Allow senders to include Spark recovery data in transactions about firo HOT 2 OPEN

Note the correctness and consistency of the sender-encrypted data (like that of existing recipient-encrypted data) cannot be checked by the network. This means that if a user does not wish to have the data included, it can simply encrypt empty or junk data of the expected length.

This doesn't change the guarantees that payment proofs provide, however. If junk or incorrect sender-encrypted data were decrypted during payment proof verification, the verification would fail.

from firo.

Interestingly, this could in certain cases replace payment proofs. Payment proofs reveal coin information (value, memo, and recipient address) to the verifier, but also assert that the prover had spend authority for the transaction that created the coin. If the verifier doesn't care about the spend authority assertion, but only the coin information, it could simply ask the prover to provide k_disc without an accompanying payment proof. This is much easier for the prover to produce than a full payment proof.

from firo.