Modern applications lack the ability to separate sensitive information (e.g. private keys) from unimportant data (e.g. welcome messages) in diﬀerent memory spaces, making it easy for attackers to steal sensitive data via memory disclosure attacks. A recent study has devised an isolated execution environment to support a privileged layer between user and kernel using protection rings: a privuser mode. However, the mode is in 32-bit, which leads to a degradation in performance. This paper presents privuser64, an additional execution mode between user and kernel that is fast and portable, but in 64bit. Kernel Page Table Isolation (KPTI) is employed to prevent the layer from accessing the kernel. The framework was faster up to 57% than the previous work, 32-bit privuser. In addition, to show the feasibility of this work, it has been applied to the widely used web server Nginx with the cryptographic library LibreSSL to secure private keys, and it showed near 0% of overhead as the response header size of HTTP grew larger.