  Randpay is a technology developed in Emercoin for blockchain micropayments
that can be more effective in some scenarios than the Lightning Network as we
show in the paper. The protocol is based on the concept of Ronald L. Rivest and
published in the paper "Electronic Lottery Tickets as Micropayments" (1997).
The "lottery ticket" was designed for centralized systems where a trusted third
party is required to provide payments, and in some scenarios is also a lottery
facilitator. The existing blockchain protocol cannot accommodate peer-to-peer
"lottery" micropayments at least without the need to create payment channels,
which is analysed in the paper. Therefore, the implementation required the
development of an update to the blockchain core. In the result, RandpayUTXO was
introduced - infinitely spendable zero output that requires the payee's
signature to be published in the blockchain. Randpay is considered to be the
first blockchain protocol to require the payee to sign the transaction by their
private key. This is a significant feature to improve not only
microtransactions but also extend the use of the blockchain for legal deeds
that require a payee's consent to be recognised in legal applications. The
second important innovation of this research is the implementation of Blum's
"coin flipping by telephone" problem to design a "lottery ticket" that does not
require any third party to facilitate the lottery. The paper offers an API
description, an analysis of the mathematical model, and proof of how "lottery"
can be beneficial. There is also an attack analysis and overview of existing
solutions.
