'Return a bool indicating  whether the client sent
us a valid signature in the payment page request.'

@staticmethod
def _is_signature_valid(post_params):

    signed_fields = post_params.get('signed_field_names').split(',')
    hash_val = ','.join(['{0}={1}'.format(key, post_params[key]) for key in signed_fields])
    public_sig = processor_hash(hash_val)
    return (public_sig == post_params.get('signature'))
