What is certificate-based authentication?
Certificate-based authentication (CBA) has been a widely-used security measure for decades. It provides a reliable and secure way to authenticate users much more securely than passwords. As the digital landscape grows increasingly complex and cyber threats evolve, the significance of CBA in protecting sensitive resources becomes even more pronounced.
Certificate-based authentication defined
Certificate-based authentication is a technique that uses digital certificates to verify the identities of users, devices or servers before granting access to a network or application. Think of a digital certificate as an electronic passport that contains information about the entity it represents (e.g. name or organization), a public key and a digital signature.
CBA is generally considered tamper-proof (or phishing-resistant) because digital certificates are difficult to forge or alter without detection. The use of strong encryption and a reliable Public Key Infrastructure (PKI) ensures that any attempt to modify the certificate will invalidate it.
Unlike password-based credentials, which can be easily guessed, shared or phished, certificates provide a much higher level of security. Since they are mathematically generated, cryptographically signed and unique to each user or device, gaining unauthorized access becomes more challenging.
How certificate-based authentication works
- A user requests a digital certificate from a trusted certificate authority (CA). After verifying the identity of the requester, the CA issues a certificate that contains the CA’s digital signature and a public key.
- The issued certificate is installed on the user’s device or server. A private key that corresponds to the public key in the certificate is also generated and stored securely.
- When a user attempts to access a protected resource, they present their digital certificate to the system.
- The system verifies the certificate through the CA's digital signature. It also confirms that the certificate is not invalid, expired or revoked.
- If the certificate is valid, the system generates a random challenge and then sends it to the user. The user must sign this challenge using their private key.
- The signed challenge is sent back to the system. The system uses the public key in the certificate to verify the signature. This is possible because public and private keys are mathematically linked; only the private key can decrypt data encrypted with the corresponding public key.
- If the signature is verified, the system grants access to the network or application.
Types of certificate-based authentication
Certificate-based authentication comes in various forms. Here are some examples of authentication methods that rely on certificates:
- Client certificate authentication
This type of certificate-based authentication involves a certificate issued to a client (user or device) that must be presented to a server to establish identity. It's often used in secure web transactions, VPNs and Wi-Fi networks. - Server certificate authentication
Servers present SSL certificates to clients to verify the server’s identity. This is commonly used in HTTPS connections to ensure that users are connecting to legitimate websites. - Mutual TLS authentication
Both the client and server authenticate each other by exchanging certificates. This mutual verification guarantees that both parties are trusted. - Smart card authentication
Users are issued smart cards that have digital certificates embedded within them. To log in, users insert the card into a reader and enter a PIN. - SSH certificate authentication
Instead of traditional SSH keys, certificates are used to authenticate SSH connections. During login, the client presents the certificate, which the server verifies against a trusted certificate authority. - Email certificate authentication
Digital certificates are used to sign and encrypt emails. This ensures that the email sender is authenticated and that the content remains confidential and untampered. - DNS-based authentication of named entities (DANE)
A certificate-based authentication method that uses DNSSEC (Domain Name System Security Extensions) to secure DNS data and validate digital certificates. It allows organizations to specify which CAs are authorized to issue certificates for their domains.
Pros and cons of certificate-based authentication
Next, let’s discuss the pros and cons of using certificates for authentication:
Pros
- Digital certificates are virtually impossible to forge, therefore they offer a significantly higher level of security compared to traditional passwords. This elevates your security posture and reduces your attack surface.
- CBA eliminates the need for users to remember complex passwords, which often leads to weak passwords or bad password practices.
- Certificates can also be used to implement Single-sign on (SSO) mechanisms. Once a certificate is installed and verified, users can access multiple services without needing to re-enter credentials.
- CBA methods are also scalable by design. Since certificates are managed centrally, it becomes easier to issue, renew and revoke certificates for large numbers of users or devices.
- CBA can be used as both a primary authentication mechanism and as a second factor for multi-factor authentication (MFA).
- Certificates provide both authentication and encryption, which means that communications are secure, and identities are verified.
Cons
- Setting up a PKI and managing certificates can be complex and requires specialized knowledge and resources.
- CBA requires ongoing maintenance. Certificates have expiration dates and need to be renewed regularly. Failure to do so can lead to access issues and security risks.
- Some older systems or applications (e.g. those using legacy protocols like LDAP) may not support certificate-based authentication, leading to integration challenges.
- Some certificate-based authentication methods may require additional hardware, such as smart cards or tokens, which can be costly.