Zero-Knowledge Encryption Explained: How SecretDrop Protects Your Data

When you share files through most cloud services, the provider can technically access your data. They hold the encryption keys, manage the storage, and could — if compelled by a court order, a rogue employee, or a data breach — read your files.

Zero-knowledge encryption eliminates this risk entirely. With zero-knowledge architecture, the service provider cannot access your data under any circumstances, because they never have the information needed to decrypt it.

This article explains how zero-knowledge encryption works, why it matters for sensitive file sharing, and how SecretDrop implements it.

What Is Zero-Knowledge Encryption?

Zero-knowledge encryption means the service provider has zero knowledge of your data’s contents. The encryption and decryption happen on your device — your browser, your laptop, your phone — and the server only ever stores encrypted blobs that it cannot read.

The key distinction from standard encryption: with most cloud services, the provider encrypts your data on their servers using keys they control. They can decrypt it whenever they choose. With zero-knowledge encryption, the provider never touches the unencrypted data or the encryption key.

How SecretDrop Implements Zero-Knowledge Encryption

Here is what happens when you create an encrypted bundle on SecretDrop:

1. Key derivation happens in your browser

When you set a password, SecretDrop uses PBKDF2 (Password-Based Key Derivation Function 2) to derive a strong encryption key from your password. This happens entirely in your browser using the Web Crypto API. The password itself is never transmitted to the server.

2. Files are encrypted client-side

Your files are encrypted using AES-256-GCM (Advanced Encryption Standard with 256-bit keys in Galois/Counter Mode) before they leave your browser. AES-256 is the same encryption standard used by governments, financial institutions, and military organizations worldwide. The GCM mode provides both confidentiality and integrity verification — if even a single bit of the encrypted data is tampered with, decryption will fail.

3. Only encrypted data reaches the server

The server receives and stores only the encrypted blob. It does not receive the password, the derived key, or the unencrypted files. The server has no mechanism to decrypt the data because it never possesses the key.

4. Decryption happens in the recipient’s browser

When your recipient opens the shared link and enters the password, the same key derivation and decryption process runs in their browser. The encrypted blob is downloaded from the server and decrypted locally. The server facilitates the transfer but cannot read the contents.

Why Zero-Knowledge Matters for File Sharing

Protection against server breaches

If an attacker compromises the server, they obtain only encrypted blobs. Without the passwords (which are never stored on the server), the encrypted data is computationally useless. There is no key database to steal, no master decryption key to extract.

Protection against insider threats

Employees, contractors, and administrators who have access to the server infrastructure cannot read your files. This eliminates an entire category of risk that exists with traditional cloud storage.

Protection against legal compulsion

In some jurisdictions, authorities can compel service providers to hand over user data. With zero-knowledge encryption, the provider can comply with such requests by handing over the encrypted blobs — but the data remains unreadable without the user’s password, which the provider does not have.

No trust required

You do not need to trust SecretDrop to protect your data. The encryption is verifiable: the Web Crypto API calls run in your browser, and you can inspect the network traffic to confirm that only encrypted data is transmitted. Security through architecture is stronger than security through policy.

Zero-Knowledge vs. Standard Encryption

Property	Standard cloud encryption	Zero-knowledge encryption
Who holds the encryption key?	The service provider	Only the user
Can the provider read your files?	Yes	No
Protection if the server is breached?	Depends on key management	Files remain encrypted
Can an insider access your data?	Potentially	No
Where does encryption happen?	On the server	In your browser

The Role of AES-256-GCM

AES-256 is a symmetric encryption algorithm that uses a 256-bit key. It is considered computationally secure against all known attacks, including theoretical quantum computing attacks (which would require Grover’s algorithm to reduce the effective key strength to 128 bits — still far beyond feasible brute-force).

GCM (Galois/Counter Mode) adds authenticated encryption, meaning the decryption process verifies that the ciphertext has not been modified. If anyone tampers with the encrypted data in transit or at rest, the decryption will fail rather than producing corrupted output. This protects against both accidental corruption and deliberate tampering.

Automatic Expiry Strengthens the Model

Zero-knowledge encryption protects the confidentiality of your data. SecretDrop’s automatic expiry limits the window during which encrypted data exists at all. After the expiry date, the encrypted bundle is permanently deleted from the server.

This combination — zero-knowledge encryption plus automatic deletion — ensures that your shared files are both unreadable to the server and ephemeral. Even in a worst-case scenario where an attacker obtains the encrypted blob and later acquires the password, the data no longer exists on the server.

Verifying the Encryption Yourself

Because SecretDrop’s encryption runs in your browser, you can verify it independently:

1. Open your browser’s developer tools (F12 or Cmd+Shift+I)
2. Go to the Network tab before creating a bundle
3. Create a bundle and inspect the outgoing requests
4. Verify that the request body contains only encrypted data — not your plaintext files or password

This transparency is a fundamental advantage of client-side encryption. You do not need to take the provider’s word for it.