Basic (static) mechanism

The Eternal Resource Locator

Basic (static) mechanism
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It would be clumsy to insist on the signature of whole web pages, so instead we propose to use HTML elements [HTML] to define the borders of the hashed section of the document as well as other features of the hashing mechanism:

The HASHBODY element denotes the hashed section of an HTML document. All the text and HTML code in this section will be hashed with various hashing algorithm specified in the HASH element.
The HASH element is an extensible container for use in identifying hash document information. It has three main functions: define the hashing algorithm used, store the corresponding hash value and link the element to a parent. The HASH element should be used both inside the HASHBODY section and outside; the purpose is to bind the protected section to its hash and its parent.
There can be as many HASH elements as hashing algorithms. Attributes of the HASH element:
- METHOD specifies the hashing algorithm. A number of algorithms may be used in parallel in order to give reassurance against cryptanalytic progress;
- VALUE specifies the value of the hash;
- PARENT provides a pointer to another HTML document, called parent and specified by its URL. This enables a browser that wishes to check the page's integrity to follow the hash chain to a suitable root. The name of the root may be given for performance reasons. If there is no parent (i.e. the document is a root) the attribute should not be omitted but instead should be set to NO.
- The URL attribute optionally specifies where the page normally lives, and can provide basic protection against attacks involving the copying of pages to false hosts. Care is needed not to get entangled with HTTP server's interpretation of URLs like https://www.foo.com/ and https://www.foo.com/index.html.
We also propose to add new attributes to the existing A element. This will allow to define the hash of the resource specified by the HREF attribute. It is assumed that the linked resource is different from the current resource as the hashed body cannot include the hash value itself. Obviously, these can be two different fragments of the same web page.
HASHMETHOD, HASHVALUE and HASHPARENT have the same meaning as the METHOD, VALUE and PARENT attributes of the HASH element.

Figure 1: Publishing medical information. The publisher issues a catalogue every few months which lists all titles published, together with their hash values. The hash of the catalogue has to be distributed a trusted way, by being published in a paper journal, and signed using a long-term key.

Simplified, the way to protect part of the web page will look like:

<HTML> ... <HASHBODY> ... The examination results for the second MB degree examination are as follows: ... ... <HASH URL="https://www.med.abc.ac.uk/examresults" METHOD="SHA-1" PARENT="https://www.cert.bma.org.uk"> <HASH URL="https://www.med.abc.ac.uk/examresults" METHOD="TIGER" PARENT="https://www.cert.med.ac.uk"> </HASHBODY> <HASH METHOD="SHA-1" VALUE="12345678..." PARENT="https://www.cert.bma.org.uk"> <HASH METHOD="TIGER" VALUE="987654321..." PARENT="https://www.cert.med.ac.uk"> ... </HTML>

One of the URLs that refer to this page might look something like:

...see <A HREF="https://www.med.abc.ac.uk/examresults" HASHMETHOD="TIGER" HASHVALUE="987654321..." HASHPARENT="https://www.cert.med.ac.uk">here </A>for the list of candidates who have satisfied the requirements for the degrees of MB and BS.

Checking a hash involves computing the hash value on all the bytes of an HTML document between the hash-input border tags and comparing the HTML document's URL against the value specified within the hash-input. This value is then verified against the value held in the reference in the parent document.

We call this URL-with-hash combination an ERL or `eternal resource locator' as it makes static objects unique for ever. Dynamic objects are slightly more complex.

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Fabien A.P. Petitcolas, Computer Laboratory, University of Cambridge