A general purpose encryption scheme for key/value stores.

Usage

use matrix_sdk_store_encryption::StoreCipher;
use serde_json::{json, value::Value};

fn main() -> anyhow::Result<()> {
    let store_cipher = StoreCipher::new()?;

    // Export the store cipher and persist it in your key/value store
    let export = store_cipher.export("secret-passphrase")?;

    let value = json!({
        "some": "data",
    });

    let encrypted = store_cipher.encrypt_value(&value)?;
    let decrypted: Value = store_cipher.decrypt_value(&encrypted)?;

    assert_eq!(value, decrypted);

    let key = "bulbasaur";

    // Hash the key so people don't know which pokemon we have collected.
    let hashed_key = store_cipher.hash_key("list-of-pokemon", key.as_ref());
    let another_table = store_cipher.hash_key("my-starter", key.as_ref());
    let same_key = store_cipher.hash_key("my-starter", key.as_ref());

    assert_ne!(key.as_ref(), hashed_key);
    assert_ne!(hashed_key, another_table);
    assert_eq!(another_table, same_key);

    Ok(())
}

⚠️ Security Warning: Hazmat!

This crate only implements the low-level block cipher function, to be used only as a building block for higher-level constructions. It is NOT intended for direct use in applications.

USE AT YOUR OWN RISK!

Encryption scheme

The central component of the encryption scheme is the StoreCipher type, used for both obfuscating keys and encrypting values of the key/value store. A StoreCipher object consists of two randomly-generated 32 byte secrets.

The first secret is used to encrypt values. XChaCha20Poly1305 with a random nonce is used to encrypt each value. The nonce is saved with the ciphertext.

The second secret is used as a seed to derive table-specific keys, used to key a keyed hash construction, which is in turn used to hash table data. Currently we use blake3 as the keyed hash construction.

                ┌───────────────────────────────────────┐
                │             StoreCipher               │
                │   Encryption key |    Hash key seed   │
                │      [u8; 32]    |       [u8; 32]     │
                └───────────────────────────────────────┘

The StoreCipher has some Matrix-specific assumptions built in, which ensure that the limits of the cryptographic primitives are not exceeded. If this crate is used for non-Matrix data, users need to ensure:

1. That individual values are chunked, otherwise decryption might be susceptible to a DOS attack.
2. The StoreCipher is periodically rotated/rekeyed.

WASM support

This crate relies on the random and getrandom crates which don't support WASM automatically.

Either turn the js feature on directly on this crate or depend on getrandom with the js feature turned on. More info can be found in the getrandom docs.