Google Trusted Publishing docs (#15195)

* Google Trusted Publishing docs

* Update docs/user/trusted-publishers/security-model.md

Co-authored-by: Facundo Tuesca <facundo.tuesca@trailofbits.com>

* Update docs/user/trusted-publishers/security-model.md

Co-authored-by: Facundo Tuesca <facundo.tuesca@trailofbits.com>

* Apply suggestions from code review

Co-authored-by: William Woodruff <william@yossarian.net>

* Apply suggestions from code review

---------

Co-authored-by: Facundo Tuesca <facundo.tuesca@trailofbits.com>
Co-authored-by: William Woodruff <william@yossarian.net>

docs/user/trusted-publishers/adding-a-publisher.md

@@ -45,7 +45,41 @@ each.
 
 === "Google Cloud"
 
-    TODO
+    For Google Cloud, you **must** provide the email address of the account or
+    service account used to publish. [You can learn more about Google Cloud
+    service accounts
+    here](https://cloud.google.com/iam/docs/service-account-overview).
+
+    For example, if you have created a service account named
+    "SERVICE_ACCOUNT_NAME" in the project "PROJECT_NAME" which is in use by
+    the environment where you would like to publish to PyPI from, your service
+    account email would take the form
+    `SERVICE_ACCOUNT_NAME@PROJECT_NAME.iam.gserviceaccount.com`, and you would do
+    the following:
+
+    ![](/assets/trusted-publishing/google/project-publishing-form.png)
+
+    !!! warning
+
+        Google Cloud also provides [default service
+        accounts](https://cloud.google.com/iam/docs/service-account-types#default)
+        for various products:
+
+          *  Compute Engine: `PROJECT_ID-compute@developer.gserviceaccount.com`
+          *  App Engine: `PROJECT_NAME@appspot.gserviceaccount.com`
+
+        However it is **not** recommended that these be used for publishing, as
+        they are provided by default to every service when they are created.
+
+    !!! note
+
+        Configuring the subject is optional. The subject is the numeric ID that
+        represents the principal making the request. While not required, providing the
+        subject further restricts the identity which is used for publishing, ensuring
+        that only a specific instance of a service account can publish, not any service
+        account with the configured email. See
+        <https://cloud.google.com/docs/authentication/token-types#id-contents>
+        for more details
 
 === "ActiveState"
 

docs/user/trusted-publishers/creating-a-project-through-oidc.md

@@ -41,8 +41,16 @@ provide the name of the PyPI project that will be created.
 
 === "Google Cloud"
 
-    TODO
+    If you have a service account named
+    `SERVICE_ACCOUNT_NAME@PROJECT_NAME.iam.gserviceaccount.com`, which is in use by
+    the environment where you would like to publish to PyPI from, then you would do
+    the following:
+
+    ![](/assets/trusted-publishing/google/pending-publisher-form-filled.png)
+
+    !!! note
 
+        Like with "normal" trusted publishers, configuring the subject is optional.
 
 === "ActiveState"
 

docs/user/trusted-publishers/security-model.md

@@ -133,7 +133,23 @@ own security model and considerations.
 
 === "Google Cloud"
 
-    TODO
+    ### Security Model
+
+    If a trusted publisher is configured for a given PyPI project, any service
+    that uses the configured service account can request an OpenID Connect token
+    from Google's identity provider on behalf of that identity. That token can be
+    exchanged for a PyPI API token with the ability to publish to the PyPI project.
+    The identity used for publishing can be optionally constrained further by
+    specifying the subject, an ID that represents the principal making the request.
+
+    ### Considerations
+
+    When using trusted publishing with Google Cloud, you must trust the service account
+    and _any service which uses it as the default ephemeral identity_.
+
+    Specifically, it is not recommened to configure the [default service
+    accounts](https://cloud.google.com/iam/docs/service-account-types#default), as
+    they are provided by default to every service when they are created.
 
 === "ActiveState"
 

docs/user/trusted-publishers/using-a-publisher.md

@@ -214,7 +214,59 @@ below describe the setup process for each supported trusted publisher.
 
 === "Google Cloud"
 
-    TODO
+    You can use the <https://pypi.org/project/id/> tool to automatically detect
+    and produce OIDC credentials on Google Cloud services.
+
+    First, ensure that `id` and `twine` are installed in the environment you
+    plan to publish from:
+
+    ```
+    $ python -m pip install -U id twine
+    ```
+
+    If you're unsure what the email address is for the service account your
+    service is using, you can verify it with:
+
+    ```
+    $ python -m id pypi -d | jq 'select(.email) | .email'
+    ```
+
+    Generate an OIDC token from within the environment and store it. The
+    audience should be either `pypi` or `testpypi` depending on which index you are
+    publishing to:
+
+    ```
+    $ oidc_token=$(python -m id pypi)
+    ```
+
+    !!! note
+        `pypi` is only correct for PyPI. For TestPyPI, the correct
+        audience is `testpypi`. More generally, you can access any instance's expected
+        OIDC audience via the `{index}/_/oidc/audience` endpoint:
+
+        ```console
+        $ curl https://pypi.org/_/oidc/audience
+        {"audience":"pypi"}
+        ```
+
+    Finally, we can submit that token to PyPI and get a short-lived API token
+    back:
+
+    ```bash
+    resp=$(curl -X POST https://pypi.org/_/oidc/mint-token -d "{\"token\": \"${oidc_token}\"}")
+    api_token=$(jq -r '.token' <<< "${resp}")
+    ```
+
+    !!! note
+
+        This is the URL for PyPI. For TestPyPI, the correct
+        domain should be is `test.pypi.org`.
+
+    This API token can be fed into `twine` or any other uploading client:
+
+    ```bash
+    TWINE_USERNAME=__token__ TWINE_PASSWORD=${api_token} twine upload dist/*
+    ```
 
 === "ActiveState"