IACR News item: 06 February 2013
Elena Andreeva, Andrey Bogdanov, Yevgeniy Dodis, Bart Mennink, John P. Steinberger
ePrint ReportKA_t(K,m)= k_t + P_t(... k_2 + P_2(k_1 + P_1(k_0 + m))...),
where (k_0,...,k_t) are obtained from the master key K using some key derivation function.
For t=1, KA_1 collapses to the well-known Even-Mansour cipher, which is known to be indistinguishable from a (secret) random permutation, if P_1 is modeled as a (public) random permutation. In this work we seek for stronger security of key-alternating ciphers --- indifferentiability from an ideal cipher --- and
ask the question under which conditions on the key derivation function and for how many rounds t is the key-alternating cipher KA_t indifferentiable from the ideal cipher, assuming P_1,...,P_t are (public) random permutations?
As our main result, we give an affirmative answer for t=5, showing that the 5-round key-alternating cipher KA_5 is indifferentiable from an ideal cipher, assuming P_1,...,P_5 are five independent random permutations, and the key derivation function sets all rounds keys
k_i=f(K), where 0
Additional news items may be found on the IACR news page.