Public snapshots are the ones created by your permission. User can do it by modifying the permissions of a snapshot, if you can share it with the AWS accounts that you specify. Users that you will authorize can then use the snapshots you share by creating their own EBS volumes, while your original snapshot created by you will remains unaffected. Now,If you choose, you can also make your unencrypted snapshots available publicly to all the AWS users. But you can't make your encrypted snapshots available publicly, this is not allowed. This is the concept of Public Snapshot. Hope it helps.