How to Implement Two CAs on One Domain

While it's not common to use multiple Certificate Authorities (CAs) on a single domain, there are certain scenarios where this might be necessary. This typically involves having different certificates for various subdomains or using a certificate chain that includes multiple CAs. Below are a few methods to implement two CAs on one domain:

1. Using Different CAs for Subdomains

You can use different CAs for different subdomains of the same domain. For example:

• www.example.com  could use a certificate from CA1.
• shop.example.com  could use a certificate from CA2.

This is a straightforward approach where each subdomain gets its own SSL certificate issued by a different CA. You’ll need to configure the web server to use the correct certificate for each subdomain.

Steps:

1. Generate a CSR (Certificate Signing Request) for each subdomain.
2. Submit the CSR to the respective CAs.
3. Install the certificates on the server, ensuring the correct certificate is associated with each subdomain.

Use case: This method is common when you want to use different CAs for various subdomains for redundancy or compatibility purposes.

2. Using a Multi-Domain (SAN) SSL Certificate

A multi-domain SSL certificate (also called a Subject Alternative Name or SAN certificate) can be issued by different CAs for different domains or subdomains. This allows multiple domain names to be covered under a single certificate from one CA or across multiple CAs.

Steps:

1. Obtain the SAN certificate from CA1 for the primary domain.
2. Add additional domain names (subdomains or completely different domains) from CA2.
3. Use the server configuration to map the correct domains to the corresponding certificates.

3. Certificate Chain with Multiple CAs

Another way to implement multiple CAs is through a certificate chain where one certificate is signed by a primary CA, and another is signed by an intermediate or secondary CA. This method is typically used for extended validation (EV) certificates or for ensuring broad compatibility.

Steps:

1. Get a certificate from CA1 for your domain.
2. Obtain an intermediate certificate or additional certificate from CA2.
3. Combine the certificates in the certificate chain, ensuring that your server recognizes both certificates.

4. Using Cross-Signing

In cross-signing, one CA (for example, CA1) issues a certificate that is signed by another CA (CA2). This method is useful for trust purposes and ensuring that certificates are recognized by a wider range of devices or browsers.

Steps:

1. Obtain a certificate from CA1.
2. Have the certificate cross-signed by CA2.
3. Install the full certificate chain (including intermediate certificates) on your server.

Key Considerations

• Server Configuration: You need to configure your web server (e.g., Apache, Nginx) to handle multiple certificates properly. This includes setting up virtual hosts for subdomains or configuring SSL settings for different certificates.
• Certificate Management: Managing certificates from different CAs can increase complexity, especially when handling renewals and ensuring proper installation.
• Browser Compatibility: Ensure that all CAs you choose are trusted by the browsers you want to support. Some CAs may have limited trust across certain devices or browsers.