As far as I understand the S3 API of Cloud Storage is meant as a temporary solution until a proper migration to Google's APIs.
The S3 keys it produces are tied to your developer account. This means that if someone gets the keys from your NAS, he will have access to all the Cloud Storage buckets you have access to (e.g your employer's).
I use Google Cloud but not Amazon. Once I wanted a S3 bucket to try with NextCloud (then OwnCloud). I was really frightened to produce a S3 key with my google developer account.
The HMAC credential that you'd use with the S3-compatible GCS API, also called the "XML API", does need to be associated with a Google account, but it doesn't need to be the main account of the developer. It can be any Google user account. I suggest creating a separate account and granting it only the permissions it needs. It'd be nice if service accounts (aka robot accounts) could be given HMAC credentials, that's not supported. Service accounts can, however, sign URLs with RSA keys.
As another option, you can continue using the XML API and switch out only the auth piece to Google's OAuth system while changing nothing else.
Thanks for the advice. I think it would be even nicer if the HMAC credentials could be assigned to a specific bucket via an ACL.
I like GCS (and the gsutil tool) but occasionally a S3 style bucket is needed. For example you need a S3 bucket or a webdav server in order to send alerts with images from Grafana to Slack. A minor issue but nice to have if possible without having to deal with Amazon's control panel.
Is there any equivalent to the Bucket Policies that AWS provides (http://docs.aws.amazon.com/AmazonS3/latest/dev/example-bucke...). Cloud Storage seems to be limited to relatively simple policies without conditionals. For a few AWS IAM keys I set up a policy that limits write/delete access to a range of IPs (among other things). Something like that doesn't seem possible with what Google offers. Or do I miss something?
I am not familiar with AWS bucket policies, but AFAIK there isn't a way to set IP based access to GCS buckets.
To be honest, I do find the GCS permissions a bit complex. You have IAM, you have ACLs and you have S3 keys. Everything is set in a different place and ACLs aren't fully represented on the developers console. S3 keys give full access to everything, IAM service accounts give access per project and ACLs are fine grained (per bucket/object). On the other hand, IIRC, IAM has a write only setting, while ACLs do not. So I can have an account that can write only to all the buckets of my project but not an ACL (not that useful).
The S3 keys it produces are tied to your developer account. This means that if someone gets the keys from your NAS, he will have access to all the Cloud Storage buckets you have access to (e.g your employer's).
I use Google Cloud but not Amazon. Once I wanted a S3 bucket to try with NextCloud (then OwnCloud). I was really frightened to produce a S3 key with my google developer account.