Add appendix on externally verirfiable transaction properties

[r3domfox]

Following discussion with Adhara about the need to verify a specific property of the Corda transaction in the Adhara cross-chain swap protocol, an appendix explains how this works, what the drawbacks of the approach are, and how a future improvement might address it.

[gendal]

Looks good - thanks Dom.

My main comment would be perhaps to move the 'motivation' pieces higher up. Or insert something like the following immediately before the paragraph that introduces the 'externally verifiable transaction property' concept?

(immediately after the para explaining how expensive/difficult it is to parse Corda transactions)

"However, it turns out that the relative opacity of serialised Corda transactions is not a major issue in practice. This is because the reason a Solidity contract needs to inspect a Corda transaction is usually to ensure that it is associated with the same 'business transaction' (eg a trade in the real world) that the solidity contract is managing. In some cases, the the Corda Transaction ID can be securely furnished to the Solidity contract, which then merely needs to satisfy itself that the Transaction has been signed and notarised by the expected parties. In other cases, the Solidity contract may not know or care what the Corda Transaction ID is; it just needs to know that all the parties to a Corda Transaction associated with the overarching business transaction (eg trade) signed and notarised it. Is is this second scenario we are concerned with here"

[gendal]

cc @petermunnings - would also be good to get your review.