International Association for Cryptologic Research

International Association
for Cryptologic Research

CryptoDB

Côme Berbain

Publications

Year
Venue
Title
2008
EPRINT
Understanding Phase Shifting Equivalent Keys and Exhaustive Search
Côme Berbain Aline Gouget Hervé Sibert
Recent articles~\cite{kucuk,ckp08,isobe,cryptoeprint:2008:128} introduce the concept of phase shifting equivalent keys in stream ciphers, and exploit this concept in order to mount attacks on some specific ciphers. The idea behind phase shifting equivalent keys is that, for many ciphers, each internal state can be considered as the result of an injection of a key and initialization vector. This enables speeding up the standard exhaustive search algorithm among the $2^n$ possible keys by decreasing the constant factor of $2^n$ in the time complexity of the algorithm. However, this has erroneously been stated in~\cite{isobe,cryptoeprint:2008:128} as decreasing the complexity of the algorithm below $2^n$. In this note, we show why this type of attacks, using phase shifting equivalent keys to improve exhaustive key search, can never reach time complexity below $2^n$, where $2^n$ is the size of the key space.
2007
ASIACRYPT
2007
FSE
2007
EPRINT
Generic Attacks on Unbalanced Feistel Schemes with Expanding Functions
\begin{abstract} Unbalanced Feistel schemes with expanding functions are used to construct pseudo-random permutations from $kn$ bits to $kn$ bits by using random functions from $n$ bits to $(k-1)n$ bits. At each round, all the bits except $n$ bits are changed by using a function that depends only on these $n$ bits. C.S.Jutla \cite{Jut} investigated such schemes, which he denotes by $F^d_k$, where $d$ is the number of rounds. In this paper, we describe novel Known Plaintext Attacks (KPA) and Non Adaptive Chosen Plaintext Attacks (CPA-1) against these schemes. With these attacks we will often be able to improve the result of C.S.Jutla. We also give precise formulas for the complexity of our attacks in $d$, $k$ and $n$. \end{abstract}
2006
ASIACRYPT
2006
EUROCRYPT
2006
FSE
2005
FSE