A payment ID may be encrypted using the tx secret key and the
receiver's public view key. The receiver can decrypt it with
the tx public key and the receiver's secret view key.
Using integrated addresses now cause the payment IDs to be
encrypted. Payment IDs used manually are not encrypted by default,
but can be encrypted using the new 'encrypt_payment_id' field
in the transfer and transfer_split RPC calls. It is not possible
to use an encrypted payment ID by specifying a manual simplewallet
transfer/transfer_new command, though this is just a limitation
due to input parsing.
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