The MPEG-1 layer-III compressed audio format, which is widely known as
MP
3
, is the most popular for audio distribution. However, it is not equipped with security features to protect the content from unauthorized access. Although encryption ensures content security, the naive method of encrypting the entire
MP
3
file would destroy compliance with the MPEG standard. In this paper, we propose a low-complexity partial encryption method that is embedded during the
MP
3
encoding process. Our method reduces time consumption by encrypting only the perceptually important parts of an
MP
3
file rather than the whole file, and the resulting encrypted file is still compatible with the MPEG standard so as to be rendered by any existing
MP
3
players. For full-quality rendering, decryption using the appropriate cryptographic key is necessary. Moreover, the effect of encryption on audio quality can be flexibly controlled by adjusting the percentage of encryption. On the basis of this feature, we can realize the try-before-purchase model, which is one of the important business models of Digital Rights Management (DRM): users can render encrypted
MP
3
files for trial and enjoy the contents in original quality by purchasing decryption keys. From our experiments, it turns out that encrypting 2-10% of
MP
3
data suffices to generate trial music, and furthermore file size increasing after encryption is subtle.
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