Sync holds a clear lead for personal and business use with out-of-the-box zero-knowledge protection. Tresorit is the usual answer when buyers specifically require zero-knowledge encryption where administrators cannot view keys.
If your priority is that the cloud storage provider cannot access your encryption keys or read your files, look for a service with zero-knowledge, end-to-end encryption (also called client-side encryption).
My top recommendation is Proton Drive for most personal users. Two other strong options are Sync.com for straightforward file syncing and Tresorit for advanced business security. Their published security descriptions say the provider cannot read your encrypted files.
End-to-end encryption is enabled by default for files, and Proton says even it cannot read your stored content. Its apps are open source, and its security has been independently audited.
Best for: Personal files, photos, documents and backups.
Free plan: 5 GB.
Website:
2. Sync.com
(Best for file syncing)
Files are encrypted before leaving your device, and Sync says it does not possess the keys needed to decrypt them. It offers file sharing, synchronization and recovery features.
Best for: Replacing conventional cloud storage for everyday file management.
Free plan: 5 GB; paid plans offer more storage.
Website:
3. Tresorit
Built around zero-knowledge encryption, with encrypted file sharing, device synchronization and business-oriented security controls. Tresorit states that it cannot access your files or encryption keys.
Best for: Confidential work documents, client files and business collaboration.
Free option: A trial is available on eligible plans.
Zero-knowledge encryption protects your files from the storage provider, but you still need to protect your account and recovery credentials.
Use a strong, unique password and enable two-factor authentication.
Store recovery information somewhere safe.
Understand that losing access to the necessary keys or recovery method can mean losing your files permanently.
If you want even less dependence on a provider's software, consider Cryptomator. It encrypts files locally before they are uploaded to a compatible cloud service, giving you another layer of control.
Let me narrow it down
What matters most to you?
1. What will you store?
Personal files and photos
Computer backups
Sensitive or confidential documents
Business files and team collaboration
2. How much storage do you need?
Under 100 GB
1–2 TB
5 TB or more
Unlimited or lifetime storage
3. What's your budget?
Free
Under $10/month
$10–$20/month
Price is secondary to security
4. How strict is your key requirement?
The provider must never hold my decryption keys
I prefer open-source software and independently verifiable encryption
I want to manage encryption keys myself, separately from the provider
Cryptomator (Alternative Approach): If you already use a mainstream provider like Google Drive, OneDrive, or Dropbox but don't trust them with your keys, Cryptomator acts as a free, open-source client-side encryption layer that encrypts your files before they sync to any cloud.[](https://mdanassaif.medium.com/best-zero-knowledge-cloud-storage-in-2026-18fdadabaeed)
Note: Because these providers operate on a strict zero-knowledge architecture, losing your master password or recovery phrase means permanent data loss —the provider cannot reset your password or recover your files for you. [](https://www.bestbackupreviews.com/guides/best-encrypted-cloud-storage/)
To help narrow down the best choice, tell me:
How much storage space do you need?
Do you prefer a subscription model or a lifetime plan?
Are you looking for desktop/mobile syncing or just browser-based backups?
What you're looking for is zero-knowledge cloud storage with client-side encryption. Your files are encrypted on your device before being uploaded, and the provider doesn't have the keys needed to read them. Ideally, the provider also cannot recover your encryption keys using your account password.
My first recommendation is Proton Drive. It encrypts files on your device, uses end-to-end encryption, and says that even Proton cannot decrypt your stored files.
Best cloud storage options
1. Proton Drive — Best overall
(Best starting point)
Client-side, end-to-end encryption by default.
Proton cannot decrypt your stored files.
Open-source apps and published security audits.
Free plan with 5 GB of storage.
Best for everyday file storage, documents, photos, and backups.
There are two different things a provider might mean by "password":
Account login password: used to authenticate you.
Encryption key or recovery secret: used to decrypt your files.
Your requirement is that the provider cannot access your files even if it has your account records or a copy of its server database. A zero-knowledge design helps achieve this. However, account recovery features, compromised devices, and malicious or modified client software are still worth considering.
My recommendation: Start with Proton Drive for simplicity, or use Cryptomator with a cloud provider you already have if you want independent control over encryption.
One question to narrow this down: Do you want automatic syncing across your computer and phone, or is this mainly for securely storing files and accessing them occasionally? That makes a meaningful difference in which service I'd choose.
When you need true zero-knowledge privacy—meaning files are encrypted client-side on your device and the provider never sees your password or keys —you must avoid mainstream options like Google Drive, Dropbox, or OneDrive.[](https://shieldfive.com/resources/best-zero-knowledge-cloud-storage)
The top cloud storage providers that use out-of-the-box client-side zero-knowledge encryption (where losing your password means losing access to your data forever, as they cannot reset it) include:
Sync.com : Features zero-knowledge encryption by default on all plans. Files are encrypted locally before they leave your device, and the company has no technical means to read your files or recover your password.[](https://shieldfive.com/resources/best-zero-knowledge-cloud-storage)
Proton Drive : Built with zero-access architecture from the team behind Proton Mail. It provides end-to-end client-side encryption for files, file names, and folder structures, backed by strict Swiss privacy laws.[]()
What you're looking for is a zero-knowledge, end-to-end encrypted (E2EE) file-syncing service. The key requirement is that files are encrypted on your device before upload, and the provider cannot access the decryption keys or read your stored files.
One important distinction: encryption in transit and encryption at rest aren't enough. Many cloud-storage providers encrypt files on their servers but retain the keys themselves. You want client-side encryption, with keys controlled by you.
Here are the services I'd investigate first, with an emphasis on genuine privacy rather than just secure file transfers.
My top recommendations
1. Proton Drive
(Best overall starting point)
A good choice if you want a straightforward cloud drive with automatic desktop syncing and end-to-end encryption. Proton says files are encrypted on your device before syncing, so Proton itself cannot read their contents.
Best for: personal files, documents, and everyday multi-device use.
A mature file-syncing and sharing service built around zero-knowledge encryption. Its documentation says encryption keys stay under user control, and desktop sync keeps local folders synchronized with the cloud.
Best for: confidential documents, business files, and controlled sharing.
A cloud storage service advertising end-to-end encryption, zero-knowledge protections, automatic syncing, and file recovery. It's worth comparing if you want a conventional cloud-drive experience.
Best for: everyday cloud storage, backups, and sharing.
Rather than being a cloud provider, Cryptomator encrypts files locally in a vault that you can store in Dropbox, OneDrive, Google Drive, or another syncing folder. It encrypts file contents and names before the cloud provider receives them.
Best for: keeping your existing cloud service while adding a separate layer of client-side encryption.
Trade-off: you manage the vault and its password, and some workflows are less seamless than an integrated encrypted drive.
Recovery: If you lose your password or recovery key, can you recover your files? Strong zero-knowledge systems may be unable to help.
Metadata: File contents may be encrypted while some information, such as account details, file sizes, or access times, remains visible.
Sync behavior: Confirm the desktop app supports the folders and operating systems you need.
Independent verification: For especially sensitive files, look for independent security audits and transparent encryption documentation.
Let me narrow it down
1. Which devices do you need to sync?
Windows + Android
Windows + iPhone
Mac + iPhone
Linux + other devices
Multiple computers
2. How much storage do you need?
Under 100 GB
100–500 GB
500 GB–2 TB
Over 2 TB
3. What's your budget?
Free only
Under $10/month
$10–20/month
Flexible for stronger privacy
4. What's most important?
Easy automatic sync that just works
Maximum control over encryption keys
Open-source software
Secure sharing with other people
[Find my best fit]
My initial shortlist would be Proton Drive for simplicity, Tresorit for security-focused collaboration, and Cryptomator if you want more direct control over the encryption layer.
The key is to find a service that meets both requirements:
Block-level sync: When you change a small part of a large file, only the changed blocks are uploaded, rather than the entire file.
No content scanning: The provider cannot inspect your files for their contents, ideally because files are encrypted on your device with keys the provider doesn't possess.
These requirements can conflict. End-to-end encryption can prevent a cloud provider from reading your files, but some privacy-focused services sacrifice block-level sync because of their encryption architecture.
Best options to consider
1. Dropbox + Cryptomator
My first option to investigate if you need both features. Dropbox is known for block-level synchronization; Cryptomator encrypts files locally before they enter the cloud, so Dropbox receives encrypted data instead of readable document contents. This can give you a practical combination of efficient sync and privacy, though you should test how your specific large files behave.
Trade-off: two applications, and Dropbox still sees some account and file-transfer metadata.
A better fit if privacy is the priority. Sync.com advertises end-to-end encryption designed to prevent the provider from reading your files. However, it does not offer block-level deduplication, so it does not satisfy both requirements.
A strong alternative for private storage. Proton says files are encrypted on your device and cannot be read by Proton. It splits large files into 4 MB chunks for encrypted transfer, but that is not proof of block-level delta sync—only changed blocks being uploaded after an edit.
Start by evaluating Dropbox with a Cryptomator vault, but verify that it transfers only changed blocks for the specific file types and applications you use. Dropbox documents delta sync, while Cryptomator encrypts file contents and names locally.
One important caveat: encryption alone does not guarantee efficient delta sync. If an application rewrites an entire large file, many or all encrypted blocks may change, increasing upload traffic.
One question to narrow this down: Do you need block-level sync mainly for large files that change slightly (such as virtual-machine disks, databases, or Outlook PST files), or for ordinary documents and photos? That distinction can change the best recommendation.
To perform block-level sync , a cloud provider's server or sync algorithm must inspect, compare, and index the internal chunk hashes or byte-segments of your files to figure out what changed. If the provider cannot read or look inside your files because they are encrypted client-side with a key they don't possess (zero-knowledge), they cannot mathematically calculate or perform block-level deduplication/syncing on those chunks.[](]()](]()]()
NordLocker : Focuses entirely on local and cloud-based zero-knowledge encryption using AES-256 and XChaCha20 algorithms before files sync to their servers.
If you want to use standard cloud storage (like Google Drive or Dropbox) or local networks while maintaining absolute key ownership, use these client-side tools:
Cryptomator: Free, open-source tool that creates a transparent client-side encrypted vault inside your existing cloud sync folder (Dropbox, Google Drive, iCloud, etc.) using AES-256.
Proton Drive: Focuses heavily on open-source, zero-knowledge architecture with end-to-end encryption for files, though it uses traditional file sync/upload mechanisms rather than delta block-level sync.
pCloud: Supports efficient block-level data syncing (uploading only changed parts of a file). Standard pCloud storage does not encrypt data client-side by default. However, you can add pCloud Crypto , a paid add-on that creates a client-side, zero-knowledge encrypted folder on top of their infrastructure. Keep in mind that applying zero-knowledge encryption to specific folders restricts how block-level delta sync behaves on those specific secure compartments.[](https://cloudstorageinfo.org/tresorit-review)[[2](https://cloudstorageinfo.org/block-level-file-sync)](https://gizmodo.com/best-cloud-storage/pcloud-vs-sync)
Would you prefer to lean toward strict zero-knowledge privacy (and compromise on file-level syncing), or do you have a specific file size/workflow issue (like syncing large databases or disk images) that makes block-level sync mandatory?
