How is my data encrypted?


This document is not a spec, only an explanation of the encryption process.

Algorithms & cryptographic library

  1. XChaCha-Poly1305-IETF (for encryption/decryption)
  2. Argon2 (for password hashing & PKDF)
    1. argon2i for PKDF
    2. argon2id for password hashing
  3. libsodium

On all three platforms we use the same exact library for all cryptographic functions. This ensures data integrity across platforms.

Fun story

When we first added encryption, we used AES-GCM-256 across platforms but the cross-platform compatbility was abyssmal. That is when I found out about the great libsodium. Written in C, wrappers available for all platforms…what more could I want?


1. Sign up & sign in

When you sign up for an account, the app takes your password and hashes it using Argon2 with a predictable per user salt.

This predictable salt is generated using a fixed client salt + your email.

Your password never leaves your device

Sending the hash over sending your plain text password ensures that there is no way for us (or anyone else) to get your password.

After the hash is generated, it is sent to the server. This hash is used as a password and is hashed again to mitigate password passthrough attacks.

This process is repeated every time you sign in.

2. Key generation

After you are signed in, the app requests your user data which includes, among other things, your salt.

Salt generation

When you create an account, the server generates a cryptographically secure random salt for you. This salt is used for key generation.

You password & salt is then used to derive a strong irreversible key using Argon2 as the password key derivation function (PKDF).

3. Encryption key storage

Instead of storing the key as plain text (and allowing anyone to copy/move it), we use browser’s IndexedDB to store the key as a CryptoKey.

CryptoKey is stored securely by the browser and cannot be exported, viewed, copied except by the app & browser.

On iOS and Android, the encryption key is stored in the phone’s keychain.

4. Data encryption

Encryption only takes place when you sync. Each item in the database is encrypted seperately using XChaCha-Poly1305-IETF.

How it works

  1. The item is read from the database as JSON object and stringified (i.e. converted to a string).
  2. The string is encrypted using the encryption key generated earlier.
  3. The result is a JSON object which contains:
    1. A base64 encoded cipher
    2. A 192-bit nonce (iv)
    3. A random salt
    4. Algorithm id alg
    5. ItemId id

See the whole process in action here.

This object is then sent to the server for storage. The server performs no further operation on this data (because it can’t).