How to create openid.sig after authorization via steam?

Question

PHP

When logging in, steam redirects me to the /login page with a bunch of GET parameters.
Here are all these parameters (!Slightly changed):

{
  "ns": "http://specs.openid.net/auth/2.0",
  "mode": "id_res",
  "op_endpoint": "https://steamcommunity.com/openid/login",
  "claimed_id": "http://steamcommunity.com/openid/id/*******************",
  "identity": "http://steamcommunity.com/openid/id/*******************",
  "return_to": "https://blablabla.bla/login",
  "response_nonce": "2015-11-29T10:47:06Z8Nm3b4doRBhzzFG+N+o3G5ThhbU",
  "assoc_handle": "1234567890",
  "signed": "signed,op_endpoint,claimed_id,identity,return_to,response_nonce,assoc_handle",
  "sig": "U29I31Ab1Dg/66cHf7dYUoqnd1M"
}

instead of asterisks, confidential data (my account).
So, I know that " sig " is a certain key by which the user is checked (or something like that).
The question itself: How to use this key, and by what formula is it created ???

Answer 1

it is strange that there is no "assoc_type" among the parameters - there must be a hashing algorithm, without knowing which hash cannot be checked