How does one distribute Data Protection keys with a .NET Core web app?

Question

I am a bit unclear on how Data Protection keys work in a web farm environment. I don't have a common location that all servers can use (and don't want to deal with permissions). Therefore I'd like to generate a key and distribute it with the web application.

I am doing the following, but not sure if that is right. So I generate a key file locally on my dev pc with:

var specialFolder = Environment.SpecialFolder.CommonApplicationData;
var appDataPath = Path.Combine(
     Environment.GetFolderPath(specialFolder),
    "company", 
    "product"
);
services.AddDataProtection().PersistKeysToFileSystem(new DirectoryInfo(appDataPath));

This creates a key-some-guid.xml file. I then distribute this file with my web application.

Now, when I run my web app, in the Startup.Configure services, copy this file to the appDataPath location (defined above) and call services.AddDataProtection().PersistKeysToFileSystem(new DirectoryInfo(appDataPath));.

Will that work? Or am I fundamentally missing something?

Answer 1

Answering my own question. The following seems to work across a web farm. Call the method below from Startup.ConfigureServices. It assumes that the key (that was generated on a dev machine) resides in the Keys folder off the root of the project.

public Startup(IHostingEnvironment env)
{
    /* skipping boilerplate setup code */

    Environment = env;
}

private IHostingEnvironment Environment { get; set; }

private void ConfigureDataProtection(IServiceCollection services) {
    // get the file from the Keys directory
    string keysFile = string.Empty;
    string keysPath = Path.Combine(Environment.ContentRootPath, "Keys");
    if (!Directory.Exists(keysPath)) {
        Log.Add($"Keys directory {keysPath} doesn't exist");
        return;
    }

    string[] files = Directory.GetFiles(keysPath);
    if (files.Length == 0) {
        LLog.Add($"No keys found in directory {keysPath}");
        return;
    } else {
        keysFile = files[0];

        if (files.Length >= 2) {
            LLog.Add($"Warning: More than 1 key found in directory {keysPath}.  Using first one.");
        }
    }

    // find and optionally create the path for the key storage
    var appDataPath = Path.Combine(
        System.Environment.GetFolderPath(System.Environment.SpecialFolder.CommonApplicationData),
        "companyName",
        "productName"
    );

    if (!Directory.Exists(appDataPath)) {
        try {
            Directory.CreateDirectory(appDataPath);
        } catch (Exception ex) {
            Log.Add($"Error creating key storage folder at {appDataPath}.  Error: {ex.Message}");
            return;
        }
    }

    // delete any keys from the storage directory
    try {
        foreach (var file in new DirectoryInfo(appDataPath).GetFiles()) file.Delete();
    } catch (Exception ex) {
        Log.Add($"Error deleting keys from {appDataPath}.  Error: {ex.Message}");
        return;
    }

    try {
        string targetPath = Path.Combine(appDataPath, new FileInfo(keysFile).Name);
        File.Copy(keysFile, targetPath, true);
    } catch (Exception ex) {
        Log.Add($"Error copying key file {keysFile} to {appDataPath}.  Error: {ex.Message}");
        return;
    }

    // everything is in place
    services.AddDataProtection()
        .PersistKeysToFileSystem(new DirectoryInfo(appDataPath))
        .DisableAutomaticKeyGeneration();
}