Validate that IPN call is from PayPal?

Question

How can I validate that a PayPal IPN POST request to my specified notifyURL is indeed coming from PayPal?

I don\'t mean comparing the data to what I sent earlier, bu

Answer 1

The IPN protocol consists of three steps:

1. PayPal sends your IPN listener a message that notifies you of the event
2. Your listener sends the complete unaltered message back to PayPal; the message must contain the same fields in the same order and be encoded in the same way as the original message
3. PayPal sends a single word back, which is either VERIFIED if the message originated with PayPal or INVALID if there is any discrepancy with what was originally sent

https://cms.paypal.com/us/cgi-bin/?cmd=_render-content&content_ID=developer/e_howto_admin_IPNIntro