I am a newbie in blockchain and smart contract. I was reading the following paper.
FADB: A fine-grained access control scheme for VANET data based on blockchain https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9086009
The paper stored private key in Identity Blockchain (Section IV.A.4, page 5-6). I was somewhat confused here. Since blockchain is public, is it not insecure? I think I am missing some basic point. Can somebody help me understand how storing private key in blockchain here is secure?