Dynamic Paypal button encryption

From your post you seem very confused about what encryption means and what to apply it to. What is the threat model? (i.e. how can it be subverted).

There is no way you should expect that paypal will always process the order you sent to the client's browser. You MUST check what Paypal did process.

You can be better assured of the integrity of the order after it leaves your site, e.g. by adding a hash of the order to the order number (and a salt!) you send to Paypal. This should allow you to verify the order without reference to the PLU/stored order (as long as the script processing the return from paypal knows the salt).

What you need to do is fairly complex, first, the intro, paypal encrypted buttons have the following layout:

    <form action="https://www.paypal.com/cgi-bin/webscr" method="post">
<input type="hidden" name="cmd" value="_s-xclick">
<input type="hidden" name="encrypted" value="-----BEGIN PKCS7-----MIIIEQYJKo...Encrypted stuff...IF5ioje8JH0LAA+5U7P+tabAMOL37k=-----END PKCS7-----">
<input type="image" src="https://www.paypalobjects.com/es_XC/MX/i/btn/btn_buynowCC_LG.gif" border="0" name="submit" alt="PayPal, la forma más segura y rápida de pagar en línea.">
<img alt="" border="0" src="https://www.paypalobjects.com/es_XC/i/scr/pixel.gif" width="1" height="1">
    </form>

The cmd field indicates an encrypted Buy Now button (check the values for the buttons you want to create), and the encrypted field is the actual content of the button in the following layout:

    cert_id=ZQCMJTZS27U4F
    cmd=_xclick
    [email protected]
    item_name=Handheld Computer
    item_number=1234
    custom=sc-id-789
    amount=500.00
    currency_code=USD
    tax=41.25
    shipping=20.00
    no_note=1
    cancel_return=http://www.company.com/cancel.htm 

Note, these are in the pair=value format, for a full reference look here: https://cms.paypal.com/us/cgi-bin/?cmd=_render-content&content_ID=developer/e_howto_html_Appx_websitestandard_htmlvariables.

Now the theory, to get the encrypted field, well encrypted, you need to sign these values with your certificate (x509 certificate) and you private key, then you need to encrypt this signed message with paypal's public certificate.

Going to the practice, for doing it you can (need) to use the following two PHP functions (part of the OpenSSL extension): openssl_pkcs7_sign and openssl_pkcs7_encrypt.

I found this last part very tricky to setup, so i recommend you to download the PHP SDK for PayPal avalaible here: https://www.x.com/community/ppx/sdks#WPST and directly here: https://cms.paypal.com/cms_content/US/en_US/files/developer/PP_PHP_WPS_Toolkit.zip, this SDK comes with the class EWPServices who contains the method encryptButton which gives you the encrypted button pretty easy; if you want to look at the bones then look in the PPCrypto class who offers you the signAndEncrypt method which give you only the encrypted string you need for the field and does show you the process of encrypting the button.

By the way, if you don't know how to get your certificate and your private key (and/or the Paypal's certificate) look here: https://cms.paypal.com/us/cgi-bin/?cmd=_render-content&content_ID=developer/e_howto_html_encryptedwebpayments#id08A3I0N30Y4