MTOM combined with WS-Security (XML signature)

Question

I'm testing the support of MTOM together with WS-Security (XML-DSIG) on OEG. When verifying the XML signature I noticed I had to add the "Insert MTOM attachments"-filter first. Is this the right way? Shouldn't the signature verification do this transparently?
My other question is how OEG handles the attachments? Does it page them to disk? What happens if my attachments are very large? With the default setup of OEG I encountered out-of-memory issues with attachments above 200MB
Edited by: wsalembi on Sep 22, 2011 12:45 AM

Answer 1

If you just sign the <xop:Include> element, you are effectively only signing the reference to the attachment, i.e. the value of the href attribute. This will only prevent someone changing the href to point to a different attachment.
If you in-line the base64 encoded contents of the attachment into the XML message and only sign the base64 encoded string, you are only preventing anyone from changing the contents of the attachment.
You are not stopping somebody from changing what the <xop:Include> href attribute points to.
So I think there is value in signing BOTH the contents AND the <xop:Include> element so that:
- The integrity of the contents of the attachment is ensured, and
- The integrity of the reference to the attachment in the <xop:Include> element is ensured.
Interestingly, the XOP spec acknowledges this issue in Section 6.1:
http://www.w3.org/TR/xop10/#package_integrity
6.1 XOP Package Integrity
The integrity of Infosets optimized using XOP may need to be ensured. As XOP packages can be transformed to recover such Infosets (see 3.2 Interpreting XOP Packages), existing XML Digital Signature techniques can be used to protect them. Note, however, that a signature over the Infoset does not necessarily protect against modifications of other aspects of the XOP packaging; for example, an Infoset signature check might not protect against re-ordering of non-root parts.
In the future a transform algorithm for use with XML Signature could provide a more efficient processing model where the raw octets are digested directly.
In OEG, it would be possible to use 2 XML Signature Validation filters with an Insert MTOM Attachment filter to validate both signatures.
The flow in the policy would be as follows:
1. 1st XML Signature Filter :- Validate the Signature over the <xop:Include> element
2. Insert MTOM Attachment Filter :- Inline the base64 encoded contents of the attachment
3. 2nd XML Signature Filter :- Validate the Signature over the element now containing the in-lined base64 encoded data.
This policy would ensure the integrity of the attachment contents AND the reference to this attachment in the <xop:Include> element.