enormity 0.1 API

The Enormity package is for normalising XML, and signing and verifying it using GPG signatures.

See:
Description

Packages

uk.me.nxg.enormity	Provides facilities for signing, verifying, and before that normalising, XML
uk.me.nxg.enormity.esis	Writes out a SAX stream in a format based on the sgmls ESIS output.
uk.me.nxg.enormity.gpg	Handles the interaction with the GPG program.
uk.me.nxg.enormity.util	Various utility classes

The Enormity package is for normalising XML, and signing and verifying it using GPG signatures.

The text below describes the need for a normalised form for XML, describes a normalised form which is simple to generate and is reversible, and which can be signed by GPG in a natural way.

Normalising and signing XML

Both normalising and signing XML appear to be hard problems, given the size and complexity of the work of the W3C Signature working group, which has produced recommendations on creating signatures for XML, as well as on the necessary problem of canonicalising XML prior to signature. Normalisation (or canonicalisation) is necessary because for every XML document, there is a set of other documents with trivially different syntax, but which mean ‘the same thing’ – that is, they might use single-quotes rather than double-quotes for marking attributes, or have the attributes in a different order, or have ‘unimportant’ whitespace differences between elements, or appear in a different encoding, while still being usefully regarded as the same document.

The key problem with attempting to sign XML is that all cryptographic signature mechanisms are designed to sign a bag of bytes. XML documents are not just bags of bytes, so there's a fundamental dislocation here between what's wanted and what's available.

One solution is not to normalise at all, but instead to regard the on-disk or on-the-wire XML document as the bag of bytes to be signed. This works, but throws away the mutability of XML, which means that if you want to actually do anything with the XML other than simply admire it, and if you want to round-trip the XML into and out of a system which doesn't know about your signature, you're presented with the dilemma of either abandoning the signature, or else worrying about how to reproduce exactly the same bag of bytes when the XML is serialised at some later stage.

It is part of the point of XML that XML documents are not just bags of bytes, and that there is a well-defined distinction between important content and meaninglessly mutable syntax. XML processors and editors freely take advantage of this, and this mutability is reflected in the fact that applications typically do not operate on the bytes of a document or stream, but instead on the abstracted content of a document, as exposed via an API such as SAX, DOM, or an XSL node-set. An XML database is free to store an XML document in any way it likes, as long as it produces an equivalent document when required.

These notions have been formalised in the concept of the XML Information Set, and the much simpler SAX model (the SAX model is defined in terms of Java, but there are exactly analogous APIs in other languages, which can consequently support the same model). Viewed through a SAX lens, an XML document is a rather simple thing, which is consequently very simple to normalise, serialise and thus sign.

A simple-enough normalisation procedure

We here define a simple normalisation mechanism, which straightforwardly turns an XML document into a stream of bytes, in a well-defined, streamable and reversible way. The resulting stream can be signed by GPG in a natural way, and the signature embedded into the XML equally naturally.

The simple normalisation turns the XML:

<doc><p class='foo'>Hello</p>
  <p> there,
chum
</p>
</doc>

into the normalised form:

(doc
Aclass foo
(p
-Hello
)p
(p
-there,\nchum
)p
)doc

This normalised form can then be signed, and the signature reinserted into the original XML, or else included as the parsed XML is passed downstream.

This Java library provides support for the various steps involved. See guidance on using the library below.

This is a rather aggressive normalisation, meaning that it defines a large class of XML documents which are equivalent in the sense that they produce identical normalisations. This normalisation (at least in this release of the software) is not appropriate for documents with mixed XML content (for example ‘text with <em>embedded</em> markup').

Using the library

You may sign and verify XML documents using the GPGSigner and GPGVerifier classes, and perform the normalisation step (for debugging purposes, for example) using the EsisHandler class. However the most usual way of using the library will be using the SigningXMLReader class.

Author:

Norman Gray, http://nxg.me.uk