This script reproduces Google Analytics' (analytics.js) allowLinker/autolinker logic, resulting in a function that returns the GA client id, but only if the autolinker parameter is valid. Uses the "old" linker plugin format. You can also pass a string as an argument to check if that string is a valid linker parameter.

allowlinker-ga.js

  // Mirrors analytics.js logic to validate autolinker param, and returns cid if valid.
  // Returns false if autolinker param found, but invalid.
  // Otherwise returns undefined.
  var getCidFromAutolinker = function(str) {
    var joiner = function (cidGid, offset) {
      var a = new Date, 
          b = window.navigator, 
          c = b.plugins || [];
      var d = [cidGid, b.userAgent, a.getTimezoneOffset(), a.getYear(), a.getDate(), a.getHours(), a.getMinutes() + offset];
      for (var e = 0; e < c.length; ++e) {
        d.push(c[e].description);
      }
      return jumble(d.join('.'));
    };
    var joiner2 = function (cidGid, offset) {
      var a = new Date, 
          b = window.navigator,
          c = a.getHours() + Math.floor((a.getMinutes() + offset) / 60);
      return jumble([cidGid, b.userAgent, b.language || "", a.getTimezoneOffset(), a.getYear(), a.getDate() + Math.floor(c / 24), (24 + c) % 24, (60 + a.getMinutes() + offset) % 60].join("."));
    };
    var jumble = function (arr) {
      var b = 1, c;
      if (arr) {
        for (b = 0, c = arr.length - 1; 0 <= c; c--) {
          var d = arr.charCodeAt(c);
          b = (b << 6 & 268435455) + d + (d << 14);
          d = b & 266338304;
          b = 0 != d ? b ^ d >> 21 : b
        }
      }
      return b.toString();
    };
    var linker = /[?&#]_ga=([^&#]+)/.exec(str||location.href);
    if( ! linker ) return;
    linker = linker[1];
    var a = linker.indexOf('.'), b, c, d, fingerprint, cidGid, cid;
    if (a > -1) {
      b = linker.substring(0, a);
      c = linker.substring(a + 1);
      d = c.indexOf(".");
      fingerprint = c.substring(0, d);
      cidGid = c.substring(d + 1);
      cid = cidGid.split('-')[1];
    }
    if (cidGid) {
      cidGid = cidGid.split('-').join('');
      return ( fingerprint === joiner(cidGid, 0) ||
        fingerprint === joiner(cidGid, -1) ||
        fingerprint === joiner(cidGid, -2) ||
        fingerprint === joiner2(cidGid, 0) ||
        fingerprint === joiner2(cidGid, -1) ||
        fingerprint === joiner2(cidGid, -2) ) ? cid : false;
    }
  };