In the pre-cloud era, to permanently delete data, the sectors on the physical disk must be overwritten multiple times with zeros and ones to make sure the data is unrecoverable. if the device will not be re-used, it must be degaussed. The Department of Defense standard, DoD 5220.22-M, goes so far as destroying the physical disk through melting, crushing, incineration or shredding to completely get rid of the data.

But these techniques do not work for data in the cloud. First, cloud customers probably will not have access to the provider’s data centers to access the physical disks. Second, cloud customers do not know where they are written, i.e. which specific sectors of the disk, or which physical disks for that matter. In addition, drives may reside on different arrays, located in multiple availability zones, or data might even be replicated in different regions.

The only way to permanently erase data in the cloud is via crypto-shredding. It works by deleting the encryption keys used to encrypt the data. Once the encryption keys are gone, the data cannot be recovered. So it is imperative that even before putting data in the cloud, they should be encrypted. Unencrypted data in the cloud will be impossible to permanently delete. As a cloud customer, it is also important that you own and manage the encryption keys and not the cloud provider.