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Friday, 19 August 2016

My first file format signature

As part of Filling the Digital Preservation Gap we've been doing a lot of talking about the importance of accurate file format identification and the challenges of doing so for research data.

Now we are thinking about how we can help solve the problem.

As promised in a post last month, I wanted to have a go at file format signature creation for PRONOM to see whether it is something that an average digital archivist could get their head around. Never before had I created my own signature. In the past I had considered this to be work that only technical people could carry out and it would be foolhardy to attempt it myself.

However, given the extent of the file formats identification challenge for research data wouldn't it be great if the community could engage more directly? Also, shouldn't file signature development be something every digital archivist should have a good understanding of?

Encouraged by Ross Spencer's blog post Five Star File Format Signature Development and a meeting with the PRONOM team at The National Archives in which the tricks and challenges of signature creation were explained, I decided to give it a go.

Where to begin?

  • First I read TNA's How to research and develop signatures for file format identification. This is an accessible and readable guide which tells you how to get started with signature development - from gathering samples, doing internet research on the format and using a Hex Editor to spot patterns. You don't need to be very technically minded to get your head around it.
  • Then I downloaded and installed a Hex editor. Though it is possible to view files as hexadecimal within Quick View Plus, I followed TNA's advice and used HxD Hex Editor as this allows you to compare files thus partially automating the process of spotting sequences.
  • Once I'd spotted a pattern which could be used to create a signature, I planned to use PRONOM's Signature Development Utility to create it. 
  • Once the signature is created I'm told it is possible to test this using DROID. Within DROID, go to Tools... Install signature file and replace the current signature with your new one (but remember to put it back again once you are done otherwise you may wonder why DROID isn't working properly!). Run this over your directory of sample files to see if they are all correctly identified using the signature you have developed. 


I decided start off with something that had been at the back of my mind for a while...those Wordstar 4.0 files from the Marks and Gran archive that I blogged about some time ago and had struggled to identify. When I wrote that post three years ago, Wordstar 4.0 files were not represented in PRONOM. They have more recently been added and the files can be identified but this is by extension only - not the more accurate file signature. I thought it would be fun to try and create a file signature for them.

I was very wrong.

My attempts to see a pattern within the files using the Hex Editor were unsuccessful. I decided to send the sample files to the experts at TNA to see if I was missing something. It was quickly confirmed that this was a rather awkward file type and not one that lent itself well to being automatically identified. Disappointing but at least it confirmed that my own investigations were not lacking.

For my next attempt I decided to tackle some of the unidentified research data that I had highlighted in my previous post Research data - what does it *really* look like?

I looked through the top ten most frequent unidentified file extensions in my sample and started to dig out the files themselves and assess whether they were a good candidate for me to work on. Ross Spencer suggests that PRONOM lends itself best to the creation of signatures for binary formats so this is what I wanted to focus on. No point in trying to make it hard for myself!

  • dat - 286
  • crl - 259
  • sd - 246
  • jdf - 130 (a signature for these JEOL NMR Spectroscopy files is now available)
  • out - 50
  • mrc - 47
  • inp - 46
  • xyz - 37
  • prefs - 32
  • spa - 32
Unfortunately, looking through the list (and digging out some samples) I discovered that many of these are ASCII formats rather than binary. It is possible to create signatures to identify ASCII files but it can be challenging (involving quite complex regular expressions) and not a great place for a first timer to start. I certainly did not want to start to tackle the confusing landscape of .dat files either!

After a little bit of investigation I discovered that the .spa files were something I could work with. I knew nothing about this format but found the relevant files and started doing some internet research looking for more information and perhaps some additional samples. I soon discovered they were one of many formats for optical spectroscopy and are known as Thermo Fisher’s OMNIC file format or Thermo Scientific OMNIC or Nicolet/Thermo OMNIC.

Looking at some of the files using a Hex Editor it was immediately apparent that there was a consistent pattern of bytes at the start of each file. A string which read 'Spectral Data File' which was represented by 53 70 65 63 74 72 61 6C 20 44 61 74 61 20 46 69 6C 65 in hexadecimal. Note that I actually thought the pattern was longer but advice received from the PRONOM team suggested that it was better to cut it down.

I also looked at the end of each file and at first sight there appeared to be consistency here too with each file ending with the same few bytes. This hypothesis was blown out of the water when I looked at a sample file that I had discovered online which did not display this pattern (but luckily did have the same bytes at the start of the file).

This is why it is so important to have sample data that comes from more than one source. A set of files from a single researcher may have misleading patterns that have occurred just because of the consistent way in which they work, rather than this being a true feature of the format itself.

So, once I'd looked at all 33 files and had convinced myself the hypothesis was solid, I went to the online signature development tool provided by The National Archives and created my signature.

PRONOM signature development tool

This was relatively easy to use but there were areas where more guidance was needed (so questions were fired off to the PRONOM team and a speedy response was received). I'm hoping that in the future there will be more documentation to help guide the completion of this form - so that people know how best to name the signature, where to find a definitive list of Mimetypes (this is the list they suggested I looked at), and what the Version field should contain (it is for the version of the file format if this is apparent/relevant - not the version of the signature you are creating).

Once I was done, I clicked on the 'Save Signature File' button and I was presented with the finished XML file:

Ta daaaaa!

I briefly admired my handiwork before sending it off to The National Archives for feedback.*

How long did it take me to do all of this? I would say one full day is a fair estimate (that would include reading the guidance, downloading the Hex Editor and a few false starts as I tried to find a format that I thought I could handle). The next signature would be much quicker.

The biggest challenges:

  1. It took me a while to find a binary format that I could work with. Much of the research data we hold appears to be ASCII formats ....which has benefits from a digital preservation perspective, but wasn't what I was looking for with regard to this exercise
  2. I did not really understand the file format I was working with. I am not a chemist. I have never heard of the .spa format. I struggle to even say 'Spectroscopy' let alone understand it. When I started to research it online I found the results quite confusing. If I knew more about the format in the first place it would have made life much easier. 
  3. There are limitations with the metadata we get from researchers when they deposit data with Research Data York. Reading the brief descriptions of the dataset that are provided did not really help me work out what the individual files are or what software and hardware was used to create them.
  4. I could not locate the file format specification online - I think next time I try this I may approach the software vendor direct and ask them for help. 
  5. Available documentation for creating and testing signatures could be enhanced. I had several questions as I went along and these were answered promptly by the PRONOM team, but if the information was all online then this would certainly help other newbies.


Despite the challenges this exercise has been both enjoyable and useful. The thing I like about being a digital archivist is being able to get hands on with the data and solve problems. Over the last few years I've done very little of this type of work so it was great to get stuck in. On top of the obvious benefit that after the next signature release these .spa files will now be recognised by DROID and other PRONOM-based file identification tools, I have also increased my knowledge and understanding of the process and this is a positive result.

I would definitely encourage other digital archivists, repository managers and research data managers to try this out for themselves.



* Feedback from the PRONOM team was positive. With a couple of modifications they were happy to include the signature in the next PRONOM signature release

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