Keyhub is described as 'Certificate Management on autopilot'. There are three alternatives to Keyhub for Linux, Online / Web-based, Self-Hosted solutions and Windows. The best alternative is Userify. Other great apps like Keyhub are Key Manager Plus (Paid) and Keyper (Free Personal, Open Source).
ManageEngine Key Manager Plus is a web-based key management solution that helps you consolidate, control, manage, monitor, and audit the entire life cycle of SSH (Secure Shell) keys and SSL (Secure Sockets Layer) certificates.
Keyper is an Open Source SSH Key and Certificate-Based Authentication Manager that acts as an SSH Certificate Authority (CA), standardizes and centralizes the storage of SSH public keys and SSH Certificates for all Linux users in your organization.