Impersonated credentials should implement `IdTokenProvider` interface

[salrashid123]

Currently there is no easy way to acquire an id_token for a service account that was impersonated.

For example, if you run an application as SA1 but you would like to get an id_token for SA2, you would have to first use Impersonated module to get a raw access_token for SA2 and then use that in iamcredentials.generateIdToken() api call manually.

in another example, if you run running workload identity federation (WIF) in AWS and need an id_token to access Cloud Run, you would need to acquire the access_token for the service account for WIF on aws and then use that same generateIDToken() api call.

This FR is to allow the Impersonated module to acquire its own id_token

---

the workaround i tried was to edit the following

• node_modules/google-auth-library/build/src/auth/impersonated.d.ts

export declare class Impersonated extends OAuth2Client implements IdTokenProvider {}

    /**
     * Fetches an ID token.
     * @param targetAudience the audience for the fetched ID token.
     */
    fetchIdToken(targetAudience: string): Promise<string>;    

• node_modules/google-auth-library/build/src/auth/impersonated.js

     async fetchIdToken(targetAudience) {
        try {
            await this.sourceClient.getAccessToken();
            const name = 'projects/-/serviceAccounts/' + this.targetPrincipal;
            const u = `${this.endpoint}/v1/${name}:generateIdToken`;
            const body = {
                delegates: this.delegates,
                audience: targetAudience,
                includeEmail: true,
            };
            const res = await this.sourceClient.request({
                url: u,
                data: body,
                method: 'POST',
            });
            const tokenResponse = res.data;

            return tokenResponse.token
        }
        catch (error) {
     ...
        }
    } 

then the usage would be

    const scopes = 'https://www.googleapis.com/auth/cloud-platform'
    const auth =  new GoogleAuth({
        scopes: scopes
    });
    const client = await auth.getClient();

    // First impersonate
    let targetCredentials = '[email protected]'
    let targetClient = new Impersonated({
        sourceClient: client,
        targetPrincipal: targetCredentials,
        lifetime: 30,
        delegates: [],
        targetScopes: [scopes]
    });

    // then get an ID Token
    let idClient = new IdTokenClient({
        targetAudience: 'https://foo.bar',
        idTokenProvider: targetClient
    })

    const res = await idClient.request({
        method: 'GET',
        url: 'https://httpbin.org/get',
      });
    console.log(res.data);

---

Finally, please note the following:

ref:

• https://github.com/googleapis/google-auth-library-nodejs/issues/1305
• https://github.com/googleapis/google-auth-library-nodejs/issues/1210 (The last issue describes the limits of using the impersonated module with google cloud client libraries)

[bcoe]

@salrashid123 do you have a testing project where you're experimenting with this functionality, I'd be happy to help add this functionality, but it would be nice to have a real world environment to test against.

[salrashid123]

certaiinly. i've added your corp user cred access to impersonate the service account below so you could directly impersonate an SA and get its id_token like the gcloud command below

export ID_TOKEN=`gcloud auth print-identity-token --audiences="https://myapp-jyosxg6puq-uc.a.run.app" --impersonate-service-account=target-serviceaccount@fabled-ray-104117.iam.gserviceaccount.com  --format="value(id_token)"`

curl -v -H "Authorization: Bearer $ID_TOKEN" https://myapp-jyosxg6puq-uc.a.run.app/

the curl command uses that id token to access a Cloud Run instance which will only allow that svc account's idtoken through.

If you would rather,i can grant project access or create a test user within the cloud org i own. LMK

[bcoe]

@bcoe I'll make an effort to prod at this next week during the slow week, feel free to message me if you feel like wokring on this patch together.

[m0ar]

@bcoe This is a blocker for us, is there an update?

[danielbankhead]

Hey @m0ar, @salrashid123, & @FrodoTheTrue: found some time to work on this today. Here's the PR:

• https://github.com/googleapis/google-auth-library-nodejs/pull/1439

It should be merged shortly after review 👌🏽