| ##### Update file format signatures

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DROID can update its file format signatures using the PRONOM technical registry service hosted at the National Archives. To manually update your file format signatures, select the Tools / Check for signature updates... menu item.

Either enter the file name and path of the signature file you want to install, or press the Browse... button to bring up a file selection dialog. If you check the "Use this signature file as default", it will become the default for new profiles. If not, the signature will be available to select in the preferences should you want to use it at some point in the future, but will not be used by default. When you have selected the file to install, press the Install button, or Cancel to exit the dialog making no changes.

It is also possible for DROID to automatically check for and update its signatures on a regular schedule. This can be configured in the preferences section.

If you have a binary signature file locally that you want to use, you can install this file in DROID using the Tools / Install signature file... menu item. This will bring up a file open dialog from which you can select the signature file that you want DROID to use. Currently this feature cannot be used to install container signatures, only binary signature files.

Alternatively, you can manually copy a binary or container signature file into your droid working area (which is by default located in a folder called ".droid", underneath your home folder). For binary signature files, place them in the "signature_files" sub-folder. For container signature files, place them in the "container_sigs" sub folder. Note that container signature files have a number appended to them, which is the date on which they are updated, in the format year, month date. For example, the file "container-signature-20110114.xml" is the container signature file last updated on January 14th, 2011.

### Binary signatures

Most file formats have either "magic numbers" embedded within them, or they have regular structures which always appear in files of that format. The binary signatures define what patterns to find in order to identify a file format. The signatures can specify simple sequences of bytes, alternate sequences, or several different possible bytes at the same position in the file. These patterns may be found at known positions (e.g. at the start or 20 bytes in from the end of the file), or they may appear at variable positions, either within a known range of possible positions, or anywhere in the file at all. Signatures can combine all of these features, sometimes more than one in the same signature.

Taking a very contrived example, a signature may specify that there are three bytes at the start of the file: 01, 02 and 03. Then 5 bytes on there is either 04 or 05. After that, somewhere between 10 and 1000 bytes on is either 06 07 08 09, or 10, 11, 12, 13. Finally, the bytes ff, ee, dd are found right at the end of the file.

### Container signatures

Some file formats are actually collections of embedded files, each with their own formats. For example, Microsoft Office 2007 files are actually zip files, which contain various other files. Taken together, they define an Office 2007 file. Binary signatures are not very good at identifying files like this, as the contents of the embedded files are obscured by the containing format (in this case, zip). In order to provide more accurate identifications, DROID can open files which use either the ZIP format, or the OLE2 format, and look inside them to see what embedded files are present.

Container signatures are just specifications of which files to look for inside the containing formats, and optionally, one or more binary signatures to run against those files if they are found.

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