PayPal is upgrading the certificate for www.paypal.com to SHA-256. This endpoint is also used by merchants using the Instant Payment Notification (IPN) product.
This upgrade is scheduled for 9/30/2015; however, PayPal may need to change this date on short notice to you to align to the industry security standard.
Please follow below steps to check if your SSL certificate qualified for PayPal changes:
Section A
1. Go to https://www.digicert.com/help/index.htm
2. Enter your domain name (eg: www.vpcart.com) in Server Address Textbox and then click CHECK SERVER button.
3. Under SSL Certificate > Key Lenght: Check if it is 2048 bit, if not, please go to Section B.
4. Under SSL Certificate > Signature algorithm: check if it has SHA256, it not, please go to Section B.
5. If both check are Ok, then your site should be fine with PayPal changes.
Section B
1. You need to ask your hosting provider to upgrade your SSL certificate based on
https://www.paypal-knowledge.com/resources/sites/PAYPAL/content/live/FAQ/1000/FAQ1766/en_US/2015%20Merchant%20Security%20System%20Upgrade%20Guide%20%28AU%20English%29.pdf