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To access your VMs in public cloud providers, you must have an RSA key pair.
See your cloud provider documentation for more details. For example, for AWS, see http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/AccessingInstances.html.
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When you generate a key pair for public cloud, you should use a pass phrase. In some cloud providers you can use a key pair that the cloud provider generates for you.
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For GCP and OCI, you can use a 2048 bit RSA key, which you can could generate with a command like this one.
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