  Uniquely identifying individuals across the various networks they interact
with on a daily basis remains a challenge for the digital world that we live
in, and therefore the development of secure and efficient privacy preserving
identity mechanisms has become an important field of research. In addition, the
popularity of decentralised decision making networks such as Bitcoin has seen a
huge interest in making use of distributed ledger technology to store and
securely disseminate end user identity credentials. In this paper we describe a
mechanism that allows one to store the COVID-19 vaccination details of
individuals on a publicly readable, decentralised, immutable blockchain, and
makes use of a two-factor authentication system that employs biometric
cryptographic hashing techniques to generate a unique identifier for each user.
Our main contribution is the employment of a provably secure input-hiding,
locality-sensitive hashing algorithm over an iris extraction technique, that
can be used to authenticate users and anonymously locate vaccination records on
the blockchain, without leaking any personally identifiable information to the
blockchain.
