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.
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