Rejectable SoulBound Tokens for Credentials Assignment and Acceptance of Terms

Digital credentials are being issued by authorized entities to facilitate the digital identification of their users. They can be classified in identity credentials, academic and professional credentials, and access credentials. Blockchain offers some inherent features that are highly advantageous for the management of credentials. Non-Fungible Tokens, or NFTs, might seem to be a perfect fit for the implementation of digital credentials. However, some crucial requirements for credentials are the non-transferability of the credential and that the authorized entity can receive explicit acceptance from the user who will own the new credential. Nevertheless, there is a new type of token, named Soulbound tokens (SBT), that has been designed to ensure the non-transferability of assets. This paper presents a protocol that is focused on the issuance of digital access credentials, with the additional ability to allow the association of terms and conditions that users must accept when receiving these credentials. We use an enhanced version of the SBTs, called RejSBTs, to represent the credentials and associated terms, providing non-repudiation of reception and origin proofs.