  This article surveys the literature over the period 2003-2021 on heart-based
biometric protocols. In particular, we focus on how the heart signal is
transformed from a continuous wave to discrete values to be used afterwards in
authentication protocols. We explain and classify the surveyed proposals
according to three main parameters: i) the dataset they use for testing their
results; ii) the delineation algorithms they use to extract the fiducial
points, and; iii) the cryptographic tests they run (if any) to validate how
random the extracted token is.
