  Privacy concerns in healthcare have gained interest recently via GDPR, with a
rising need for privacy-preserving data collection methods that keep personal
information hidden in otherwise usable data. Sometimes data needs to be
encrypted for several authentication levels, where a semi-authorized user gains
access to data stripped of personal or sensitive information, while a
fully-authorized user can recover the full signal. In this paper, we propose a
compressive sensing based multi-level encryption to ECG signals to mask
possible heartbeat anomalies from semi-authorized users, while preserving the
beat structure for heart rate monitoring. Masking is performed both in time and
frequency domains. Masking effectiveness is validated using 1D convolutional
neural networks for heartbeat anomaly classification, while masked signal
usefulness is validated comparing heartbeat detection accuracy between masked
and recovered signals. The proposed multi-level encryption method can decrease
classification accuracy of heartbeat anomalies by up to 50%, while maintaining
a fairly high R-peak detection accuracy.
