[Zcash] [Bug] Fees are incorrect for transactions with shielded components

[daira]

This explorer is calculating fees incorrectly for Zcash transactions with shielded components.

Example: https://zecblockexplorer.com/tx/63679fc7f9628bc5065636813ee6adbe89f6dfdf3635524ac0792b344a9fd588 shows a fee of 72.56179835 ZEC. In fact this is a shielding transaction which transfers 72.56024835 ZEC to the shielded pool. The remaining 72.56179835 - 72.56024835 = 0.00155 ZEC is the fee.

That transaction is shown correctly by other block explorers at https://mainnet.zcashexplorer.app/transactions/63679fc7f9628bc5065636813ee6adbe89f6dfdf3635524ac0792b344a9fd588 and https://3xpl.com/zcash/transaction/63679fc7f9628bc5065636813ee6adbe89f6dfdf3635524ac0792b344a9fd588 .

In order to calculate the fee correctly in all cases:

• for each JoinSplit description, add vpub_new and subtract vpub_old.
• add valueBalanceSapling.
• add valueBalanceOrchard.

That is, a negative valueBalanceSapling or valueBalanceOutput, or a nonzero value in any vpub_old, represents a net transfer into the corresponding shielded pool, and is treated like an output of that magnitude when calculating the fee. A positive valueBalanceSapling or valueBalanceOutput, or a nonzero value in any vpub_new, represents a net transfer out of the corresponding shielded pool, and is treated like an input of that magnitude when calculating the fee.

(The transaction given as an example is a v4 transaction that only involves Sapling. I can give other examples that involve the Sprout and Orchard pools if needed.)