Client-side encryption vs server-side encryption for uploads

Why encryption choices matter for uploaded documents

If your app lets people upload files, you’re not just storing “documents.” You’re often storing a second identity for the user: signed contracts, passport or driver’s license photos, medical forms, and lab results. The files are small. The risk attached to them isn’t.

A leaked contract can trigger legal and financial fallout: pricing becomes public, signatures get copied, and disputes get ugly fast. A stolen ID scan can lead to identity theft and account takeovers. Medical documents can expose private health details people never expected to share beyond a small circle. One mistake can damage trust for years.

Teams often say “encrypted storage” while meaning different things. Sometimes they mean the upload connection is encrypted (HTTPS). Sometimes they mean the file is encrypted on disk (encryption at rest). Sometimes they mean the file is encrypted before it leaves the user’s device, so the server never sees plaintext (client-side encryption). These aren’t interchangeable. They protect against different failures.

Security choices also shape daily usability and support. More privacy can mean more friction: extra steps to view a file, harder sharing, limited search and previews, and painful recovery when someone loses a device or key. Easier access can enable indexing, antivirus scanning, previews, and password resets, but it also increases what your server (and anyone who compromises it) can see.

Picture a small clinic uploading insurance IDs and medical PDFs. Staff need to find the right file quickly, but the clinic also wants to reduce damage if an admin account gets hacked. The “right” model depends on what failure would hurt most, and what inconvenience users will tolerate.

Quick definitions: client-side vs server-side encryption

The practical question is simple: when is the file encrypted, and who can decrypt it later?

Server-side encryption means the file reaches your backend in readable form, and your server encrypts it before saving it. This is encryption at rest: if someone steals a drive or gets raw access to a storage bucket, they see scrambled data. Your server can still decrypt the file when needed.

Client-side encryption means the file is encrypted on the user’s device (browser or mobile) before upload. Your server stores only ciphertext. In normal operation, the server can’t read the document unless it also has access to the decryption key.

Key ownership is the real dividing line:

• With server-side encryption, keys are managed by your backend (often via a key management service), and the backend decrypts files for authorized users.
• With client-side encryption, keys are controlled by the user, their device, or a client-held secret, and the backend mainly enforces storage and access rules.

In both models, you still need authentication and permissions. Encryption doesn’t replace access control.

People also use “end-to-end encryption” to mean: the file is encrypted on the sender’s device, stays encrypted on the server, and is decrypted only on an approved recipient’s device. That can improve confidentiality, but it makes server-side preview, full-text search, virus scanning, and easy recovery much harder unless you’re willing to change the threat model.

Threat models: what you’re actually trying to stop

Encryption only helps when you’re clear about the risks you want to reduce. “Nobody except the intended user can read the file” leads to different choices than “reduce damage if storage is leaked.”

Common threats for apps that store contracts, IDs, or medical documents include:

• Stolen or reused passwords (account takeover)
• Insider access that’s broader than it should be (support, admin, contractors)
• A compromised cloud account or misconfigured storage bucket
• A bug or leak exposing databases or backups
• Malware on a user’s device

It also helps to separate where exposure can happen:

• In transit: the file moving from device to server
• At rest: object storage, database rows, backups, logs
• During viewing/processing: previews, OCR, search, conversions

Here’s the core difference. With server-side encryption, your system can decrypt, so it can preview, scan, and index. With client-side encryption, the server stores ciphertext and can’t read content without user-held keys. That usually reduces the blast radius of server compromises and insider access.

Encryption doesn’t stop everything. If a device is infected, malware can grab the file before encryption or after decryption. If someone can view a document, they can still screenshot it, re-share it, or print it.

What each model protects against (and what it doesn’t)

A useful way to compare these approaches is to ask: who can see the file in plaintext at any point? That answer shapes breach impact, insider risk, and the safety of backups.

With server-side encryption, a server breach can still expose a lot. The backend usually has access to decryption keys (or can request them) because it needs to generate previews, run antivirus checks, support search, or handle sharing. In the worst case, an attacker who gets both stored data and key access can decrypt everything.

With client-side encryption, an attacker who breaches your infrastructure typically steals encrypted blobs. Without user-held keys, those blobs are far less useful.

Insider access is where the difference becomes most visible. With server-side encryption, a privileged employee, cloud admin, or compromised support account can often access documents by impersonating the app or querying storage. With client-side encryption, your infrastructure can store and move files, but can’t read them unless keys are provided.

Client-side encryption doesn’t fix a hacked device. For high-risk uploads like IDs and medical PDFs, device security and account protection matter as much as storage encryption.

Also watch for leaks outside the “file store.” Many incidents happen through:

• Backups and snapshots that capture decrypted files or keys
• Logs that record filenames, metadata, or extracted text
• Thumbnails, previews, and search indexes
• Exports to email, chat, or ticketing tools
• Temporary files created during upload or conversion

UX tradeoffs users notice right away

The biggest difference isn’t the math. It’s what users can do easily, and what becomes hard.

Server-side encryption often feels invisible. Users log in, upload, and preview immediately. Support can reset access. Admins can usually reassign permissions when someone is out sick.

Client-side encryption changes onboarding and daily work. Users may need a stronger passphrase, a local key stored on a device, or an extra unlock step. Switching devices, clearing a browser, or reinstalling an app can break access unless you planned for key backup and recovery.

Account recovery is where teams get surprised. If only the user holds the decryption key, “Forgot password” can turn into “lost access.” You can add a recovery key or escrow flow, but you need to be honest about what that implies and explain it clearly.

Sharing also gets more complicated. With server-side encryption, sharing is mostly permissions. With client-side encryption, sharing often involves key sharing too, along with questions like revocation and what happens to old copies.

Search and convenience features can force decryption somewhere. Full-text search, previews, and OCR are easiest when the server can read the file. If you want end-to-end style privacy, you may need client-side OCR, local indexing, or limited search (for example, filenames and tags only).

Example: a clinic uploads lab PDFs and wants OCR to find patient names inside scans. Server-side encryption supports fast OCR and search. Client-side encryption pushes that work to user devices, which can be slower and harder to support across web and mobile.

Document-specific needs: contracts, IDs, and medical records

The best choice depends less on file type and more on workflow: who needs to read it, how fast, how often, and for how long.

Contracts

Contracts often involve review, redlines, approvals, and audit trails. Teams also want reliable search, sharing, and retention rules.

If your app supports collaborative review inside the product, server-side encryption is often the practical default because the system can render previews, run search, and enforce role-based access without asking users to manage keys.

Client-side encryption can fit if the app is mostly a vault, such as storing final signed PDFs for a small executive group. The tradeoff is weaker built-in convenience unless you add client-side tooling.

IDs (passports, driver licenses)

IDs are high risk but often short-lived. Many teams only need them long enough to verify a person, then delete them. The workflow is quick viewing and tight handling rules, not collaboration.

A common approach is server-side encryption paired with strict access control, strong audit logs, and short retention. Client-side encryption can fit if support staff should never be able to view IDs at all, but then you need another way to complete verification.

Medical documents

Medical records carry stronger privacy expectations. Users often assume only the minimum set of people can see them.

Client-side encryption can reduce exposure if the server is breached or admin access is misused. But it can also create real UX and operational problems: password resets, device changes, and emergency access become complicated.

Before choosing, map each document type to the workflow:

• Who must read it (and from where)
• How fast it must load
• How long you keep it
• Which product features matter (preview, search, automated deletion)

How to choose: a step-by-step decision process

Start by writing down what you store and who touches it. A folder called “uploads” isn’t a policy.

Step 1: map access, not just “users”

List roles and answer two questions: who must be able to open the file, and who must never be able to open it (including admins)? Include retention while you’re here.

Step 2: choose your threat assumptions

Be direct about what you’re defending against.

• If insider risk is a top concern, client-side encryption becomes more attractive.
• If device loss and password resets are common, you’ll pay for it in recovery complexity.

Then pressure-test the experience:

1. Recovery and support: What happens when someone forgets a password or loses a phone? If you must recover access, pure client-side encryption may not fit.

2. Must-have features: Do you need previews, search, OCR, e-signing, or API-based processing? Many of these are simpler with server-side decryption somewhere in the flow.

3. Audit and compliance: Do you need clear logs of who accessed what and when? Both models can log access, but client-side designs need extra care to avoid “we can’t help you” outcomes.

Most teams end up with a hybrid: server-side encryption for most documents, and client-side encryption for a small set of “never visible to staff” uploads.

Common mistakes and traps to avoid

The biggest trap is treating encryption as the whole story. Privacy and compliance also depend on who can access data, how access is approved, what gets logged, how long files are kept, and how deletion requests are handled.

A close second is building client-side encryption without a recovery plan. If a user loses a device, forgets a passphrase, or leaves the company, can you restore access without breaking your security promise? “We can’t help you” might be acceptable for a personal vault, but it usually fails in business apps.

These mistakes cause real leaks and workarounds:

• Giving admins a hidden “view everything” path, or letting support impersonate users, without strict logs and second-person approval.
• Decrypting in the browser or app and leaving copies behind (cached previews, temporary downloads, synced folders).
• Sending “secure” documents through insecure channels later (emailing PDFs, pasting screenshots into chat, attaching files to tickets).
• Making the secure flow so hard that users move to personal drives or take photos of the screen.
• Assuming “encrypted at rest” automatically satisfies consent, access history, retention, and breach reporting requirements.

Example: a clinic encrypts intake forms, then staff export a billing report that creates a local unencrypted folder. That folder gets backed up to a shared drive. The leak happens in the workflow, not the crypto.

Quick checklist before you commit

Write one plain sentence: who should be able to read these files, and who should never be able to, even if someone gets access to your servers.

Then check:

• Who can decrypt, and when? List exact roles and conditions. If your policy says “only the uploader,” don’t quietly add a backdoor through shared keys.
• Can you revoke access quickly? Offboarding is the real test. Decide if access is tied to an account, a device, or a group.
• What happens after device loss or password resets? If you need recovery, protect it with strong approvals.
• Do you need previews, virus scanning, or OCR? If yes, plan where decryption happens and who can trigger it.
• Are your audit logs specific enough? Define what counts as “viewed” vs “downloaded,” and capture user, time, device, and reason.

Example scenario: a small team storing IDs and medical PDFs

A small clinic has an app where staff upload patient referrals (PDFs) and photos of insurance IDs. The goal is to move documents to the right people quickly, while reducing damage from lost devices, compromised accounts, or cloud mistakes.

One workable approach is server-side encryption with strict roles. Files are encrypted at rest, and access is controlled by permissions: front desk can upload, billing can view IDs, and clinicians can view referrals. This tends to be easier day to day because staff can open documents on desktop or mobile without extra steps, and supervisors can reassign access during absences.

Another approach uses client-side encryption for the most sensitive items. The file is encrypted on the device before upload, so the server stores ciphertext. This can shrink the impact of server breaches and insider access, but it changes operations:

• Password resets can restore access normally with server-side encryption; with client-side encryption, resets can lock users out unless keys are backed up safely.
• Staff turnover is simpler with server-side encryption; with client-side encryption, you need a plan to transfer or escrow keys so documents remain accessible.

Users feel friction around sharing, mobile access, and recovery after a lost phone. Those details matter as much as the encryption model itself.

Next steps: decide, document the policy, and implement

Start by writing your threat assumptions in plain language. Pick the simplest approach that meets your risk, then tighten it only where the documents truly justify it.

Put the decision into a short internal rule set people can follow:

• Which file types are allowed and where they can be stored
• Who can access and share them, and how access is approved
• Retention and deletion rules
• How recovery works after password resets and device loss
• How exports (downloads, email, messaging) are handled, and when they’re blocked

Then design the implementation around those rules: role checks, audited actions (view, download, share), safe previews, and careful key handling.

If you’re building on a no-code platform like AppMaster (appmaster.io), it helps to model roles and approval flows early, then adjust as requirements change without rewriting the whole app. The important part is making the secure path the easy path for real users.