Validate secure https callback

I have a simple rails app that is interacting with a payment processing service.

I am looking to validate a https callback and all I am given is a SHA1 in order to do so.

Is it possible to verify the remote identity with this SHA1 string alone?

I have looked at some openssl code samples and I am not quite sure how to do this?

Do I need to put this SHA1 in a file and read it in? Do I need to obtain the cert of the payment processor and put a copy on my server?

Anyone got any snippets?