SSH (Secure Shell) is a secure communication protocol widely used to enable secure access to remote devices and servers over an unsecured network like the Internet. stands as a strong and reliable guardian of data integrity and confidentiality. It has been used for decades to enable secure access to remote devices and servers over an unsecured network like the Internet. Traditionally, SSH authentication has been widely implemented using either passwords or cryptographic keys. However, password insecurity, SSH key management challenges, and evolving security threats have led organizations to explore a safer, less complex, and easier to manage approach to SSH authentication – SSH certificates.

How are SSH Certificates Different from SSH Keys?

SSH keys and SSH certificates both help in securing SSH connections, but they differ in their structure and functionality. Traditional SSH keys are simply made up of a public and private key pair that are used for SSH key-based authentication. SSH certificates, on the other hand, consist of a public key along with additional identity information and access permissions that are signed by a trusted SSH Certificate Authority (CA). From a functionality point of view, SSH certificates can be thought of as an ID card given they contain identity information like a user’s name, SSH public key, and expiration date. SSH certificates also have a corresponding private key that remains stored privately on the user/client machine. In this capacity, SSH certificates can be used for SSH certificate-based authentication.

SSH Certificate-based Authentication Vs. SSH Key-based Authentication

SSH certificate-based authentication has advantages over key-based authentication because of the identity information contained in the certificate and the trusted Certificate Authority involved. Instead of verifying public keys (in SSH key-based authentication), SSH certificate-based authentication focuses on checking the validity and authenticity of a certificate signed by a trusted CA. Additionally, unlike SSH keys, SSH certificates will have an expiration date which enables SSH certificates to be better managed than SSH keys that never expire.

How Certificate-based Authentication Works

When a client attempts to connect to an SSH server, it sends its SSH certificate to the server. The server verifies the client’s authenticity by validating the digital signature in the certificate against the CA’s public key configured in the server. Additionally, the server checks if the certificate is within its expiry date and meets any specified access controls. If everything checks out, the client is granted access to the server.

Advantages of Using SSH Certificates Over SSH Keys

1. Higher Security

One of the biggest challenges with SSH keys is that they do not expire. They continue to enable system access until explicitly removed. As SSH keys can be easily created on the fly, the absence of an expiry date and lack of oversight often lead to key sprawl, increasing the risk of key theft, misuse, and security breaches.

Unlike SSH keys, SSH certificates are issued and signed by a central and trusted SSH Certificate Authority (CA) through an enrollment process. Each SSH certificate includes the user’s public key and additional identity information as certificate metadata. Host servers trust the central SSH CA, allowing users to authenticate using an SSH Certificate signed by that CA.

SSH certificates also come with an expiration date, solving the challenge of perpetual access associated with SSH keys. Additionally, certificates support session-based access, where SSH certificates are issued with limited validity, so they can only be used for a particular task and a specific period. These certificates automatically expire after the set date, preventing unwarranted access and exposure. Regular certificate expirations, renewals, and custom validity significantly reduce the likelihood of attackers compromising an SSH certificate for unauthorized access. This enhances the overall security of SSH connections.

2. Greater Access Control

While the ease of SSH key generation is great, it also contributes to the risk of privilege sprawl . With so many keys being generated on an adhoc basis, IT teams struggle to track, manage and control who owns SSH keys, what they are used for, and what level of access they provide. Poor access control coupled with the sheer volume of keys together increases the risk of unauthorized access and key compromises.

One of the major advantages of certificates over keys is the granularity of access control they provide. SSH certificates are issued by CAs to users only on an on-demand basis. This ensures that new access is granted only when there is a need for one. Certificates can also include specific access rights and restrictions, such as permitted hosts, source IP addresses, and allowed commands. As these specific settings within the certificate cannot be modified at will by users, the possibility of unauthorized access is reduced by default. The level of granular control offered by certificates enables network administrators to enforce least privilege principles, restricting access and limiting the scope of potential security breaches.

3. Easier to Manage and Operate at Scale

As the number of users and servers grows, SSH key management can become a logistical nightmare for IT teams. Getting key approvals, provisioning keys individually to client devices, distributing the authorized public keys to the host servers for verification, and maintaining the ~/.ssh/authorized_keys file for every user is a highly strenuous and time-consuming process. Further, creating and distributing keys for new employees and revoking access for ex-employees significantly increases the administrative workload. Imagine having to do this regularly with millions of SSH keys!

On the other hand, SSH certificates work with CAs to minimize operational complexity and notably simplify SSH management. As SSH certificates are issued by a trusted CA, there is no need for administrators to distribute clients’ public keys to the host servers. When a client requests server access with an SSH certificate, the server verifies the client’s identity by validating the CA’s signature in the certificate against the list of CAs it has (in /etc/ssh/sshd_config file). This essentially makes the list of CAs the single source of truth for servers and eliminates the need for IT teams to add and manage millions of SSH keys on servers.

SSH certificate lifecycle management is also more straightforward compared to key lifecycle management. As mentioned earlier, SSH keys do not expire and must be rotated on a regular basis. They should also be deleted when the intended task is complete or the user changes role or leaves the company. Key rotations are a complex process as is and particularly so when there are shared private keys, as IT teams often struggle to map these keys with their host servers.

Certificates on the other hand, naturally expire and require that new or renewed certificates are issued for continued access. With this SSH certificate lifecycle concept, key rotation is facilitated automatically by the CA. Administrators can issue new certificates signed by the CA, ensuring smooth transitions and reducing exposure to security vulnerabilities. Additionally, as SSH certificates come with an expiry date that can’t be modified, it’s not mandatory for administrators to revoke them and there is less risk of theft or misuse. This simplification improves operational efficiency, mitigates risk, and reduces the possibility of misconfigurations and unauthorized access.

Many organizations already utilize certificates and certificate lifecycle management for various identity and access management (IAM) purposes, such as SSL/TLS authentication and VPN access. Leveraging similar certificate lifecycle management practices for SSH authentication fosters seamless integration, further simplifying management and enhancing interoperability across diverse security domains.

4. More User-Friendly

When it comes to onboarding an SSH user, SSH keys make the whole process slow and inconvenient. Additionally, when a client initiates an SSH connection with a host server for the first time, the client is expected to authenticate the server by verifying the host’s public key. Sometimes, when a client fails to authenticate the host server (either due to a change in the host’s name, IP address, or public key), users are alerted with security warnings. While these TOFU (trust on first use) warnings are justified from a security perspective, they still cause confusion and impact user experience. Users also tend to ignore the warning and proceed with the connection rather than cross-verifying the host’s name and the public key with the administrator. This can actually create a security risk if the host server is compromised.

In certificate-based authentication, as the host SSH certificate contains the host’s current name and the CA’s public key, clients are able to trust the host servers at all times by verifying the CA (which is common to both the host and the client). This method eliminates TOFU warnings and associated risks even when a server is replaced, or the public key is changed, enhancing the overall SSH user experience.

5. Simplified Audits and Compliance

As SSH keys are typically generated on the fly, many SSH keys go unmanaged and often have excess privileges, including root access. Not having visibility and insights into these unmanaged keys is a major challenge during audits. Some SSH keys are also shared between multiple servers making it difficult for IT teams to identify their owners.

SSH certificates facilitate robust compliance and auditing practices by embedding metadata such as user names, expiration dates, and usage permissions. This metadata enables organizations to track and audit SSH access with greater precision, ensuring adherence to regulatory requirements, and bolstering the overall security posture of the infrastructure.

While traditional key-based authentication has been a cornerstone of SSH security for decades, adopting certificates will be the next step forward in bolstering cyber resilience. With SSH certificates, organizations can ensure easy, reliable, and secure remote access to distributed workforces while simplifying SSH management for IT teams.

Despite their advantages, SSH certificates still need to be managed efficiently. Without visibility, automation, and policy control, SSH certificates can also present challenges and pose a threat to enterprise security. To ensure successful implementation and operations, organizations must implement effective SSH certificate lifecycle management.

How AppViewX Can Help

AppViewX helps organizations simplify and modernize certificate and key lifecycle management with holistic visibility, end-to-end automation, and policy-driven control. AppViewX streamlines both SSH key and certificate lifecycle management, allowing organizations to discover, monitor, manage, and renew certificates and keys seamlessly at scale to prevent privilege sprawl, security vulnerabilities, and compliance issues.

AppViewX can help you on your SSH journey, whether you’re looking to fortify your SSH access with robust SSH Key Lifecycle Management or move to the more modern and scalable SSH Certificate Lifecycle Management. To learn more about gaining complete visibility, end-to-end automation, and continuous control of your SSH Infrastructure with AppViewX, visit https://www.appviewx.com/solutions/ssh-access-control/, or contact us today.

*** This is a Security Bloggers Network syndicated blog from Blogs Archive - AppViewX authored by Krupa Patil. Read the original post at: https://www.appviewx.com/blogs/why-ssh-certificates-can-be-a-better-option-for-remote-access-than-ssh-keys/