It seemed to me that the ability to compute something like an MD5 hash has amazing potential as a unique identifier system for digital content. Anyone can generate and publish them, and they are more unique than fingerprints. Is there any major reason why hashes arn't published as unique identifier metadata for digital objects?
Other than the answer that getting everyone to agree on a single way is a very difficult (probably impossible) task, perhaps one answer might be that people sometimes want to be able to group digital objects using an identifier scheme.
I'm assuming you're suggesting hashing the digital object itself, rather than metadata about the digital object (which might change as authorized headings change over time)...
Imagine a dataset that has been submitted to the institutional repository. You could hash an uploaded spreadsheet, but what do you do when the author realizes there was a mistake in it and wants to upload a modified version? You could hash the modified version, but then you have two objects, that have a relationship, with no indication of that relationship in their identifiers (which some would argue is a good thing, granted).
Some folks want to be able to embed this sort of relationship in the identifier. One example is how Dryad (a data repository for scientific datasets) uses DOIs:
Another example would be people who choose to use Cool URIs as their identifiers. They may create URIs that have a structure indicating relationships between different items.
I think you can definitely argue that opaque identifiers are better, but that some people want to embed information in their identifiers is one example of why hashs wouldn't work for everyone.
Hashes only really work to identify a particular packaging of the content, not the content itself.
So, take for instance if we have an MS Word document and a Pages file with the exact same content, same metadata, etc -- they're going to be written to disk differently, and thus, generate different hashes. (and MS Word is particularly tricky, as it stores the content out of order, so it can handle undo) Yet, it's perfectly normal to assign a DOI that takes you to a page asking you what format you want to download in item in. (typically PDF + something else)
PDF's another problematic one, as it's more a layout language ... this word gets placed in a particular spot on a page (which is why you'll try to select a block of text in columns, but it'll select across multiple columns)... you could have multiple files that generate an identical printed page. The whole thing might be an image ... or an image + tagged text ... or text laid out any number of ways.
The stuff that I work with is multi-dimensional scientific data. We have lots of problems regarding serialization which may or may not be the 'same' content (eg, if you convert integer datum to floats, so there's no loss of precision or addition of error, is it still the 'same'?) But even without that, we have column-major form vs. row-major form. Eg:
You can store this as either
Another problem I've run into are people suggesting that we include checksums of some sort in data citations. (eg, Micah Altman's 2007 paper in DLib ... which doesn't work for massive datasets that are 100s of TB.
... and for those wondering why I kept quoting 'same'. The terms 'same' and 'unique' are used a lot, but you need to define what properties / characteristics we care about when we define 'same'ness. We typically care about 'equivalent' rather than 'same' or 'unique', and use of the term acknowledges that they may not be 100% identical.
See also, reasons not to use MD5:
But assuming you had an adequate algorithm, the more fundamental problem others spoke to is defining and maintaining "sameness" for digital content. Consider how rapidly the file formats themselves change (and break!), as typified by early PDFs and the ongoing evolution of various audio/video codecs.
In general a hash is still very useful metadata for content, if for nothing else than verifying integrity after retrieval or transfer. I'd enjoy seeing it more widely employed for identification also. Maybe when you get your whole library's e-content store up on p2p....
MD5 is officially considered broken, as is SHA-1. SHA-2 is ok for now, and the winner of the SHA-3 contest is due to be chosen soon.
Peer-to-peer systems that use distributed hash trees for locating items use cryptographic hashes to generate identifiers for use as hash keys for locating content. For archival purposes, this is the approach I wanted to use for SCHMEER (Several Copies Help Make Everything Eventually Reachable). It's robust and scalable.
MD5 was a hash function used in cryptography. Wikipedia has a good overview of crypto hash functions and properties desirable for them. Most relevant to your question:
Put differently, MD5 will give two inputs with very similar but unequal content (1st and 2nd editions of the same thing) very dissimilar hashes. The amount of information you can determine about the input behind the hash is as low as possible, by design.
This doesn't bode well for anyone trying to learn something about a set of inputs from their hashes, and it shouldn't.