Expose ZIP 317

[LukasKorba]

No description provided.

[str4d]

On the Android side (Electric-Coin-Company/zcash-android-wallet-sdk#1359), I'm working on creating new public SDK APIs that expose the proposal state (from which the total required fee will be accessible). Once the PR for that is ready, I'll attempt to replicate it in this SDK.

[str4d]

In a pairing today, @nuttycom and I decided that we will change Synchronizer.proposeShielding in three ways:

• It will take an optional transparent receiver, corresponding to a specific diversified UA (or the default transparent address) that should have its transparent funds shielded.
  • If the transparent receiver is not part of the specified account, an error will be returned.
  • If no receiver is provided, the current behaviour of "shield transparent funds in whichever transparent receiver under the account has them" will be used.
• It will return an error if there are transparent funds in more than one transparent receiver in the given account.
  • This fixes an address linkability problem that currently exists with Synchronizer.shieldFunds, wherein a transaction spending funds from multiple transparent receivers in the same account can be created which indelibly links them on-chain.
  • We are not fixing the problem in Synchronizer.shieldFunds because working around it would require changing the shieldFunds API, but we are deprecating and soon removing the method instead.
• It will return nil if there are no funds to shield (i.e. its proposal will be "do nothing"), a la zcash_client_backend: Change propose_shielding to return Option<Proposal> zcash/librustzcash#1200.

The transparent receiver argument serves two functions:

• It is a (more complicated but safer) escape hatch to use when the new "multiple transparent receivers" error is encountered.
• It enables SDK users to implement their own mecahnism for avoiding address linkability when shielding funds, in the interim while we do not have our own solution (zcash_client_backend: Propose multiple transactions when shielding funds from multiple transparent receivers zcash/librustzcash#1201).