* Upgrade to new Cloud KMS client libraries
We recently released the new Cloud KMS client libraries which use GRPC
instead of HTTP. They are faster and look nicer (</opinion>), but more
importantly they drastically simplify a lot of the logic around client
creation, encryption, and decryption. In particular, we can drop all the
logic around looking up credentials and base64-encoding/decoding.
Tested on a brand new cluster (no pre-existing unseal keys) and against
a cluster with stored keys from a previous version of Vault to ensure no
regressions.
* Use the default scopes the client requests
The client already does the right thing here, so we don't need to
surface it, especially since we aren't allowing users to configure it.
This changes the behavior of the GCPCKMS auto-unsealer setup to attempt
encryption instead of a key lookup. Key lookups are a different API
method not covered by roles/cloudkms.cryptoKeyEncrypterDecrypter. This
means users must grant an extended scope to their service account
(granting the ability to read key data) which only seems to be used to
validate the existence of the key.
Worse, the only roles that include this permission are overly verbose
(e.g. roles/viewer which gives readonly access to everything in the
project and roles/cloudkms.admin which gives full control over all key
operations). This leaves the user stuck between choosing to create a
custom IAM role (which isn't fun) or grant overly broad permissions.
By changing to an encrypt call, we get better verification of the unseal
permissions and users can reduce scope to a single role.
