Dear Bangaly Cisse,

I would suggest that you explore the Use of Blockchain to increase the Security of some Types of Digital Twins (DTs) - such as Human Health DTs (HHDTs).

General Information about Digital Twins can be found at the Address:

https://www.researchgate.net/publication/366167747_Digital_Twins_in_IoT

My small List of Literature is intended to give you the Basics for using Blockchain in System Solutions with DTs.

Best regards

Anatol Badach

Haya R. Hasan, Khaled Salah, Raja Jayaraman, Mohammed Omar, et al: A Blockchain-Based Approach for the Creation of Digital Twins; IEEE Access, Vol. 8, Feb 2020, DOI: 10.1109/ACCESS.2020.2974810

Sabah Suhail, Rasheed Hussain, Raja Jurdak, Choong Seon Hong: Trustworthy Digital Twins in the Industrial Internet of Things with Blockchain; arXiv:2010.12168v1, Oct 2020; DOI:10.1109/MIC.2021.3059320

Haya R. Hasan, Khaled Salah, Raja Jayaraman, Mohammed A Omar, Ibrar Yaqoob, Sasa Pesic, Todd Taylor, Dragan Boscovic: A Blockchain-Based Approach for the Creation of Digital Twins; IEEE Access, Feb 2020, DOI: 10.1109/ACCESS.2020.2974810

Ibrar Yaqoob, Khaled Salah, Mueen Uddin, Raja Jayaraman, Mohammed Oma, Muhammad Imran: Blockchain for Digital Twins: Recent Advances and Future Research Challenges; IEEE Network, Vol. 34, Issue 5, Sep/Oct 2020; DOI: 10.1109/MNET.001.1900661

For your master thesis, I would suggest conducting a literature review on security and privacy challenges of block chain. It helps you build an understanding about state-of-the-art research and then in your PhD studies you can continue it further and make your own contribution.