One possible PhD topic that could link cyphersecurity with deep learning is "Using Deep Learning for Secure Encryption of Sensitive Data". The aim of this research project would be to develop an end-to-end system using deep learning algorithms that can be used to securely encrypt sensitive data. The system should be able to identify potential attack vectors and prevent them from being successful. In addition, the research should also focus on improving the efficiency of the system and increasing its scalability.