I recently got a new machine at work. Before I got it I spent a bit of time preparing and figuring out which essential tools I need to do my daily work. But, also to minimize the amount of time I would have to spend installing everything.

As you the reader might know I develop mobile Apps with Xamarin and .NET. So this blog post will be geared towards that. However, you would be able to install anything using the same approach.

My colleague pointed out that there is this super cool feature of ZSH called dotfiles. ZSH ships with macOS ans is the shell that you see when you open a terminal. It has a set of very powerful interactive tools but is also a script interpreter. Similar to other shells you might know such as bash, fish and csh.

The configuration for ZSH happens with a file called .zshrc, which is a list of settings, aliases for commands, styles and more.

Dotfiles in ZSH is a way to organize the settings you would normally put it .zshrc, but not only that, you can organize other configuration files for other tools, and also organize scripts.

There are multiple attempts to build on top of these dotfiles for easily managing the tools you need for your daily work. The version I went for is Oh Your dotfiles. With this tool you can make a logical folder structure with scripts and dependencies you want to have installed.

Let’s dig into it!

Start off by cloning Oh Your dotfiles:

git clone https://github.com/DanielThomas/oh-your-dotfiles ~/.oh-your-dotfiles

Then you run

ZDOTDIR=~/.oh-your-dotfiles zsh

Followed by:

dotfiles_install

Now you have oh your dotfiles installed. Time to create your own repository to customize your installation. For my own use I’ve created a dotfiles repository, with the tools I need. I’ve called it dotfiles on purpose as Oh Your Dotfiles automatically looks for paths containing dotfiles in the name, call it anything you want, just make sure you clone it into a folder starting with . and containing dotfiles in your user directory.

Now with the stuff cloned, you can start add content.

For instance. I have a folder called dev with a script called install.homebrew with the following contents:

ca-certificates
git
gh
scrcpy
[email protected]
mitmproxy
gawk
imagemagick
curl
wget
tree

This will install all those tools from homebrew.

Similarly if you want to install stuff from homebrew casks you can create a install.homebrew-cask file and add stuff there. For instance I have this in one of my folders:

firefox
amethyst
anydesk
little-snitch
spotify
gimp
signal
macfuse
keka
hot

When you’ve added some files and stuff to download. Simply run dotfiles_update and it will install everything.

Remember to synchronize your dotfiles somewhere. Then next time you get a new machine you can simply do:

git clone https://github.com/DanielThomas/oh-your-dotfiles ~/.oh-your-dotfiles
git clone https://github.com/Cheesebaron/dotfiles ~/.dotfiles
ZDOTDIR=~/.oh-your-dotfiles zsh
dotfiles_install

Then you can wait and everything is installed. Super convenient.

Had the pleasure of hitting a bug in the net6.0-android TFM when targeting Android 31. This bug had been fixed in a preview version of the net6.0-android workload, targeting Android 32 or higher.

So question is, how would you set up your CI or even your local environment to run this?

In my specific case I wanted:

• Android platform for android-Tiramisu
• .NET workload for android, containing the fix, android-33

The steps to install these are fairly simple. Use Android SDK manager to install the platform. Then use dotnet to install the specific workload.

sdkmanager --install "platforms;android-Tiramisu"

dotnet workload install android-33

This enables me to use net6.0-android33.0 as TFM.

sdkmanager is located in your Android SDK folder to invoke it, you might need to add it to your path or cd into the folder <android-sdk>/cmdline-tools/latest/bin and run ./sdkmanager.

Setting up steps in Azure Pipelines is super easy too:

- script: |
    ${ANDROID_SDK_ROOT}/cmdline-tools/latest/bin/sdkmanager --install "platforms;android-Tiramisu"
  displayName: Install Android SDK for Tiramisu

- task: UseDotNet@2
  displayName: 'Use .NET Core SDK 6.0.x'
  inputs:
    version: 6.0.x

- script: |
    dotnet workload install android
    dotnet workload install android-33
  displayName: Install Android Workloads

Now you should be able to build .net6.0-android33.0 TFMs!

I’ve been debugging some network issues in one of my Apps recently. To help me I acquired the help of the excellent macOS Application Charles Proxy. I needed to see what was sent to and from the servers the App communicates with and also check the contents. One issue though, all the calls are through SSL, so without a little bit of setup, you will not get far checking the contents of the calls.

The information served in this blog post, may also work with Fiddler and mitm-proxy if you prefer those tools.

Charles Proxy has a way to do SSL proxying, which can show you the text contents of SSL requests and responses. You can specify specific sites to include. To get this to work with Xamarin.Android or .NET6 Android, we need to tell the AndroidClientHandler which certificates we trust. In addition to the regular set up specifying Charles to be the proxy in the WiFi Settings.

For some reason using the built in Android functionality specific to setting networkSecurityConfig in the Android manifest and allowing to use trusted user certificates doesn’t work with AndroidClientHandler, so we have to do some manual grunt work.

1. Setting Up Device Proxy

First we need to modify the device settings to point towards Charles Proxy. You can do that through WiFi settings on your device. Go to Settings -> Network & Internet -> Wi-Fi. Press the Gear icon next to the connected network and edit it using the pencil icon.

In the Proxy drop down select Manual

Proxy hostname will be the IP of the machine you are running Charles Proxy on. You can find this using Charles in the Help -> Local IP address menu. Proxy port will be the port defined in Charles Proxy settings, default is 8888

Doing this step, you should start seeing traffic flowing into Charles from your device. You will see that all SSL requests/responses have garbled data. Not something you can read. We will fix that in next step.

You can also verify the proxy works by going to http://chls.pro/ssl in a browser on the device, and it will download the Charles Root Certificate. For this setup, it is optional to install it.

2. Adding Trusted Certificates to AndroidClientHandler

Since AndroidClientHandler doesn’t grab user installed certificates, we need to add them ourselves.

In Charles Proxy go to Help -> SSL Proxying -> Save Charles Root Certificate…. Save it as Binary certificate (.cer).

In your Android project add it to the Assets folder and remember to mark the file Build Action as AndroidAsset.

Now to setting up a AndroidClientHandler with the certificate.

private AndroidClientHandler CreateClientHandler()
{
    CertificateFactory? factory = CertificateFactory.GetInstance("X.509");
    var clientHandler = new AndroidClientHandler();
    if (factory == null)
        return clientHandler;

    try
    {
        using Stream? stream = ApplicationContext?.Assets?.Open("charles-ssl-proxying.cer");
        var cert = (X509Certificate?)factory.GenerateCertificate(stream);

        if (cert != null)
        {
            clientHandler.TrustedCerts ??= new List<Certificate>();
            clientHandler.TrustedCerts.Add(cert);
        }

        return clientHandler;
    }
    catch (Exception e)
    {
        _logger.LogError(e, "Failed to get Charles Certificate");
        throw;
    }
}

Basically what happens here is that we load the charles-ssl-proxying certificate from assets and create a certificate that we add to AndroidClientHandler.TrustedCerts. This is where it expects extra certificates to be added.

Now with this AndroidClientHandler you can create your HttpClient with that handler:

var client = new HttpClient(CreateClientHandler());

In my latest project I started using HttpClientFactory, which allows me to set this handler once and forget about it.

Anyways, when creating requests with HttpClient now, you should be albe to see the data sent and received even when using SSL!

Note: remember to add domains to the SSL Proxy Settings in Proxy -> SSL Proxy Settings in Charles. A good practice is to limit these to the domains you expect and want to debug.

All the code in this post can be found on GitHub

A StackOverflow question sparked my interest in playing around with the SafetyNet API on Android.

You might ask, what is this SafetyNet thing? Let us say you are writing an Application, where trust and security is a top concern. You want to verify that the device your App is running on has not been tampered with. It will check both the hardware and the software to see what the condition it is in. Google provides SafetyNet to help with detecting this. It is not bulletproof and there are ways of circumventing this. So it should be kept in mind that it should not be used as the only way to detect and prevent any abuse. There is some very good documentation with a lot more details you can read in the Android Developer docs.

The SafetyNet API can be queried through Google Play Services libraries that you add to your App. This means, the device you are running on will need to have a recent enough version of Google Play Services as well. Devices without Google Play Services, will not be able to pass this check, as such devices are usually not certified by Google.

You will need to add the following NuGet package:

Xamarin.GooglePlayServices.SafetyNet

This package has a ton of dependencies that it pulls in. Additionally if you want to check the response from the API you might also want to install:

System.IdentityModel.Tokens.Jwt

The response is a JWT token that you can verify and its claims will contain the result of the attestation.

Checking for Google Play Serivces

Before you can do anything, you need to check if the Device has Google Play Services installed. This is fairly simple and Google provides an easy way to do this through the Play Service libraries that are pulled in as dependencies.

var code = GoogleApiAvailability.Instance.IsGooglePlayServicesAvailable(context, 13000000);

The code returned he can be checked for whether the services are available, if not it can also be used to show a message to the user of what they can do to resolve the issue, this can be done like so:

if (code == ConnectionResult.Success)
   // bingo!

If we have an error showing a message to the user can be done with:

var instance = GoogleApiAvailability.Instance;
if (instance.IsUserResolvableError(errorCode))
{
    instance.ShowErrorDialogFragment(context, errorCode, 4242);
}

This should show a dialog looking like so. But it varies depending on the error:

Once you’ve checked and verified Google Play Services are available, you can then start checking the SafetyNet attestation.

Acquiring an SafetyNet API key

In your Google Cloud Console you will need to enable the Android Device Verification service.

Once you have enabled it, you will be prompted to create an API key. I highly suggest that you create it where you tie it to the application identifier, such that it is only usable by the App. You can create another API key for your validation server. Once you have an API key you are ready to verify your device.

Checking SafetyNet Attestation

With a API key in your hand and Google Play Services ready and installed, you can now use the SafetyNet client to acquire an attestation.

SafetyNetClient client = SafetyNetClass.GetClient(context);
var nonce = Nonce.Generate(24);

var response = await client.AttestAsync(nonce, attestationApiKey).ConfigureAwait(false);

The nonce here is very important, the more information it contains, the harder it will be for attackers to make replay attacks on your App. Google recommends you add stuff like:

• hash of username
• timestamp of the nonce

The example of nonce above is not recommended to use

With the response in your hand, you can already decode it as a JWT token to figure out what SafetyNet thinks about the device:

var result = response.JwsResult;
var jwtToken = new JwtSecurityToken(result);

var cts = jwtToken.Claims.First(claim => claim.Type == "ctsProfileMatch").Value;
var basicIntegrity = jwtToken.Claims.First(claim => claim.Type == "ctsProfileMatch").Value;

The ctsProfileMatch is a verdict of the device integrity. This will be true if your device matches a profile of a Google-certified Android device. While, the basicIntegrity is more lenient and is telling you whether the device the App is running on has been tampered with.

There is a table in the Android documentation which tells what it means if these values are true or false.

Note emulators will always return false for both values

Ideally, you would send this JWT token to be verified on your server to check if the nonce matches and the signatures are valid. Your server would either call Google’s API to validate the JWT token. Which can be done as follows.

public async Task<bool> VerifyAttestationOnline(string attestation)
{
    using var request = new HttpRequestMessage(HttpMethod.Post,
        $"https://www.googleapis.com/androidcheck/v1/attestations/verify?key={attestationApiKey}");
    var data = new JWSRequest { SignedAttestation = attestation };
    var json = JsonSerializer.Serialize(data);
    request.Content = new StringContent(json, Encoding.UTF8, "application/json");

    using var response = await httpClient.SendAsync(request).ConfigureAwait(false);
    response.EnsureSuccessStatusCode();

    using var responseData = await response.Content.ReadAsStreamAsync().ConfigureAwait(false);
    var attestationResponse = await JsonSerializer.DeserializeAsync<AttestationResponse>(responseData).ConfigureAwait(false);

    return attestationResponse.IsValidSignature;
}

public class JWSRequest
{
    [JsonPropertyName("signedAttestation")]
    public string SignedAttestation { get; set; }
}

public class AttestationResponse
{
    [JsonPropertyName("isValidSignature")]
    public bool IsValidSignature { get; set; }
}

Alternatively Google provides a sample to validate the JWT token yourself offline.

Make sure you go through [this checklist] before you ship your SafetyNet validation to make sure you have covered all cases.

All the code can be found in a Sample Application I have made and published on GitHub.

I was lucky to be tasked to get Universal AppLinks working, for a cool feature in the TrackMan Golf App I work on at work. I thought this would be super easy to do. But, oh how naive and wrong I was.

So in essence, the App is supposed to open if I hit a URL with a specific pattern for one of our domains. Something like: https://trackman.com/pin/123456. Where the numbers can differ.

I’ve done this before with a custom scheme like trackman://pin/123456, which is just a simple entry in the Info.plist for your App. However, for Universal AppLinks, there are many more moving parts involved.

1. Associate your App(s) to a specific domain

I guess, in order to not be able to open arbitrary URLs in your App. Apple, requires you to host a file called apple-app-site-association on your Web Server. This file should not have any file extensions. Make sure you also serve it with the Content-Type: application/json. Also, there should be no redirects to get to this URL.

This file should be in either the root or in a .well-known folder. For instance:

• https://trackman.com/apple-app-site-association
• https://trackman.com/.well-known/apple-app-site-association

Apple will check the .well-known folder first, then fall back to the root.

This check happens when you install the App on your device. There is a process called swcd that will run and check these associasions on your device during install. If any of this stuff fails, this swcd process is what you look for in the device log. The errors will look something like this.

Request for '<private>' for task AASA-80AD262A-3EF6-42A2-B992-AC97234187647 { domain: *.tr....com, bytes: 0, route: cdn } denied because the CDN told us to stop with HTTP Status...

You will need to deduct yourself which domain it relates to, because the console entries redact some of the values. However, it will tell you what is wrong. Some common issues are:

• SSL certificate doesn’t match the domain entry you’ve registered
• Apple CDN cannot access the file
• The JSON is in a wrong format

Yes, you read it right. From iOS 14 and up, Apple will hit the file through their CDN. So you have to make sure their CDN can reach the file.

The contents of this JSON file will look as follows. Also refer to Listing 6-1 in these Apple docs. for more details of the format.

{
    "applinks": {
        "apps": [],
        "details": [
            {
                "appID": "<app id prefix>.<bundle id>",
                "paths": [ "/pin/*"]
            }
        ]
    }
}

The JSON file format that they suggest in the Supporting Associated Domains documentation, did not work for me! If something doesn’t work, try swiching between the two. Unfortunately Apple does not provide a JSON-Schema for this, so it is not easy to validate if you entered stuff correctly, except that you are returning valid JSON.

The App id prefix and bundle name can be located in the developer portal where you have created your App in the Identifiers section. The App Id will be the same as your Team ID usually. Bundle id, is the Id of your App that you have registered in there. So appID that goes into the JSON file will look something like: 2V9BB354QZ.my.awesome.app.

The paths is an array of paths. So anything that goes after your domain. You can use wildcards like:

• * will match any substring
• ? will match a single character

Examples:

• /tamarin will match a specific path
• /cats/* will match everything in the “cats” section of your website
• /cats/archives/202? will match everything in “cats/archives” and 2020-2029

You can combine the wildcard characters. The paths array can have multiple, comma delmited paths for a App Id.

To exclude a path you can write NOT /cats/*.

2. Enabling the Associated Domain entitlement in provisioning profiles

Go to Certificates, Identifiers & Profiles in the Apple Developer Portal. Here you will need to enable the “Associated Domains” entitlement for your App.

In the list of Identifiers find your App and Edit it. Enable the “Associated Domains” entitlement

Now go to any Provisioning profiles you have an regenerate them. You might be using Fastlane or something like that. Read the docs for your tool to download or regnerate provisioning profiles.

After regenerating the Provisioning profiles, you might need to update any CI pipeline and machines that you build on with these new profiles.

3. Adding Associated Domains to your Entitlements.plist files

If you don’t have a Entitlements.plist file, it is now time to create one. Microsoft provides good docs for you to read more about how to create one. You will need to add all the domains you wish your Apps to open. These of course need to match the domains you added in the first step in the apple-app-site-association file. This will look something like:

<key>com.apple.developer.associated-domains</key>
<array>
    <string>applinks:trackman.com</string>
    <string>applinks:ostebaronen.dk</string>
    <string>applinks:sub-domain.ostebaronen.dk</string>
</array>

You can also use wildcards here:

<string>applinks:*.ostebaronen.dk</string>

This will match any sub-domain, but not include the raw ostebaronen.dk domain. If you can avoid it, I recommend not to use the wildcard here. I had issues with the Apple CDN not being able to process the apple-app-site-association file, since it seems like it expects a wildcard entry in the SSL certificate too. I did not have that. If you are going to use it, make sure to test this thoroughly.

4. Extending AppDelegate.cs to handle the URL requests to your App

Luckily it seems like how our App is triggered is that first it launches the App fully. So FinishedLaunching is allowed to complete. Then ContinueUserActivity gets triggered with the URL. For good measures I also added an override for UserActivityUpdated. So in your AppDelegate.cs override these two methods.

public override bool ContinueUserActivity(UIApplication application, NSUserActivity userActivity, UIApplicationRestorationHandler completionHandler)
{
    CheckForAppLink(userActivity);
    return true;
}

public override void UserActivityUpdated(UIApplication application, NSUserActivity userActivity)
{
    CheckForAppLink(userActivity);
}

To get the URL from the NSUserActivity I got inspired from the AppDelegate.cs that Xamarin.Forms provides.

The code looks like:

private void CheckForAppLink(NSUserActivity userActivity)
{
    var strLink = string.Empty;
    if (userActivity.ActivityType == "NSUserActivityTypeBrowsingWeb")
    { 
        strLink = userActivity.WebPageUrl.AbsoluteString;
                
    }
    else if (userActivity.UserInfo.ContainsKey(new NSString("link")))
    {
        strLink = userActivity.UserInfo[new NSString("link")].ToString();
    }

    // do something clever with your URL here
    NavigateToUrl(strLink);
}

5. Debugging all of this

This part is a huge pain. First of all, it appears to me, when you deploy an Application using Xcode or Visual Studio, the swcd process doesn’t run and check the apple-app-site-association file. This means when you hit your URL in Safari or through a QR code, it will not suggest to open your App. I had to grab builds from my CI pipeline for this to work 😢.

But things to check:

• apple-app-site-association is reachable
• the URL you are hitting is not returning 404 (although it might actually still work)
• entries in your Entitlements.plist match the association json file
• if you have multiple Entitlements for different configurations, make sure you are using the correct one
• check the console output on your device
  • Xcode -> Window -> Devices & Simulators -> Devices -> Select device -> Open Console
  • Click Errors and Faults
  • add swcd in the search field

6. Bonus: Handling custom schemes

If you also need to handle custom URL schemes. The only thing you need to do is to register the schemes in your Info.plist file:

<key>CFBundleURLTypes</key>
<array>
    <dict>
        <key>CFBundleURLName</key>
        <string>my.bundle.id</string>
        <key>CFBundleURLSchemes</key>
        <array>
            <string>awesomescheme</string>
            <string>tamarin</string>
        </array>
    </dict>
</array>

Then override OpenUrl in your AppDelegate.cs file:

public override bool OpenUrl(UIApplication app, NSUrl url, NSDictionary options)
{
    if (url.ToString().Contains("/my-pattern"))
    {
        NavigateToUrl(url.ToString());
        return true;
    }

    return base.OpenUrl(app, url, options);
}