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uncrypt can read a file on an encrypted filesystem and rewrite it to
the same blocks on the underlying (unencrypted) block device.  This
destroys the contents of the file as far as the encrypted filesystem
is concerned, but allows the data to be read without the encryption
key if you know which blocks of the raw device to access.  uncrypt
produces a "block map" file which lists the blocks that contain the file.

For unencrypted filesystem, uncrypt will produce the block map without
touching the data.

Bug: 12188746
Change-Id: Ib7259b9e14dac8af406796b429d58378a00c7c63