Yi et al. have proposed a cipher called the fast encryption algorithm for multimedia (FEA-M). Recently Mihaljevic and Kohno pointed out that FEA-M is insecure. However, their attacks are not efficient: their chosen plaintext attack and known plaintext attack require 2~(37)-bit chosen plaintext and 2~(60)-bit known plaintext, respectively. In this paper we give an efficient known plaintext attack against FEA-M. Our attack requires only 2~(28)-bit known plaintext and about 2~(33) XOR operations.
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