  The popularity of Deep Learning (DL) makes the privacy of sensitive data more
imperative than ever. As a result, various privacy-preserving techniques have
been implemented to preserve user data privacy in DL. Among various
privacy-preserving techniques, collaborative learning techniques, such as Split
Learning (SL) have been utilized to accelerate the learning and prediction
process. Initially, SL was considered a promising approach to data privacy.
However, subsequent research has demonstrated that SL is susceptible to many
types of attacks and, therefore, it cannot serve as a privacy-preserving
technique. Meanwhile, countermeasures using a combination of SL and encryption
have also been introduced to achieve privacy-preserving deep learning. In this
work, we propose a hybrid approach using SL and Homomorphic Encryption (HE).
The idea behind it is that the client encrypts the activation map (the output
of the split layer between the client and the server) before sending it to the
server. Hence, during both forward and backward propagation, the server cannot
reconstruct the client's input data from the intermediate activation map. This
improvement is important as it reduces privacy leakage compared to other
SL-based works, where the server can gain valuable information about the
client's input. In addition, on the MIT-BIH dataset, our proposed hybrid
approach using SL and HE yields faster training time (about 6 times) and
significantly reduced communication overhead (almost 160 times) compared to
other HE-based approaches, thereby offering improved privacy protection for
sensitive data in DL.
