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Recently, there was a proposal of using the underdetermined blind source separation (BSS) principle to design image and speech encryption. In this paper, we report a cryptanalysis of this BSS-based encryption scheme and point out that it is not secure against known-/chosen-plaintext attack and chosen-ciphertext attack. In addition, we discuss some other security defects of the schemes: (1) it has a low sensitivity to part of the key and to the plaintext; (2) it is weak against a ciphertext-only differential attack; and (3) a divide-and-conquer (DAC) attack can be used to break part of the key. We finally analyze the role of BSS in this approach towards cryptographically secure ciphers.