If you want to add security to your button, you can create a certificate and then share it with PayPal through your profile. You'll also need to download PayPal's public certificate so they can communicate.
Before you share the certificate, you just need to give the certificate your name or company name, how long you want the certificate to last, and a password for the certificate. Then click ok and the certificate will be saved.
Once you've created it, log into PayPal and upload the certificate in your profile. And make note of the id that PayPal gives it.
Then download the PayPal public certificate.
Then you can use the certificates to encrypt the buttons.