How do I authorise an app (web or installed) without user intervention?

Solution 1:

This can be done with the Oauth2 Playground at https://developers.google.com/oauthplayground

Steps:-

1. Create the Google Account (eg. [email protected]) - Or skip this step if you are using an existing account.
2. Use the API console to register the mydriveapp (https://console.developers.google.com/apis/credentials/oauthclient?project=mydriveapp or just https://console.developers.google.com/apis/)
3. Create a new set of credentials. Credentials/Create Credentials/OAuth Client Id then select Web application
4. Include https://developers.google.com/oauthplayground as a valid redirect URI
5. Note the client ID (web app) and Client Secret
6. Login as [email protected]
7. Go to Oauth2 playground
8. In Settings (gear icon), set
   • OAuth flow: Server-side
   • Access type: Offline
   • Use your own OAuth credentials: TICK
   • Client Id and Client Secret: from step 5
9. Click Step 1 and choose Drive API v3 https://www.googleapis.com/auth/drive (having said that, this technique also works for any of the Google APIs listed)
10. Click Authorize APIs. You will be prompted to choose your Google account and confirm access
11. Click Step 2 and "Exchange authorization code for tokens"
12. Copy the returned Refresh token and paste it into your app, source code or in to some form of storage from where your app can retrieve it.

Your app can now run unattended, and use the Refresh Token as described https://developers.google.com/accounts/docs/OAuth2WebServer#offline to obtain an Access Token.

NB. Be aware that the refresh token can be expired by Google which will mean that you need to repeat steps 5 onwards to get a new refresh token. The symptom of this will be a Invalid Grant returned when you try to use the refresh token.

NB2. This technique works well if you want a web app which access your own (and only your own) Drive account, without bothering to write the authorization code which would only ever be run once. Just skip step 1, and replace "my.drive.app" with your own email address in step 6. make sure you are aware of the security implications if the Refresh Token gets stolen.

See Woody's comment below where he links to this Google video https://www.youtube.com/watch?v=hfWe1gPCnzc

. . .

Here is a quick JavaScript routine that shows how to use the Refresh Token from the OAuth Playground to list some Drive files. You can simply copy-paste it into Chrome dev console, or run it with node. Of course provide your own credentials (the ones below are all fake).

function get_access_token_using_saved_refresh_token() {
    // from the oauth playground
    const refresh_token = "1/0PvMAoF9GaJFqbNsLZQg-f9NXEljQclmRP4Gwfdo_0";
    // from the API console
    const client_id = "559798723558-amtjh114mvtpiqis80lkl3kdo4gfm5k.apps.googleusercontent.com";
    // from the API console
    const client_secret = "WnGC6KJ91H40mg6H9r1eF9L";
    // from https://developers.google.com/identity/protocols/OAuth2WebServer#offline
    const refresh_url = "https://www.googleapis.com/oauth2/v4/token";

    const post_body = `grant_type=refresh_token&client_id=${encodeURIComponent(client_id)}&client_secret=${encodeURIComponent(client_secret)}&refresh_token=${encodeURIComponent(refresh_token)}`;

    let refresh_request = {
        body: post_body,
        method: "POST",
        headers: new Headers({
            'Content-Type': 'application/x-www-form-urlencoded'
        })
    }

    // post to the refresh endpoint, parse the json response and use the access token to call files.list
    fetch(refresh_url, refresh_request).then( response => {
            return(response.json());
        }).then( response_json =>  {
            console.log(response_json);
            files_list(response_json.access_token);
    });
}

// a quick and dirty function to list some Drive files using the newly acquired access token
function files_list (access_token) {
    const drive_url = "https://www.googleapis.com/drive/v3/files";
    let drive_request = {
        method: "GET",
        headers: new Headers({
            Authorization: "Bearer "+access_token
        })
    }
    fetch(drive_url, drive_request).then( response => {
        return(response.json());
    }).then( list =>  {
        console.log("Found a file called "+list.files[0].name);
    });
}

get_access_token_using_saved_refresh_token();

Solution 2:

Let me add an alternative route to pinoyyid's excellent answer (which didn't work for me - popping redirect errors).

Instead of using the OAuthPlayground you can also use the HTTP REST API directly. So the difference to pinoyyid's answer is that we'll do things locally. Follow steps 1-3 from pinoyyid's answer. I'll quote them:

1. Create the Google Account (eg. [email protected]) - Or skip this step if you are using an existing account.
2. Use the API console to register the mydriveapp (https://console.developers.google.com/apis/credentials/oauthclient?project=mydriveapp or just https://console.developers.google.com/apis/)
3. Create a new set of credentials (NB OAuth Client ID not Service Account Key and then choose "Web Application" from the selection)

Now, instead of the playground, add the following to your credentials:

Authorized JavaScript Sources: http://localhost (I don't know if this is required but just do it.)
Authorized Redirect URIs: http://localhost:8080

Screenshot (in German):

Make sure to actually save your changes via the blue button below!

Now you'll probably want to use a GUI to build your HTTP requests. I used Insomnia but you can go with Postman or plain cURL. I recommend Insomnia for it allows you to go through the consent screens easily.

Build a new GET request with the following parameters:

URL: https://accounts.google.com/o/oauth2/v2/auth
Query Param: redirect_uri=http://localhost:8080
Query Param: prompt=consent
Query Param: response_type=code
Query Param: client_id=<your client id from OAuth credentials>
Query Param: scope=<your chosen scopes, e.g. https://www.googleapis.com/auth/drive.file>
Query Param: access_type=offline

If your tool of choice doesn't handle URL encoding automagically make sure to get it right yourself.

Before you fire your request set up a webserver to listen on http://localhost:8080. If you have node and npm installed run npm i express, then create an index.js:

var express = require('express');
var app = express();

app.get('/', function (req, res) {
  res.send('ok');
  console.log(req)
});

app.listen(8080, function () {
  console.log('Listening on port 8080!');
});

And run the server via node index.js. I recommend to either not log the whole req object or to run node index.js | less for the full output will be huge.
There are very simple solutions for other languages, too. E.g. use PHP's built in web server on 8080 php -S localhost:8080.

Now fire your request (in Insomnia) and you should be prompted with the login:

Log in with your email and password and confirm the consent screen (should contain your chosen scopes).

Go back to your terminal and check the output. If you logged the whole thing scroll down (e.g. pgdown in less) until you see a line with code=4/....

Copy that code; it is your authorization code that you'll want to exchange for an access and refresh token. Don't copy too much - if there's an ampersand & do not copy it or anything after. & delimits query parameters. We just want the code.

Now set up a HTTP POST request pointing to https://www.googleapis.com/oauth2/v4/token as form URL encoded. In Insomnia you can just click that - in other tools you might have to set the header yourself to Content-Type: application/x-www-form-urlencoded.

Add the following parameters:

code=<the authorization code from the last step>
client_id=<your client ID again>
client_secret=<your client secret from the OAuth credentials>
redirect_uri=http://localhost:8080
grant_type=authorization_code

Again, make sure that the encoding is correct.

Fire your request and check the output from your server. In the response you should see a JSON object:

{
  "access_token": "xxxx",
  "expires_in": 3600,
  "refresh_token": "1/xxxx",
  "scope": "https://www.googleapis.com/auth/drive.file",
  "token_type": "Bearer"
}

You can use the access_token right away but it'll only be valid for one hour. Note the refresh token. This is the one you can always* exchange for a new access token.

* You will have to repeat the procedure if the user changes his password, revokes access, is inactive for 6 months etc.

Happy OAuthing!