How to Choose Private Cloud Storage: 8 Questions to Ask

Almost every cloud storage service says your files are encrypted, and the smaller ones often add the word zero-knowledge. Those words cover very different designs. At one service the provider cannot open any file you upload; at another, only the files in a folder you pay extra for; at a third, the provider keeps a copy of your key so that it can reset your password by email.

Choosing private cloud storage comes down to questions that the label does not answer and the service’s own documentation does. The eight below are the ones our cloud storage reviews are built on, each with examples of what the answers look like at real services. If you are still deciding whether you need anything other than Google Drive, start with Why Not Just Use Google Drive?

1. Who holds the keys, and is that the default?

Encryption protects a file only from people who do not have the key. So the first question is key custody: does the key that opens your files exist only on your devices, or also on the provider’s servers?

There are three possible answers. At Google Drive, Dropbox and OneDrive, the provider holds the keys to personal accounts, and its systems can open your files whenever they need to. At services built the other way, the keys never reach the provider in readable form: Proton says that in Proton Drive “all keys and passphrases are generated on the client’s side and only transmitted to the server in encrypted form” [1].

The third answer is the one to watch for, because these services use the same words as the second: the keys are yours only after you change a setting, or only for part of your account.

• pCloud encrypts files on your device only in a Crypto folder, which comes with pCloud Encryption. pCloud describes the result as “both server-side encrypted, and client-side encrypted files” in one account [2], and sells the encryption at its own price, separately from storage [3].
• Icedrive offers encryption on your device on its premium plans [4], for files in a separate Encrypted section. To make previews of your other files, Icedrive sends them “to one of our conversion servers” [5].
• iCloud Drive keeps the keys with Apple unless you turn on Advanced Data Protection, which Apple calls “an optional setting” [6].
• Sync.com encrypts files on your device, but on paid plans email password reset is on by default. Sync says this gives “Sync’s automated systems temporary access to your account encryption key” [7], and its terms say that with the feature on, the account “is not configured on a strictly zero-knowledge basis” [8]. You can turn the feature off in the account settings.

What to look for: whether the encryption page says every file is encrypted on your device, or names a folder, a plan or a setting; and whether anything that is on by default keeps a copy of your key with the provider.

2. What is encrypted besides the contents?

A file’s name often says as much as the file. A scan called “passport-renewal” or a folder called “divorce” tells the provider what is inside without opening anything. Services differ widely in how much of this metadata they encrypt along with the contents.

Internxt encrypts file contents on your device, but says that file and folder names, the folder structure, file sizes and timestamps are only “server-side encrypted”, so Internxt can read them; for files whose names are sensitive, it suggests neutral names or an encrypted archive [9]. Proton Drive encrypts file names, folder names and thumbnail previews, and can see when each file was created and modified, who has which permissions and which username uploaded it [10]. Tresorit encrypts file names, but its privacy policy says some information about its top-level folders “is unencrypted, like its name, size and members” [11]. MEGA encrypts file names and thumbnails, and its security whitepaper says it chose not to protect the folder structure cryptographically [12]. Filen says it also encrypts “folder names, file metadata (such as mime type, size etc.)” [13]. Even with Advanced Data Protection on, Apple keeps the modification dates and checksums of iCloud files under keys it holds [14].

No design hides everything. Filen, which encrypts more file details than most services we review, still stores “the user’s email and IP address” [13]. Any provider knows the email address on the account, the IP address you connect from and how much space you use.

What to look for: a page that lists what is not encrypted. If a service only says “your files are encrypted”, assume that names and folders are readable until its documentation says otherwise.

3. Does the encryption survive sharing and editing?

Someone you share a file with does not have your key, so the service has to pass one to them. How it does that decides whether sharing stays end-to-end encrypted.

For links, a sound design puts the key in the part of the address after the # sign, which browsers do not send to the server. MEGA explains its links this way [15]. In 2024, researchers at ETH Zurich found that Sync.com links carried the link password in the address itself, so the server received it as soon as someone clicked [16]; Sync told TechTarget that the link issue had been fixed, the only fix it has publicly confirmed [17]. Sync’s links are end-to-end encrypted only with a setting called Enhanced privacy; without it they are “encrypted in transit with SSL”, and with it, previews, streaming and document editing are switched off [18]. Icedrive does not let you share files from its Encrypted section at all [19].

Sharing a folder with another account raises a second question: who confirms that the key you encrypt for really belongs to the recipient? At Tresorit, recipients’ keys are vouched for by certificates from Tresorit’s own certificate authority. The ETH Zurich study showed that a compromised server able to get such certificates could hand you a fake key and read the files you then share; the researchers note that they could not run the full attack without breaking into Tresorit’s systems [16].

Editing documents together is where encryption is often switched off. Sync says that editing in Microsoft’s online editor “is encrypted, but not end-to-end encrypted”, and that Microsoft’s editor “temporarily gains access to the document” [20]. Proton describes Proton Docs as “the end-to-end encrypted, real-time collaborative document editor” [21]. Our reviews say for each service whether collaborative editing runs on the provider’s server.

What to look for: which ways of sharing are end-to-end encrypted and which are not; for links, whether the key sits after the #; whether you can check a recipient’s key; and whether the document editor runs on the provider’s server.

4. What does the web version trust?

A desktop or mobile app is a program installed on your device, and its encryption code changes only when the app updates. A web app works differently: each time you open it, your browser downloads the encryption code from the provider’s server and runs it. MEGA describes this for its links, where the browser first “downloads a bunch of JavaScript code” that then fetches and decrypts the files [15]. A web app is therefore only as trustworthy as the code the server sends that day, and the server is exactly what encryption on your device is meant to protect you from.

Some providers offer a way around this. MEGA’s extension for Chrome and Edge contains the code needed to run MEGA locally, “eliminating the need to download and verify MEGA from the web each time” [22]. When you turn on Advanced Data Protection, Apple switches off access to your data on iCloud.com; if you switch it back on, your own trusted devices have to allow temporary access [23]. The web apps of Proton Drive, Filen, Tresorit, Sync.com, Internxt, pCloud and Icedrive all load their encryption code on every visit, and each review notes it.

What to look for: for your most sensitive files, the desktop or mobile app rather than the browser; and whether the provider offers an extension or another way to avoid loading its code on every visit.

5. What happens if you forget your password?

When only you hold the key, nobody else can give it back. Account recovery at such services is really two separate things, and Proton’s help page names both: a password reset lets you sign in again, and data recovery makes your files readable again. With a way to reset the password but no way to recover data, Proton says, “you’ll lose access to everything that was on your account before the password reset” [24]. Its recovery phrase covers both steps.

Many of these services give you a recovery key to save in advance. MEGA’s is a 22-character code, and if you lose both the password and the recovery key, MEGA says you “will permanently lose access to the data stored in your account” [25]. Before you turn on Advanced Data Protection, Apple asks you to set up a recovery contact or a recovery key [6]. Some services offer no recovery once you are signed out everywhere. Tresorit cannot restore a lost password, and if you are not signed in on any device, the only option is to reset the account, which erases all stored files and folders [26]. In pCloud’s Crypto folder, resetting the Crypto Pass “will delete encrypted files” [27], and Icedrive says a forgotten encryption passphrase leaves the encrypted data “unusable. Forever.” [28]

A service that resets your password by email and gives your files back can do it only because it keeps a way to your key, like Sync.com’s default setting in question 1. For a family archive that may be the right trade, but it is a different design from one where the provider cannot open your files.

What to look for: whether the service has a recovery key, and whether a password reset brings the files back or only the account. If an email reset restores everything, the provider has a way to your key.

6. Who checked the zero-knowledge claim?

Zero-knowledge is a vendor’s description of its own design. Three kinds of evidence can stand behind it, and they prove different things.

The weakest is the vendor’s documentation on its own. Filen, for example, has not published an independent audit and plans its next larger one after a rebuild of its programming interface, API v4, which has no date yet [29].

Next comes a security audit that the vendor ordered. Tresorit chose to have EY review its encryption and apps in 2019, and, according to Tresorit’s announcement, EY found its zero-knowledge claim “well founded”; the full report is not published [30]. Proton says it “subjected” its Drive mobile apps “to an independent security audit by Securitum” [31]. The auditors are outside firms, but the vendor decides what they test, when, and whether the report is published. Read each report’s scope and date, not only the announcement.

The strongest is research that no vendor ordered. In 2024, cryptographers at ETH Zurich tested Sync, pCloud, Icedrive, Seafile and Tresorit against a server controlled by an attacker and found severe vulnerabilities in the first four; in many cases the server could plant files, change their contents or read them [16]. According to the researchers, Icedrive chose not to address the issues, and Sync and pCloud had not replied to the researchers by October 2024 [16]. The study found less serious weaknesses at Tresorit: the certificate problem from question 3 and file details a server could change. That came five years after the EY review, and the two checks were looking for different things. Earlier, researchers published practical attacks that MEGA’s own servers could carry out against its users; MEGA added checks in response, and a follow-up study showed that those checks could themselves be used to recover a user’s private key [32]. MEGA says its March 2023 update closes these attacks, but it has not said whether the root cause the researchers named is fixed [43].

Open-source code is evidence of another kind: anyone can read how the apps encrypt, and the app code matters most, because the encryption happens there. Internxt publishes the code of its apps and of its server [33], Proton publishes the code of its Drive apps [34], and MEGA publishes its web client under a licence that allows use “solely for review purposes” [35]. Published code makes outside research possible; it does not mean that anyone has done it.

What to look for: who ordered each audit, what it covered, when, and whether the full report is public; whether independent researchers have studied the design, and what the vendor did about their findings.

7. Where is the company, and who owns it?

When the keys stay on your devices, a court order to the provider reaches only what the provider can read: the details it does not encrypt, your email address, your IP addresses and your payments. Which country’s law applies to that data depends on where the company you contract with is registered, and that is often not where your files are stored.

Tresorit is operated by Tresorit AG in Switzerland, but by default it stores the encrypted files of customers outside Switzerland in Microsoft Azure data centers in Ireland, and it keeps metadata in Ireland for everyone [36]. Swiss Post bought a majority stake in Tresorit in 2021 [37]. pCloud asks you to choose at sign-up whether your files and account data are stored in Luxembourg or in Dallas, Texas, and charges $19.99 to move them later [38]. MEGA’s terms name MEGA Privacy Kft, registered in Hungary, as the company you contract with for its cloud storage, unless you subscribed through the App Store or Google Play, in which case it is MEGA Networks LLC in Delaware, USA [39].

What to look for: the company named in the terms, the country of the data centers and the owner. After a takeover or a change of contracting company, read the terms again: different rules may now apply.

8. What does the storage cost after the first payment?

Compare the price of a named amount of storage at the rate you will pay on renewal, not the discount on the first payment. Internxt’s Essential plan, with 1 TB [40], is a yearly plan billed monthly with 80% off the first month only: €1.99 for the first month, then €9.99 a month [41]. Encryption can cost extra as well: pCloud sells pCloud Encryption, which gives you the Crypto folder, separately from storage [3].

Lifetime plans are paid once, and the terms define what “lifetime” means. pCloud’s terms say a lifetime plan lasts for the account owner’s life or 99 years, whichever is shorter, and that if pCloud ceases operation, the account is terminated without a refund [42]. A lifetime plan lasts as long as the company does.

What to look for: the yearly renewal price divided by the storage it buys, any add-on the encryption needs, and what the terms say happens to a lifetime plan if the company closes.

Where private cloud storage services publish the answers

Most of these answers sit on pages that the sales pages do not link to prominently. A security whitepaper or encryption page covers questions 1, 2 and 4. Help-center articles on sharing and on forgotten passwords cover questions 3 and 5. The privacy policy often lists what is not encrypted, and the terms name the company and define lifetime plans. Published audit reports and academic papers answer question 6. Our cloud storage reviews answer all eight for each service, with sources, and the comparison tool puts any two of them side by side.

Summary

Before you trust a service with private files, find out who holds the keys and whether that is the default; what is encrypted besides the contents; whether sharing, editing and the web app keep the encryption; and what a forgotten password costs you. Then check who has tested the zero-knowledge claim: the vendor alone, an auditor the vendor hired, or researchers nobody hired. Finally, find out which company and which country’s law you are dealing with, and what the storage costs after the first payment. A service that answers these questions in its own documentation gives you more to go on than one that answers with the word “encrypted”. To see two services answered side by side, read Filen vs MEGA or MEGA vs Proton Drive.