Figuring out the Goo.gl API

15 Dec 2009 · By Anant Narayanan

UPDATE: ‘Fatalis’ has pointed out in the comments that the POST should be made to http://goo.gl/api/url with User-agent set to ‘toolbar’. The code now works, Yay!

Google just announced their own URL shortening service. Their service can only be used from the toolbar or FeedBurner, and I don’t particularly like adding extra toolbars to my browser. Maybe I can figure out a way to use their service from the command line?

I downloaded the toolbar XPI, unzipped it and peeked inside. Horribly indented JS awaited me. Nothing jsbeautifier couldn’t fix though. Few minutes later, I arrived at this readable JS function:

var getUrlShorteningRequestParams = function (b) {
    function c() {
        for (var l = 0, m = 0; m  0 ? l : l + 4294967296);
        for (var o = 0, n = false, p = m.length - 1; p >= 0; --p) {
            var q = Number(m.charAt(p));
            if (n) {
                q *= 2;
                o += Math.floor(q / 10) + q % 10
            } else o += q;
            n = !n
        }
        m = m = o % 10;
        o = 0;
        if (m != 0) {
            o = 10 - m;
            if (l.length % 2 == 1) {
                if (o % 2 == 1) o += 9;
                o /= 2
            }
        }
        m = String(o);
        m += l;
        return l = m
    }
    function e(l) {
        for (var m = 5381, o = 0; o < l.length; o++)
            m = c(m << 5, m, l.charCodeAt(o));
        return m
    }
    function f(l) {
        for (var m = 0, o = 0; o < l.length; o++)
            m = c(l.charCodeAt(o), m << 6, m << 16, -m);
        return m
    }

    var i = e(b);
    i = i >> 2 & 1073741823;
    i = i >> 4 & 67108800 | i & 63;
    i = i >> 4 & 4193280 | i & 1023;
    i = i >> 4 & 245760 | i & 16383;

    var h = f(b);
    var k = (i >> 2 & 15) << 4 | h & 15;
    k |= (i >> 6 & 15) << 12 | (h >> 8 & 15) << 8;
    k |= (i >> 10 & 15) << 20 | (h >> 16 & 15) << 16;
    k |= (i >> 14 & 15) << 28 | (h >> 24 & 15) << 24;
    j = "7" + d(k);

    i = "user=toolbar@google.com&url=";
    i += encodeURIComponent(b);
    i += "&auth_token=";
    i += j;
    return i
};

So, I call getUrlShorteningRequestParams("http://www.kix.in/"); to get "user=toolbar@google.com&url=http%3A%2F%2Fwww.kix.in%2F&auth_token=78925814685". I see in their code that they do a POST request to the service to obtain a JSON return value that would contain the short URL. I punch it in using cURL:

$ curl -v -d\
   "user=toolbar@google.com&url=http%3A%2F%2Fwww.kix.in%2F&;\
   auth_token=78925814685" http://goo.gl/
* About to connect() to goo.gl port 80 (#0)
*   Trying 74.125.19.102... connected
* Connected to goo.gl (74.125.19.102) port 80 (#0)
> POST / HTTP/1.1
> User-Agent: curl/7.19.7 (i386-apple-darwin10.2.0) libcurl/7.19.7
> Host: goo.gl
> Accept: */*
> Content-Length: 77
> Content-Type: application/x-www-form-urlencoded
>
< HTTP/1.1 405 HTTP method POST is not supported by this URL

Oops! Well, not really, the URL shortener from the toolbar doesn’t work either, I just get the full URL whenever I try to “share” something. Has anybody actually generated a real goo.gl short URL yet?

Their auth_token parameter seems completely superfluous to me as it is generated from the URL itself. Don’t we all know security by obscurity doesn’t work :)

 

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