Hartung-Gorre Verlag

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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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ETH Series in Information Security and Cryptography