gcs_signed_url - Create Signed URLs for Google Cloud Storage

Important

This package works with Elixir >= 1.8 and Erlang/OTP >= 22.3

Hex Installation

Add gcs_signed_url to your list of dependencies in mix.exs:

def deps do
  [{:gcs_signed_url, "~> 0.4"}]
end

Usage

This library creates signed URLs for the Google Cloud Storage in three steps:

1. Create a string to sign (use V4 signing process - V2 signing process is stil supported but deprecated)
2. Sign the string to sign with the private key of a Google service account (GSA)
3. Form the URL including the signature

The actual signing can be done on-premise on the machine your application is executed or it can be delegated to the
Google IAM SignBlob API. The method of signing depends on your setup. Both methods work for creating V4 or V2 (deprecated) signatures.

Google IAM SingBlob API - Preferred on the GKE

If your application runs on the Google Kubernetes Engine, the preferred way of accessing Google Cloud services is
through a Workload Identity. In this
scenario, you run your application under a Kubernetes service account (KSA) which is related to a GSA. Using
Goth, you would get an OAuth2 access token and use it to create a signed URL
through the SignBlob API.

In this scenario, the GSA you get the access token for (GSA_AUTH) acts as a Google Service Account GSA_SIGNER and
signs the URL on his behalf. This requires the GSA_AUTH go have the Google IAM permission
iam.serviceAccounts.signBlob on the GSA_SIGNER, e.g. by giving it the built in
role roles/iam.serviceAccountTokenCreator on GSA_SIGNER.

GSA_AUTH and GSA_SIGNER can also be the same service account in which case he needs to have the permission
iam.serviceAccounts.signBlob on itself.

Example

iex> {:ok, %{token: access_token}} = Goth.Token.for_scope("https://www.googleapis.com/auth/devstorage.read_write")
iex> oauth_config = %GcsSignedUrl.SignBlob.OAuthConfig{service_account: "project@gcs_signed_url.iam.gserviceaccount.com", access_token: access_token}
iex> GcsSignedUrl.generate_v4(oauth_config, "my-bucket", "my-object.jpg", verb: "PUT", expires: 1800, headers: ["Content-Type": "application/jpeg"])
{:ok, "https://storage.googleapis.com/my-bucket/my-object.jpg?X-Goog-Expires=1800..."}

On-Premise Signing

In this scenario you have a service account key in form of a JSON file on your machine. The library will use the
private key to create the signature, no network calls are needed.

1. Load the client

   iex> GcsSignedUrl.Client.load_from_file("/home/alexandrubagu/config/google.json")

   or

   iex> service_account = service_account_json_string |> Jason.decode!
   iex> GcsSignedUrl.Client.load(service_account)

2. Generate signed url

   GcsSignedUrl.generate_v4(client, "my-bucket", "my-object.mp4")
   GcsSignedUrl.generate_v4(client, "my-bucket", "my-object.mp4", expires: 3 * 3600) # 3 hours