Generating App Hash key for SMS Retriever API for (Xamarin incl.)…

Trying to generate your app’s Hash Key for the implementation of new Android SMS Retriever API? Still couldn’t get it to work? Well the problem could be in the actual generation process of the has string!

And here are some ways you could actually make sure to generate the valid hash string to get the functionality working.

Specially if you’re developing on Windows, or better working on a Xamarin Android app? I’ve got some goodies to make it work as well! 😉

App Hash string for SMS Retriever API

Anyhow, to use the SMS Retriever API we need generate our App’s unique Hash string which is a combination of app’s package name and your app’s public key certificate. Once the SMS is received into the Inbox the SMS Retriever API looks for any matching hash key in the message corresponding to the app that requested to read the SMS, if it finds a perfect match it will kick in the execution of retrieving the SMS string, otherwise it will time out. So without the accurate app hash key the SMS Retriever API will not kick in at all. That is why it is very crucial we generate the correct app hash key for our app using the App package name and keystore hash.

Troublesome!

Although according to the official Google Documentation, it should be easy and straight forward, but it is not, specially if you’re using a Windows Dev Environment, which could be troublesome at times. To add some topping on to that, if you’re working on a Xamarin Android project in Windows, you’re in deep troublesome waters, since even Google Docs only provides samples for the Android Java implementation.

Then the sample command that Google Docs provide doesn’t really throw any error if anything goes wrong during, wrong parameter, or a wrong keyword, during hash string generation, it simply returns a false hash string, which makes things worse!

This is actually something I experienced while I was developing one of our Xamarin Android applications. Oh yes! that’s even more annoying since there’s no official documentation or Xamarin sample of the implementation code regarding that API. 😦

Hence Google’s doc command failed me I had to play around with the parameters we’re passing in to the command, re-arrange and modify to get it to work. But anyhow with a walk in the midst of the dark forest of troubleshooting, I figured out how to properly generate the App Hash key!

Get your pre-requisites right!

First of all prepare your keystore file path, keystore alias name and keystore password, and double check those values are being properly set. This is where most of them goes wrong. Then the obvious requirement, make sure you have Androd

Method 1: On MacOS Terminal

So this is the easiest and most straight forward way to generate the hash string, by simply executing the below command on a MacOS Terminal.

keytool -exportcert -alias <keystore alias name> -keystore <keystore file with extension> | xxd -p | tr -d "[:space:]" | echo -n <your app id> `cat` | sha256sum | tr -d "[:space:]-" | xxd -r -p | base64 | cut -c1-11
  • Replace the <keystore alias name> with your keystore alias name, ex: mykeystorename
  • Replace the <keystore file with extension> with the full path to your keystore file, ex: /Users/username/Desktop/mykeystorefile.keystore
  • Replace the <your app id> with your app id: com.mytest.app

If everything went well, next line Terminal will immediately prompt you to enter the keystore password, which will generate the accurate app hash key!

Yep simple as that! 😀

Now you might wonder how I shared the above screenshot so bravely without worrying about the security, oh well its just using the default Xamarin Android keystore generated by VS for Mac with its default settings and a fake app id! 😛

Troublesome? Get Utils installed!

Unless you got some missing utils in your mac developer machine, such as the following error: sha256sum: command not found, which indicates missing coreutils in your Mac. Therefore you need to fist install Homebrew if you don’t have already.

Install Homebrew: https://brew.sh/

Restart Terminal and run the below command!

brew install coreutils

Restart Terminal and re-run the has generation command! 🙂

Method 2: On Windows CMD Prompt

Now on Windows its going to be a little bit of more work specially most of the command parameters which are used in unix cannot be executed straight up in Windows Command Prompt.

So in hope of avoiding at least a little bit of pain I would recommend using the Visual Studio Command Prompt or the Android ADB Command Prompt in your Windows PC. Specially since the java keytool path should already be configured in the Path property there.

Step 1: Generate the hex string from keystore file

keytool -alias <keystore alias name> -exportcert -keystore "<keystore file with extension>" -storepass "<keystore password>" | xxd -p

Now the parameters that you need to replace are the same as we did on MacOS command, but as you can see keystore file path and password are provided within brackets or quotations. So make sure to add that detail.

If everything went well, it will return you the hex string from your keystore file!

Now that’s me trying out the default debug keystore file in Windows. If everything executes properly, it should return a super long hex string as shown above, actually even much longer than that lol, I just cropped it out 😛

Copy that hex string and keep it, we are gonna use it in the next step.

Step 2: Implement the Hash key generate code script

Next we’re going to port the Google Doc’s Android Java implementation of the Hash key generation execution to Xamarin Android C# code snippet, thanks to my few minutes of trail and error efforts 😛

Simply copy that snippet into a Xamarin Android project with the specific imports mentioned at the top of the snippet. You could use this as a little helper extension method to generate the app hash key by passing in the app package name and keystore hash value that we generated in the step 1, as mentioned in the parameters.

That should work like a charm! 😀

Although keep in mind once you generate the app hash string, you should remove the keystore hex and package name hex from the code before you submit it to the app store. Google does not recommend those sensitive information inside the code as magic strings due to obvious security reasons.

Now the above code snippet can be used in either Visual Studio on Windows or Mac as long as you got the keystore hex string. 😉

TADAAA! 😀 That’s it!

You’re welcome!

Share the love! 😉 ❤

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