Preserving Hidden Data with an Ever-Changing Disk

This paper presents a storage system that can hide the presence of hidden data alongside a larger volume of public data. Encryption allows a user to hide the contents of data, but not the fact that sensitive data is present. Under duress, the owner of high-value data can be coerced by a powerful adversary to disclose decryption keys. Thus, private users and corporations have an interest in hiding the very presence of some sensitive data, alongside a larger body of less sensitive data (e.g., the operating system and other benign files); this property is called plausible deniability. Existing plausible deniability systems do not fulfill all of the following requirements: (1) resistance to multiple snapshot attacks where an attacker compares the state of the device over time; (2) ensuring that hidden data won't be destroyed when the public volume is modified by a user unaware of the hidden data; and (3) disguising writes to secret data as normal system operations on public data.

We explain why existing solutions do not meet all these requirements and present the Ever-Changing Disk (ECD), a generic scheme for plausible deniability storage systems that meets all of these requirements. An ECD stores hidden data inside a large volume of pseudorandom data. Portions of this volume are periodically migrated in a log-structured manner. Hidden writes can then be interchanged with normal firmware operations. The expected access patterns and time until hidden data is overwritten are completely predictable, and insensitive to whether data is hidden. Users control the rate of internal data migration (R), trading write bandwidth to hidden data for longevity of the hidden data. For a typical 2TB disk and setting of R, a user preserves hidden data by entering her secret key every few days or weeks.