Tuesday, June 25, 2013

Validating Windows Mobile App Store Receipts Using Node.js


When your windows phone user performs an in-app purchase you can get its receipt using the API:

You need to send this receipt to your backend system to validate the signature. If that backend system happens to be in C# your life is easy as the official documentation provides exact validation instructions. If you use another platform then be aware that there are a few gotchas to this validation process.

Based on several requests I have checked the feasibility to use my xml-crytpo node.js module to perform this validation. There were two challenges that required to patch xml-crypto. Both of them caused the signature digest calculation to differ from the stated one by the store. This had failed the validation process.

White spaces
As you can see in the sample receipt above it contains white spaces. White spaces in Xml are meaningful and once an Xml has been signed it is not legal to remove or alter the white spaces. However take a look at these lines from Microsoft C# validation instructions:

The receipt is loaded into an XmlDocument. Since not defined otherwise, the PreserveWhitespace property defaults to false. This means that the actual validaiton code later on is performed not on the actuall Xml received from the network/disk but on an altered version of it which strips all white space. This is not standard and confusing. Anyway if you use node.js remember to first remove the whitespace before processing the document further. Unfortunately xmldom, the module which xml-crypto uses for xml processing, does not provide this utility. I did a quick patch for it and for now have put it in my private xmldom repo. Just initialize the parser with the ignoreWhiteSpace flag:

Debugging this was quite painfull which is why I posted this tweet shortly after:

Implicit Exclusive Xml Canonicalization
Canonicalization is probably one of the most confusing topics regarding xml digital signature. In a nutshell, before processing Xml (either when signing it or when validating the signature) we need to transform it to a canonical form in terms of attribute order, namespace definition, whitespace and etc. There are multiple standard to do so and they can be chained together so the signed xml should state which standard(s) it uses:

This is the transformation stated by the windows store receipt

In practice when trying to validate according to this transformation xml-crypto shows this validation error:

I have built a simple C# app to do this validation according to the Microsoft sample - it worked. The sample uses the high level SignedXml class. I have then built a C# snippet to do the validation in a more low level way by applying the enveloped-signature transformation myself - this failed.

I had to dig deep in the reflector to find that internally the SignedXml .net class always applies the Exclusive Xml Canonicalization standard in addition to any explicitly defined transformation. This happens in TransformChain.TransformToOctetStream() method which is used internally in the signing process when it starts with SignedXml:

The CanonicalXml class has an internal property m_c14nDoc which holds the canonicalized version.

Once I have forced xml-crypto to use c14n at all times the validation was successful.

I have not found any evidence in the xml digital signature or exclusive xml canonicalization specs that the way SignedXml works complies with the definitions.

All Together Now
I have committed the changes to the xml-crytpo windows-store branch and have not checked them in to npm since this is not necessarily the correct behavior with other platforms. So you should use xml-crypto from there (a quick way is to "npm install xml-crypto" and then override the created folder with the windows-store branch zip. There is a way to tell npm to install directly from github but not sure if this can happen from a branch. This is the usage example:

The pem file contains the raw content of the certificate which you get from the windows store provided url:

I hope your app will be successful and you will use this procedure a lot...


What's next? get this blog rss updates or register for mail updates!