Client-Side Encryption (CSE)
Client-side encryption (CSE) is the cryptographic technique of encrypting data on the sender’s side before it is transmitted to a server such as a cloud storage service. The data owner retains the encryption key and does not share it with the cloud-based service. Data stored in the cloud is therefore more secure. Client-side encryption is one potential technique to offset the rapidly escalating risk of attack against cloud-based technologies. CSE is a type of zero-knowledge service.
- Encryption keys are not available to the cloud storage service, making it very difficult / impossible to decrypt hosted data.
- Prevents man-in-the-middle (MITM) attacks because only encrypted data is transmitted, thereby ensuring data privacy and integrity.
- Provides an extra layer of security if a user’s device is lost or stolen, as the data is stored in encrypted form.
- Prevents app creators and publishers from accessing stored user data.
- Prevents third party access if a cloud storage provider is compelled to provide data access under legal or regulatory requirements.
- Ensures data security if data is leaked or stolen from cloud storage.
- Provides privacy and security for data users as storage providers cannot access stored files.
- Security relies on the data owner keeping their encryption key secret.
- If the data owner loses or forgets their key there is no easy way for it, or the data, to be recovered.
- Prevents online collaboration as team members cannot work with encrypted files.
- Prevents file sharing, as the recipient would need to be told the secret key, thereby rendering CSE potentially insecure.
- Impairs server monitoring for malware and other suspicious files.
- Hinders forensic access if there is a crucial issue or hacking.
Client-side encryption provides an extra layer of protection for important or confidential personal, business and military data.
Some providers of CSE-based cloud storage (such as pCloud) provide a hybrid storage model whereby important data is secured via CSE and less-important data is left unencrypted, in an attempt to maximise the benefits described above while reducing (but not eliminating) the downsides.
End-to-End Encryption (E2EE) is a related but different concept. In E2EE, data is encrypted during transmission between users and cloud-based service providers, but remains unencrypted on both the user’s device and the cloud service. This protects data from MITM attacks and allows file sharing and document collaboration as files on the cloud storage server are unencrypted. Because data is unencrypted on both the user’s device and the cloud server, many of the privacy and security advantages of CSE (discussed above) are lost.