Azure Functions Tips: where are our Function App keys?

Introduction

Azure Functions give you a built-in way to secure the calls for Http trigger functions. If you look at the following simple Azure Functions:

[FunctionName("Echo")]
public IActionResult Echo(
    [HttpTrigger(AuthorizationLevel.Function, "post", Route = "echo")] HttpRequest req,
    ILogger log)
{
    log.LogInformation("C# HTTP trigger function processed a request.");

    return new OkObjectResult(req.Body);
}

You can notice the argument AuthorizationLevel.Function of the HttpTrigger attribute. According to the official documentation (here for more info), that value:

is a string value that indicates the kind of authorization key that's required to access the function endpoint.

It means that, if you enable that feature, you need to invoke the Http endpoint with an URL like the following:

https://myfunctionapp.azurewebsites.net/api/echo?code=<API_KEY>

If you prefer, you can send the key using the header x-functions-key instead the query string.
The possible values for the AuthorizationLevelenumeration are:

Azure portal gives you the capability to manage both function-specific keys and master keys:

Default keys store

As you can see from the previous images, you can add more than one key both in the system group than for each single function.
But the question is: where are stored these keys?

By default, the Function App retrieves (and saves, of course) all the keys you create in a Storage Account.
The connection string of the Storage Account is stored in the AzureWebJobsStorage configuration.
The Function App uses this Storage Account for the timer triggers info, the Event Hub checkpoint and, also, as store for the different Keys you have in it.
If you open the Storage Account page, you can find a container called azure-webjobs-secrets as shown in the following picture:

In this container you find a folder for each Function App that uses it as store and inside folders you can see the JSON blobs that contain the secrets:

As you can see in the previous image, you have one blob for each function (in our sample, we have only the echo.json blob for the echo function) and one blob (host.json) for the system keys.
The echo.json has the following schema:

{
  "keys": [
    {
      "name": "default",
      "value": "CfDJ.........HaTQ",
      "encrypted": true
    }
  ],
  "hostName": "keyvaultkeysfuncapp.azurewebsites.net",
  "instanceId": "1cb21680e4253c2794483f9b01c5c800",
  "source": "runtime",
  "decryptionKeyId": "MACHINEKEY_DecryptionKey=Qzlp8wEKnbM33X1gHAREIGNJee8Ii6e6GEmnt5jT4j8=;"
}

You have the array keys that contains all the keys you create for the specific function.
The key is a JSON Web Key (JWK) key (rfc definition) and you can add, remove or modify a key changing the JSON blob (I suggest to use the Azure portal or the CLI command, but if you want you can change directly in the blob).

To generate a JWK kay you can use the following C# snippet (or you can find a lot of on-line tools):

using System.Security.Cryptography;
using System.Text.Json;

class Program
{
    static void Main()
    {
        // Create a new RSA key pair
        using (var rsa = RSA.Create())
        {
            // Export the public key as a JWK
            var jwk = Base64UrlEncode(rsa.ExportRSAPublicKey());
            Console.WriteLine(jwk);
        }
    }

    static string Base64UrlEncode(byte[] data)
    {
        return Convert.ToBase64String(data)
            .Replace('+', '-')
            .Replace('/', '_')
            .Replace("=", "");
    }
}

If you set a key with wrong format, the Azure portal doesn't show it.

Change the default storage

You can use a secondary storage for the secrets.
To do that, you can set the setting called AzureWebJobsSecretStorageSas with the Shared Access Signature of the secondary storage.

So, imagine you create the storage account called myfunckeysstorage.
You need a container inside it (to store the JSON blobs) as shown in the following picture:

Of course, you can call it as you want.
Now you need to generate the Shared Access Token to use in the Function App setting. To generate the SAS, you can open the container and use the option "Shared access tokens" as shown in the following picture:

To set the new storage as secret store for the Function App, you need to add the setting AzureWebJobsSecretStorageSas with the value of the URL with SAS you generate in the previous step.