The PayPal IPN contribution for osCommerce offers OpenSSL public/private key encryption to securely send the order information to PayPal during checkout.

Here is information on how to generate the necessary certificates:

1. You should have OpenSSL installed on your computer. Linux distrubutions should have it installed by default, and a Windows version can be downloaded from here.

2. You should generate a private key using this command:

openssl genrsa -out my-prvkey.pem 1024

This will create a 1024-bit RSA private key in the file my-prvkey.pem.

3. You should generate a public certificate using this command:

openssl req -new -key my-prvkey.pem -x509 -days 365 -out my-pubcert.pem

This will create a public certificate in the file my-pubcert.pem.

4. Then you should upload your public certificate to the PayPal website:

4.1. Log in to your PayPal account.
4.2. Click the Profile tab.
4.3. In the Seller Preferences column you should click the Encrypted Payment Settings link. The Website Payment Certificates page should appear.
4.4. Scroll down to the Your Public Certificates section and click the Add button. The Add Certificate page should appear.
4.5. Click the Browse button and select the public certificate that you want to upload to PayPal from your local computer.
4.6. Click the Add button. After you upload the public certificate successfully, it appears in the Your Public Certificates section of the Website Payment Certificates page.

5. A certificate ID will be assigned to your public certificate. You need this certificate ID to encrypt your payment buttons by using PayPal's Encrypted Website Payments program.

For more information about PayPal's certificates and their integration please check the official integration documentation.