF into a fast and key-efficient symmetric encryption scheme, secure against
chosenciphertext attacks. A general paradigm for domain extension of message
authentication codes is also given, together with an essentially optimal
extension for practical use.
Keywords: weak pseudorandom
function, symmetric encryption, block cipher, message authentication code,
domain extension, range extension, known-plaintext attack, non-adaptive
chosen-plaintext attack
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