hiberfil.sys is the file used by default by Microsoft Windows to save the machine's state as part of the hibernation process. The operating system also keeps an open file handle to this file, so no user, including the Administrator, can read the file while the system is running.
Although most of the data structures required to parse the file format are available in the Microsoft Windows debug symbols, the compression used (Xpress) was undocumented until it was reverse engineered by Matthieu Suiche. He created with Nicolas Ruff a project called Sandman is the only open-source tool that can read and write the Windows hibernation file.
In early 2008, as part of the Windows Server Protocol Program, Microsoft released documentation on the Xpress compression algorithm in the Directory Replication Service (DRS) Remote Protocol specification (section 18.104.22.168.15, "DecompressWin2k3"). However, the pseudocode given contains numerous errors, as documented by Matthieu Suiche in a blog entry.