Support workload identity and default app credentials for GCS signed URLs and or provide example of how to sign the blob yourself

[red8888]

See SO post: https://stackoverflow.com/questions/76266019/how-do-i-generate-signed-urls-for-gcs-with-workload-identity-in-the-c-sharp-sdk/76266912#76266912

Currently workload id is not supported for generating signed URLs for GCS. This is a problem because it forces a bad practice (generating a static json key instead of using workload id which is much more secure).

At the very least can someone provide a full example of how to use the SignBlob method in conjunction with SignedURL to work around this problem?

Also, I think default app creds are also not supported right? This makes the code painful to run locally for testing purposes.

[jskeet]

~~If this feature request is specific to Google.Cloud.Storage.V1, please reopen it on https://github.com/googleapis/google-cloud-dotnet, which is the repo where that library lives.~~

Also, I think default app creds are also not supported right? This makes the code painful to run locally for testing purposes.

It's unclear what you mean by that. Supported in what respect? Please be more specific.

[jskeet]

Apologies, I hadn't noticed that @amanda-tarafa had specifically requested raising an issue here instead of in the other repo. (She's worked on the signing infrastructure more recently than I have, so I trust her judgement. The sample code change would be in google-cloud-dotnet, but we can handle that.)

[red8888]

Cool thanks! Yeah I'm just looking for some kind of official-ish example snippet so I know I'm doing it right as it seems non-standard/undocumented.

[jskeet]

We can work on creating a sample - we had a sample until recently, which was removed in this commit - you may be able to use that as a starting point until we can potentially restore it in the docs.

@amanda-tarafa Can you remember why we removed the IAM service signer sample? Was it that we didn't expect it to be useful with the new support?

@red8888: If you could clarify what you would expect to be able to do with default application credentials that doesn't currently work, that would help us to help you.

[red8888]

@jskeet Sorry default app creds may work as expected so I guess I shouldn't have assumed. I just saw other somewhat related github issues saying it didn't or it behaved differently: https://github.com/octue/django-gcp/issues/10

I really just wanted to confirm that if I use sigblob the same code will run correctly on gke with workload id and locally with default app creds without needing to introduce logic in the code to support both scenarios.

[jskeet]

@jskeet Sorry default app creds may work as expected so I guess I shouldn't have assumed. I just saw other somewhat related github issues saying it didn't or it behaved differently: https://github.com/octue/django-gcp/issues/10

Right, I believe ADC credentials will work if the underlying credential supports signing (which I believe is still not the case for WIF, as Amanda indicated in the Stack Overflow answer).

I really just wanted to confirm that if I use sigblob the same code will run correctly on gke with workload id and locally with default app creds without needing to introduce logic in the code to support both scenarios.

I'd expect that to be the case if both kinds of credential have access to IAM to perform the signing remotely. But it'll be significantly slower than any local signing.

At this point we're really at the edge of my knowledge though, so I'll assign to @amanda-tarafa for further comment.

[amanda-tarafa]

We might be able to offer some out of the box sopport for URL signing with external account credentials, but we need to confirm a few things first, so please bare with us.

If we cannot offer out of the box support we'll publish a sample.

I'll update here as we know more.

[amanda-tarafa]

As described in the documentation, GoogleCredential (The type returnes by ADC) is only supported if the underlying credential type is itself supported, and user crendetial is not currently supported out of the box. For all currently unsuported credentials you can call the IAM API SignBlob operation and it should work. But we are looking into supporting this ourselves. We still need to confirm a few things from the Auth side of things, but we'll get there I believe.

I'll update when I know more.

[OneEcho]

Here's a class that I was able to use to generate signed URLs. It uses the IAM API SignBlob operation since we wanted to use default application credentials instead of a service account key.

public class IamServiceBlobSigner : UrlSigner.IBlobSigner
{
    private readonly IAMCredentialsClient _iamService;

    public string Id { get; }

    public string Algorithm => "GOOG4-RSA-SHA256";

    internal IamServiceBlobSigner(IAMCredentialsClient service, string serviceAccountId)
    {
        _iamService = service;
        Id = serviceAccountId;
    }

    public string CreateSignature(byte[] data, UrlSigner.BlobSignerParameters _)
    {
        var request = CreateRequest(data);
        var response = _iamService.SignBlob(request);
        return response.SignedBlob.ToBase64();
    }

    public async Task<string> CreateSignatureAsync(byte[] data, UrlSigner.BlobSignerParameters _, CancellationToken cancellationToken)
    {
        var request = CreateRequest(data);
        var response = await _iamService.SignBlobAsync(request);
        return response.SignedBlob.ToBase64();
    }

    private SignBlobRequest CreateRequest(byte[] data)
    {
        var account = $"projects/-/serviceAccounts/{Id}";
        var base64Data = Convert.ToBase64String(data);
        var request = new SignBlobRequest
        {
            Name = account,
            Payload = ByteString.FromBase64(base64Data)
        };
        return request;
    }
}

[mbamm]

@amanda-tarafa any updates on this feature being part of the library? Or do you have an official example? thanks

[amanda-tarafa]

@mbamm We'll work on this next year but we don't have an ETA yet. I'd expect this to happen in the first half of the year though.

As a sample starting point, you can use the sample that we originally had and was removed in this commit.

For context, we removed the sample because it was meant to demonstrate URL signing using a Compute credential, and that is currently supported by the library out of the box. It's just Workload Indentity Federation credentials that are not supported out of the box at the moment.