Make memos mandatory

[cbeck88]

In our release plan for the encrypted memos feature, we will do one release where memos are optional, and in a subsequent release, the memos will be mandatory for every transaction output.

This is intended to make the upgrade path easier for clients.

(See MCIP #3 for details on encrypted memos feature mobilecoinfoundation/mcips#3)

This ticket tracks the task of making the memos mandatory.

[UkoeHB]

Protocol rules? Are you planning to write an MCIP? (I have had pending proposals for this kind of thing for... 5 months?)

[cbeck88]

The in-flight mcip proposes that the memos are mandatory.

The e_memo field shall appear on every MobileCoin TxOut, starting with BLOCK_VERSION = 1.

But to accommodate a graceful update, we determined that there has to be a transitional period where they are optional, because we can't force all the clients to update simulatenously with the servers. In testing we discovered that if the clients update first and start writing memos, but the servers are not yet updated and drop the unknown field in the protobuf, that causes the TxPrefix hash to be computed differently, and a ring signature mismatch. So updating the clients first isn't actually an option, which we had hoped might work.

I don't think we need an additional mcip for this but we could update the mcip to describe that there is an interim period in which they are optional and in a later consensus release they become mandatory.

Thanks for pointing this out. The optional thing is not in trunk yet, it is an in-flight PR.

[jcape]

Superceeded by mobilecoinfoundation/mcips#26