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While user access control and encryption can protect valuable data from passive observers, those techniques leave visible ciphertexts that are likely to alert an active adversary to the existence of the data, who can then compel an authorized user to disclose it. We introduce StegFS, a steganographic file system that aims to overcome that weakness by offering plausible deniability to owners of protected files. StegFS securely hides user-selected files in a file system so that, without the corresponding access keys, an attacker would not be able to deduce their existence, even if the attacker is thoroughly familiar with the implementation of the file system and has gained full access to it. Unlike previous steganographic schemes, our construction satisfies the prerequisites of a practical file system in ensuring integrity of the files and maintaining efficient space utilization. We have completed an implementation on Linux, and experiment results confirm that StegFS achieves an order of magnitude improvements in performance and/or space utilization over the existing schemes.