It is 1990 at CERN, and work is beginning on a new information management system called “Mesh”. They need a uniform way to identify resources. The syntax of URLs is becoming quite complex. What if we do away with all this and just identify a Resource by its Cryptographic Hash ?
I have to say that now I regret that the syntax is so clumsy. […] But it is too late now.
Even in 1990 it would have been obvious that MD2 would be obsoleted soon, and that this would continue to occur. Tim could wisely include the hash name at the start, so a URI might look like:
These URIs can be used in documents to refer to other documents, just like we use URLs today. I’m not really interested in speculating about HTML, so we’ll just assume that it stays much the same, and links for pages and images stay in their familiar form:
<a href="md2:6c508c14308f608dd24602201cfc13fe">The World Wide Web Project</a> <img src="md2:742b8198cff94b32c92ec8a44fad9091" alt="LHC">
As a sad footnote, this leaves Sun Microsystems as “the colon in emm-dee-five-colon.”
A URL gets its uniqueness from centralized authority: the Domain Name System guarantees the uniqueness of the first part, and the filesystem of the web server guarantees the uniqueness of the second part.
This makes caching web resources rather difficult, and has led to schemes like ETags which add a revision identifier to a resource to make it possible to see if it has changed. Cryptographic hashes are frequently used to calculate ETags.
In contrast, Cryptographic Hash URIs describe resources which never change, so are easy to cache.
Resources may be modified on the server: you think you’re downloading a particular version of a particular file, but it may have been tampered with. Resources may also be modified in transit by your ISP or by other intermediaries.
A client loading a resource by Cryptographic Hash URI can tell if the file has been modified by calculating the hash of the received resource itself, and alerting the user if the calculated hash does not match the URI they asked for.
When you see URLs on grocery bags, on billboards, on the sides of trucks, at the end of movie credits just after the studio logos – that was us, we did that.
There is, of course, a problem. I can write all the documents I want on my local disk, and link them to each other, but how do I publish them so that others can see them.
There’s some stuff here I need to expand on …