What is an intermediate certificate and how does it work?

When researching digital security, you’ve probably come across the term “ intermediate certificate” more than once . But what is an intermediate certificate , and why is it important? Let’s look at how intermediate certificates fit into the SSL/TLS framework and the broader public key infrastructure (PKI).

Table of contents

What is an intermediate certificate?
How does an intermediate certificate work?
The Importance of Intermediate Certificates in SSL/TLS
Root and Intermediate SSL Certificates
Types of Intermediate Certificates
How to get an intermediate certificate
Common Use Cases for Intermediate Certificates
What is an intermediate certificate?
An intermediate certificate sits between the root certificate and the end-user certificate in the SSL/TLS certificate hierarchy. Its primary purpose is to provide a bridge of trust from the root certification authority (CA) to the leaf certificate used by the end entity, such as a website or application. Technically, it is a file containing the public key, the signature of the issuing authority, and other metadata.

To visualize this, think of a certificate mobile app development service chain. At the top of the chain is the root certificate, a trusted authority that doesn’t sign all certificates directly. Instead, it issues intermediate certificates, which then issue end-user certificates. This hierarchy helps you manage and secure digital certificates efficiently.

How does an intermediate certificate work?
Here's how an intermediate certificate works in the SSL/TLS protocol :

Issuance : The root certification authority (CA) issues intermediate certificates. These are not used directly on web servers, but act as a link between the root CA and end-entity certificates.
Certificate Signing Request (CSR) : When a website needs a certificate, it generates a CSR and submits it to a CA. The CSR contains information such as the public key and organizational details.
Signing and Verification : The CA verifies the CSR and issues intermediate certificates (CA Bundle ) and an end-user certificate . The intermediate certificates are signed with the private key of the root CA, which ensures their legitimacy.
Chain of Trust : An intermediate CA signs the end-user certificate. Browsers or clients can follow this certificate up the chain to the root certificate to verify trust.
Establishing Trust : When a user connects to a secure site, their browser checks the entire chain, from the end-user certificate to the intermediate certificate authority and finally to the root certificate. If all certificates are valid and trusted, the connection is secure.
Certificate revocation and renewal : CAs can revoke and renew intermediate and end-user certificates if they are compromised or expired. Browsers check the revocation status using certificate revocation lists (CRLs) or the Online Certificate Status Protocol (OCSP) to ensure that certificates are valid.
The Importance of Intermediate Certificates in SSL/TLS
Interim certificates are required for several reasons:

Increased Security : By using intermediate certificates, the root CA can ensure the security of its certificate. If the intermediate certificate is compromised, the root certificate will remain intact.
Efficient Management : Intermediate certificates simplify the management of large numbers of certificates. They allow for efficient delegation of certificate issuance authority and reduce the vulnerability of the root certificate.
Flexibility and Control : With multiple intermediate CAs, organizations can create different certification paths to meet different needs. This setup provides greater control and flexibility in certificate management.

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