Improving the Token ERC-721 Implementation for Selective Receipt: Rejectable NFTs.

Lots of business processes can be benefited of the use of blockchain-based technologies. The distributed nature of the blockhain overcome a part of the reliability issues related to centralised systems. Recently the use of NFTs has been spread to multiple applications. The EIP-721 standard proposal defines the interface of non-fungible tokens (NFTs). This type of token was designed with the aim of creating interchangeable tokens but with the peculiarity of being unique and non-fungible and with the impossibility to be damaged or destroyed. For this reason, the EIP-721 standard has almost limitless applications: it can represent ownership of digital or physical assets, unlock access to certain services, … In short, it can represent any single object or right that we can transfer between a sender and a receiver. In this paper we present a study on the transferability of the NFTs and its implications in the generation of secure applications. Currently, the NFTs can be transferred with the consent of the owner (by himself or by an authorised party) but the receiver of the token cannot decline the reception of the token, that will be transferred to his wallet. Therefore, we propose in this paper the enhancement of the NFT standard that allows the rejection of reception of tokens, obtaining a kind of token that can be used in a secure way in all kinds of applications that require the selective reception of the tokens.