This document proposes a key-aggregate cryptosystem (KAC) for securely and flexibly sharing encrypted data in cloud storage. KAC allows a data owner to encrypt files under different class identifiers and generate a single aggregate key encompassing decryption power for any chosen ciphertext class set. This aggregate key can be sent through limited secure channels like email, addressing disadvantages of traditional approaches that require sending many distinct secret keys. The proposed solution could help Alice securely share specific encrypted files on Dropbox with Bob by sending him a single aggregate key.