What is zero-knowledge encryption?
The Scrinium team · September 8, 2026 · 6 min read
“Zero-knowledge” is a phrase that gets thrown around a lot, but it has a precise meaning: the service that stores your data is architecturally unable to read it. Even if the company wanted to, even if a court demanded it, even if a server was fully compromised — it only ever has ciphertext.
“Encrypted at rest” isn’t the same thing
Most apps say your data is “encrypted.” Often that means the files on disk are encrypted, but the company holds the keys. If someone hacks the database, logs in as an admin, or receives a valid legal request, they can decrypt everything. That’s protecting the physical drive, not protecting you.
Zero-knowledge is different: the keys that decrypt your data never leave your device. The server stores only the encrypted blob and the pieces it needs to let you sign in. Decryption happens entirely in your browser or app, in memory, and only when you’re actively using it.
How zero-knowledge works
The trick is a key hierarchy. Your password is strengthened by a memory-hard key derivation function (Argon2id) to produce a key. That key unlocks your account key, which unlocks a vault key, which unlocks a unique key for every note. Each note is sealed with authenticated encryption, so any modification is detected.
- Your password never leaves your device.
- Decrypted keys exist only in memory while the vault is unlocked.
- The server sees ciphertext, a login verifier, and metadata — never plaintext.
Because the server never receives a usable key, it can’t read your notes, can’t serve them to anyone, and can’t comply with a data request for content it doesn’t have.
Server breaches, data leaks, insider access, subpoenas for content, and a provider reading your private notes for ads or training. Your notes are unreadable to anyone without your keys.
Device compromise. If your own device is infected, or you type your password into a fake page, your notes can be decrypted. Zero-knowledge protects the server, not your device. And if you lose your password and your recovery key, the data is unrecoverable — by design.
The honest trade-off
That last point is the real cost. Because no one else can decrypt your data, there’s no “forgot your password” reset from the company. A zero-knowledge app has to hand you a recovery key up front and make clear that losing both means losing the vault. That’s the same guarantee 1Password makes — and it’s exactly why a proper privacy product can’t offer a simple “we’ll reset it for you.”
Why it matters for notes specifically
Notes are deeply personal: journals, health thoughts, passwords for things, ideas, drafts of things you haven’t published. That’s not content you want on a server anyone can read. The problem has been that a truly private app felt like a downgrade.
How Scrinium does it
Scrinium pairs a real Markdown WYSIWYG editor with true zero-knowledge encryption: a 1Password-style key hierarchy, Argon2id key derivation, XChaCha20-Poly1305 on every note, and SRP login (your password never even reaches the server as a password). The backend is a dumb encrypted-blob store — it persists ciphertext and a login verifier, nothing more. You can read the full crypto design and threat model.
Create a vault for free — unlimited notes, encrypted sync, unlimited devices.
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