This paper reports security problems with improper implementations of an improved version of FEA-M (fast encryption algorithm for multimedia). It is found that an implementation-dependent differential chosen-plaintext attack or its chosenciphertext counterpart can reveal the secret key of the cryptosystem, if the involved (pseudo-)random process can be tampered (for example, through a public time service). The implementation-dependent differential attack is very efficient in complexity and needs only O(n 2) chosen plaintext or ciphertext bits. In addition, this paper also points out a minor security problem with the selection of the session key. In real implementations of the cryptosystem, these security problems should be carefully avoided, or the cryptosystem has to be further enhanced to work under such weak implementations. Key words: multimedia encryption, FEA-M, insecure implementation, differential attack, chosen-plaintext attack, chosen-ciphertext attack, pseudo-random process
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