Build a ConfidentialMPTSend transaction that transfers a confidential amount
from the sender's spendable balance to the destination's inbox. The amount is
encrypted under the sender, destination, issuer, and (when registered) auditor
keys with a shared blinding factor, and a single proof binds the amount
commitment, balance commitment, and per-recipient ciphertexts.
Build a ConfidentialMPTSend transaction that transfers a confidential amount from the sender's spendable balance to the destination's inbox. The amount is encrypted under the sender, destination, issuer, and (when registered) auditor keys with a shared blinding factor, and a single proof binds the amount commitment, balance commitment, and per-recipient ciphertexts.