Smartbox: Tax form signing

[semyon2105]

The current implementation handles the login scenario, but tax forms have a different structure. Tax forms differ greatly from the login one, the nodes there have varying names, structure and contents.

I'm working on another Smartbox implementation in parallel where I decided to delegate XML DSig template construction and signing to xmlsec C library. That XML-DSig standard looks pretty scary, so I chose not to attempt constructing the signed XML manually 😅. I was able to submit a valid tax form with that library. That said, all the C FFI with XmlSec/OpenSSL/NSS is pretty nasty. I'll share the code in the next few weeks on Github in case you're interested.

The easiest way to get the example XML is to submit an "OLZ" form in test.purs.gov.rs as it's the simplest one to fill out.

[semyon2105]

Nothing should be added on the protocol side. Smartbox uses the same "GetSignedXml" request for both login and tax forms.

[ubavic]

I think I cold rely on https://github.com/moov-io/signedxml I don’t like idea of using any C library, that sounds like opening a can of worms :P