Make TokenWithRefunds Production-Ready

[just-mitch]

Partial notes were used to make private refunds possible.

In short, private refunds allow Alice to fund an arbitrary transaction (possibly with public functions) using an arbitrary private token via a fee payment contract, and get her refund as private token notes.

Without this flow, Alice must fund her transactions either:

• natively with AZT herself
• using public balances and an FPC
• using private balances but forgoing a refund

Partial notes generally support patterns where we need to construct a note that is dependent on on-chain data sources that are not available in private.

Example sources are:

• price feeds
• prevailing interest rates
• weather data from IoT sensors
• time

Applications could include:

• start a point $(\text{owner} + \text{principle})*G$, complete it with the prevailing interest rate to construct a note representing a loan.
• submit a claim on your farm insurance based on regional flooding data.
• private discounts/deals based on time of day
• lock some private assets as a point, cash them out privately with interest based on the amount of time they were locked up.
• a dex where the liquidity is in public but rewards/swaps occur with notes

So the use cases are compelling, but there are problems with the current implementation. This issue tracks work to make the concept ready for production.

Tasks

Beta Give feedback

1. determine if using the x-coord of a Point for a note hash is safe #7325
   +0
2. bug(PartialNotes): Add verifiable link between to address/spending keys and the PrivateTokenNotes #7323
   A-security T-bug +2
3. bug(PartialNotes): use separate generators for the constituent pieces of PrivateToken #7320
   A-security T-bug +2
   
4. leak(PartialNotes): use different randomness for sponsored user and fee recipient in PrivateToken #7321
   A-privacy-leakage T-bug +2
5. bug(PartialNotes): Check funded_amount >= transaction_fee #7322
   A-security T-bug +2
   
6. bug(PartialNotes): account for key rotation in PrivateToken #7324
   A-security T-bug +2
7. unconstrained getter for SharedMutable #7326
   +0
   
8. Protect FPC against griefing #7650
   +0
   
9. Set flat fee to a reasonable value #7688
   +0

[benesjan]

Closing this now as #7650 is not planned to be tackled in the near future and everything else in this issue done.