The idea around hash value generation is that you are turning a digital file (pdf, tif, etc.) into a mathematical hash value.  For example, an MD5 hash will generate a 16-byte (32 hexadecimal digit) value.  Any changes to the underlying data will completely change the hash value.  If you look at the Wikipedia article (http://en.wikipedia.org/wiki/MD5), it shows an example of two very similar phrases and the different hash values they create:

"The quick brown fox jumps over the lazy dog" - 9e107d9d372bb6826bd81d3542a419d6
"The quick brown fox jumps over the lazy dog." - e4d909c290d0fb1ca068ffaddf22cbd0

This methodology can be used to authenticate that an image hasn't been modified from the time it was originally captured.  When you capture the image or file, you generate a hash value of the file and store it.  50 years later, you could retrieve the same file and generate the MD5 hash again.  If the original MD5 hash matches the new MD5 hash, you can guarantee that the file hadn't been modified.

So the question is, how do you manage all the original hash values?  When you're dealing with imaging and/or content management systems, you are potentially dealing with millions of records and you'll need to ensure that you can find the original hash value at a later date (needle in the haystack).  You'll also need to ensure you can prove that the original hash value hasn't been modified.  

For searching purposes, it's probably best to keep the data in a digital format so you can do a text search for the document in question.  However, this returns us back to the original question, how do you verify that the hash data itself hasn't been altered?  Perhaps the original hashes could be stored in an analog format (paper?) and/or stored on a WORM-compliant electronic storage device.

Mark Graves
Omaha, NE
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