How to encrypt PayPal's button

I'm little bit fighting with the PayPal button. I have the unencrypted version which works great, the problem is that the customer can change the amount etc. I found out that is possible to encrypt the data. I used these two examples. The 开发者_运维百科problem is that I'm getting 'We were unable to decrypt the certificate id.'. Maybe it looks like I don't have right PayPal public certificate? I downloaded it from the backend, the only one I found. There wasn't any reference where to download it in non of the turorials. I'm trying this on sandbox right now. Please can anyone advice?

Check this, it may help you: Dynamic Paypal button encryption; also verify you're using the current certificate and be advised that if you're doing tests in the sandbox, the sandbox uses a different certificate of the one you get in the main site (and also you must have an account in the sandbox which is different of your main account).