File hosting service

Personal file storage services are aimed at private individuals, offering a sort of "network storage" for personal backup, file access, or file distribution. Users can upload their files and share them publicly or keep them password-protected.[1]

Document-sharing services allow users to share and collaborate on document files. These services originally targeted files such as PDFs, word processor documents, and spreadsheets.[2] However many remote file storage services are now aimed at allowing users to share and synchronize all types of files across all the devices they use.

File syncing and sharing services are file hosting services which allow users to create special folders on each of their computers or mobile devices, which the service then synchronizes so that it appears to be the same folder regardless of which computer is used to view it. Files placed in this folder also are typically accessible through a website and mobile apps, and can be easily shared with other users for viewing or collaboration.[3]

Such services have become popular via consumer products such as Dropbox and Google Drive.[4]

Content providers who potentially encounter bandwidth congestion issues may use services specialized in distributing cached or static content. It is the case for companies with a major Internet presence.[5]

Deals with the question of confidentiality and availability : Will the user be able to continue accessing their data? Who else can access it? Who can change it?

Whether the user is able to continue accessing their data depends on a large number of factors, ranging from the location and quality of their internet connection and the physical integrity of the provider's data center to the financial stability of the storage provider.

The question of who can access and, potentially, change their data ranges from what physical access controls are in place in the provider's data center to what technical steps have been taken, such as access control, encryption, etc.

Many cloud storage services state that they either encrypt data before it is uploaded or while it is stored. While encryption is generally regarded as best practice in cloud storage[18] how the encryption is implemented is very important.

Consumer-grade, public file hosting and synchronization services are popular, but for business use, they create the concern that corporate information is exported to devices and cloud services that are not controlled by the organization.

Some cloud storage providers offer granular ACLs for application keys. One important permission is append-only, which is distinct from simple "read", "write", and "read-write" permissions in that all existing data is immutable.[19][20] Append-only support is especially important to mitigate the risk of data loss for backup policies in the event that the computer being backed-up becomes infected with ransomware capable of deleting or encrypting the victim's backups.[21][22]

Secret key encryption is sometimes referred to as zero knowledge, meaning that only the user has the encryption key needed to decrypt the data. Since data is encrypted using the secret key, identical files encrypted with different keys will be different. To be truly zero knowledge, the file hosting service must not be able to store the user's passwords or see their data even with physical access to the servers. For this reason, secret key encryption is considered the highest level of access security in cloud storage.[23] This form of encryption is rapidly gaining popularity, with companies such as MEGA[24] (previously Megaupload) and SpiderOak being entirely zero knowledge file storage and sharing.[25]

Since secret key encryption results in unique files, it makes data deduplication impossible and therefore may use more storage space.[26]

Convergent encryption derives the key from the file content itself and means an identical file encrypted on different computers result in identical encrypted files.[26] This enables the cloud storage provider to de-duplicate data blocks, meaning only one instance of a unique file (such as a document, photo, music or movie file) is actually stored on the cloud servers but made accessible to all uploaders. A third party who gained access to the encrypted files could thus easily determine if a user has uploaded a particular file simply by encrypting it themselves and comparing the outputs.[26]

Some point out that there is a theoretical possibility that organizations such as the RIAA, MPAA, or a government could obtain a warrant for US law enforcement to access the cloud storage provider's servers and gain access to the encrypted files belonging to a user.[27] By demonstrating to a court how applying the convergent encryption methodology to an unencrypted copyrighted file produces the same encrypted file as that possessed by the user would appear to make a strong case that the user is guilty of possessing the file in question and thus providing evidence of copyright infringement by the user.

There is, however, no easily accessible public record of this having been tried in court as of May 2013 and an argument could be made that, similar to the opinion expressed by Attorney Rick G. Sanders of Aaron | Sanders PLLC in regards to the iTunes Match "Honeypot" discussion,[28] that a warrant to search the cloud storage provider's servers would be hard to obtain without other, independent, evidence establishing probable cause for copyright infringement. Such legal restraint would obviously not apply to the secret police of an oppressive government who could potentially gain access to the encrypted files through various forms of hacking or other cybercrime.