Tweakable Block Ciphers Secure Beyond the Birthday Bound in the Ideal Cipher Model

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Presented at Asiacrypt2018 2018 by

Byeonghak Lee

We propose a new construction of tweakable block ciphers from standard block ciphers. Our construction, dubbed mathsf {XHX2}, is the cascade of two independent mathsf {XHX} block ciphers, so it makes two calls to the underlying block cipher using tweak-dependent keys. We prove the security of mathsf {XHX2} up to min {2^{2(n+m)/3},2^{n+m/2}} queries (ignoring logarithmic factors) in the ideal cipher model, when the block cipher operates on n-bit blocks using m-bit keys. The mathsf {XHX2} tweakable block cipher is the first construction that achieves beyond-birthday-bound security with respect to the input size of the underlying block cipher in the ideal cipher model.