Lightweight devices, such as radio frequency identi- fication tags, have a limited storage capacity, which has become a bottleneck for many applications, especially for security applica- tions. Ciphertext-policy attribute-based encryption (CP-ABE) is a promising cryptographic tool, where the encryptor can decide the access structure that will be used to protect the sensitive data. However, current CP-ABE schemes suffer from the issue of having long decryption keys, in which the size is linear to and dependent on the number of attributes. This drawback prevents the use of lightweight devices in practice as a storage of the decryption keys of the CP-ABE for users. In this paper, we provide an affirmative answer to the above long standing issue, which will make the CP-ABE very practical. We propose a novel CP-ABE scheme with constant-size decryption keys independent of the number of attributes. We found that the size can be as small as 672 bits. In comparison with other schemes in the literature, the proposed scheme is the only CP-ABE with expressive access structures, which is suitable for CP-ABE key storage in lightweight devices.