How to Verify if an SSL Certificate Matches its Private Key

This guide applies to the following versions of Plesk Panel:

• Plesk Panel 11.x for Linux
• Plesk Panel 10.x for Linux
• Plesk Panel 9.x for Linux/Unix

Warning Signs

If you encounter the following error while uploading an SSL certificate in the Plesk control panel:

ERROR message: Unable to set the certificate: Incompatible private key/certificate pair.

The same problem can occur even when the certificate is in .txt format.

Reason

This error occurs because the private key and the SSL certificate do not match.

Solution Overview

To resolve this issue, you need to verify the modulus of the private key and the SSL certificate to ensure they match.

Step-by-Step Guide

1. Check the Modulus of the Private Key

Save the private key file for your domain (e.g., abc.com) and run the following command in your terminal:

openssl rsa -text -noout -in abc.com -modulus | grep Modulus=

Example Output:

Modulus=A6ACD1BCD71FBAD9499D95B9F341F65980BFE13D5189CE9629642F7211E7F8C5CD42394A6F0047A51E4451647E367E36B69D8A42F62B995532F63311

2. Check the Modulus of the SSL Certificate

Next, save your certificate file (e.g., abc.com.crt) and execute the following command:

openssl x509 -text -noout -in abc.com.crt -modulus | grep Modulus=

Example Output:

Modulus=C55B529210F59C810097B854BA3816627DE68D903B85336F92E1E278B0DF31F01944FEFB7A0E7EA39D830559CBF1C40923F826FFE77C97896067F223

3. Compare the Modulus Outputs

Compare the modulus values from both commands.

• If the modulus values match, the private key and SSL certificate are compatible.
• If the modulus values do not match, the SSL certificate is incorrect or does not correspond to the private key.

Solution for Mismatched Modulus

If the modulus values do not match, the issue lies with the SSL certificate. In this case, you need to:

Contact Your Certificate Authority (CA):

Request the CA to re-issue the certificate using the same Certificate Signing Request (CSR) you originally provided.

Upload the New Certificate:

Once you receive the corrected certificate, upload it to the Plesk Panel.

Conclusion

Verifying the modulus ensures that your SSL certificate matches its private key, helping you troubleshoot compatibility issues quickly. For further assistance, contact your SSL provider or refer to the documentation available on host.co.in.

Let me know if you need further adjustments!