Within the security architecture of the 3GPP system there is a standardised encryption mode f8 based on the block cipher KASUMI. In this work we examine the pseudorandomness of the block cipher KASUMI and the provable security of f8. First we show that the three round KASUMI is not a pseudorandom permutation ensemble but the four round KASUMI is a pseudorandom permutation ensemble under the adaptive distinguisher model by investigating the properties of the round functions in a clear way. Second we provide the upper bound on the security of f8 mode under the reasonable assumption from the first result by means of the left-or-right security notion.
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