The LocalMode Encryption Stack: PBKDF2 Key Derivation, AES-256-GCM, and Encrypted Embeddings

Most "privacy-first" AI libraries mean one thing: they do not phone home. Your data stays on the device during inference. That is necessary, but it is not sufficient. If your embedding vectors sit in IndexedDB as raw Float32Arrays, anyone with access to the device -- a shared laptop, a stolen phone, a compromised browser extension -- can read them. Worse, research has shown that embedding vectors can be inverted to reconstruct the original text with surprising accuracy.

LocalMode ships a complete encryption stack that closes this gap. Every layer -- key derivation, authenticated encryption, persistent key management, vector encryption middleware -- runs in the browser through the Web Crypto API. No external cryptography libraries. No server round-trips. Zero knowledge by construction.

This post walks through every layer of that stack, from the raw cryptographic primitives to the high-level middleware that encrypts your VectorDB transparently.

Layer 1: Key Derivation with PBKDF2

The entire encryption pipeline starts with a password. Users do not think in terms of 256-bit keys; they think in terms of passwords. The bridge between a human-memorizable string and a cryptographically strong key is PBKDF2 (Password-Based Key Derivation Function 2).

LocalMode's deriveEncryptionKey() function wraps the Web Crypto API's SubtleCrypto.deriveKey() to produce an AES-256-GCM CryptoKey from a password, a salt, and an iteration count:

import { deriveEncryptionKey } from '@localmode/core';

// Derive an encryption key from a user password
const { key, salt } = await deriveEncryptionKey('user-password');

// Store the salt for future key derivation (it is not secret)
localStorage.setItem('encryption-salt', salt);

// Later, re-derive the same key with the stored salt
const { key: sameKey } = await deriveEncryptionKey('user-password', storedSalt);

Under the hood, this executes two Web Crypto operations in sequence:

1. Import the password as raw key material -- crypto.subtle.importKey('raw', encodedPassword, { name: 'PBKDF2' }, false, ['deriveKey']) converts the UTF-8 encoded password into a non-extractable CryptoKey that can only be used for derivation.
2. Derive the AES-256 key -- crypto.subtle.deriveKey() runs PBKDF2 with HMAC-SHA-256 as the pseudorandom function, the provided salt, and the configured iteration count, producing a 256-bit AES-GCM key.

The returned CryptoKey is non-extractable (extractable: false). The raw key bytes never enter JavaScript memory. They exist only inside the browser's cryptographic subsystem, inaccessible to application code, browser extensions, or in-memory inspection tools.

Salt Generation

When no salt is provided, deriveEncryptionKey() generates 16 random bytes (128 bits) via crypto.getRandomValues(). The salt is returned as a base64 string for easy storage. It is not secret -- its purpose is to ensure that the same password produces different keys for different databases, defeating precomputed rainbow table attacks.

Iteration Count: The Security-Performance Tradeoff

LocalMode defaults to 100,000 PBKDF2 iterations. Each iteration runs one HMAC-SHA-256 computation, so the total derivation cost scales linearly.

The OWASP Password Storage Cheat Sheet currently recommends a minimum of 600,000 iterations for PBKDF2-SHA-256 in server-side password hashing. Some security researchers advocate for 310,000+ iterations as a practical floor in 2025-2026.

LocalMode's default of 100,000 is a deliberate tradeoff for client-side key derivation in the browser. Key derivation happens on the user's device -- which might be a mid-range phone, not a server with dedicated hardware. At 100,000 iterations, derivation takes approximately 50-150ms on modern hardware. At 600,000, it would take 300-900ms, creating a noticeable delay on every vault unlock.

The iteration count is fully configurable:

// High-security: match OWASP server-side recommendations
const { key, salt } = await deriveEncryptionKey('password', undefined, 600_000);

// Default: 100,000 iterations (good browser-side tradeoff)
const { key: defaultKey } = await deriveEncryptionKey('password');

For applications handling highly sensitive data (medical records, legal documents, financial information), raising iterations to 300,000-600,000 is straightforward. The Keystore class (Layer 4) persists the iteration count alongside the key metadata, so the correct value is always used for re-derivation.

Layer 2: AES-256-GCM Authenticated Encryption

With a derived CryptoKey in hand, the next layer handles the actual encryption. LocalMode uses AES-256-GCM (Galois/Counter Mode) -- a NIST-standardized authenticated encryption with associated data (AEAD) cipher.

Why AES-256-GCM?

GCM provides three properties in a single pass:

This matters because IndexedDB is not a tamper-proof store. A malicious extension or XSS attack could modify stored ciphertext to inject crafted data. With GCM's authentication tag, any such modification causes decryption to fail with an explicit error rather than silently returning corrupted plaintext.

The Encrypt Function

The encrypt() function accepts strings or raw ArrayBuffer data:

// Note: encrypt/decryptString are internal utilities used by encryptionMiddleware.
// For most use cases, use encryptionMiddleware() or deriveEncryptionKey() directly.
import { deriveEncryptionKey, encryptionMiddleware } from '@localmode/core';

// Encrypt a string
const encrypted = await encrypt('Sensitive medical record', 'user-password');
// Returns: { ciphertext, iv, salt, algorithm: 'AES-GCM', version: 1 }

// Decrypt it back
const plaintext = await decryptString(encrypted, 'user-password');
// 'Sensitive medical record'

Each call to encrypt() generates:

1. A fresh 16-byte salt for key derivation (via crypto.getRandomValues)
2. A fresh 12-byte IV (initialization vector / nonce) for AES-GCM
3. A derived AES-256 key from the password + salt via PBKDF2
4. The ciphertext + authentication tag via crypto.subtle.encrypt()

The EncryptedData structure stores everything needed for decryption:

interface EncryptedData {
  ciphertext: string;      // Base64-encoded ciphertext + auth tag
  iv: string;              // Base64-encoded 12-byte IV
  salt: string;            // Base64-encoded 16-byte PBKDF2 salt
  algorithm: 'AES-GCM';   // Algorithm identifier
  version: 1;              // Schema version for future compatibility
}

Nonce Uniqueness: The Critical Invariant

AES-GCM has one absolute requirement: the (key, IV) pair must never repeat. Reusing an IV with the same key completely breaks both confidentiality and authenticity. Since LocalMode generates a fresh random salt for every encrypt() call, a different key is derived each time -- making IV collisions irrelevant even if the same 12-byte IV were drawn twice. In the encryption middleware (Layer 5), where a single derived key is reused across multiple operations, a fresh 12-byte IV is generated per operation via crypto.getRandomValues(), keeping the collision probability at approximately 2^-48 per operation -- safely below the practical threshold for any client-side workload.

Specialized Encryption Functions

Beyond string encryption, the crypto module provides type-specific wrappers:

// Vector and metadata encryption is handled automatically by encryptionMiddleware.
// The low-level encryptVector/decryptVector functions are internal to the middleware.

// Encrypt a Float32Array (embedding vector)
const vector = new Float32Array([0.1, 0.2, 0.3, /* ... 384 dimensions */]);
const encryptedVector = await encryptVector(vector, 'password');
const decryptedVector = await decryptVector(encryptedVector, 'password');
// decryptedVector is identical to the original Float32Array

// Encrypt arbitrary JSON
const metadata = { patientId: 'P-12345', diagnosis: 'confidential' };
const encryptedMeta = await encryptJSON(metadata, 'password');
const decryptedMeta = await decryptJSON(encryptedMeta, 'password');

The encryptVector() function passes the Float32Array's underlying ArrayBuffer directly to AES-GCM, avoiding any intermediate serialization. For a 384-dimensional embedding (1,536 bytes), the encrypted output is 1,536 bytes of ciphertext + 16 bytes of auth tag + 12 bytes of IV + 16 bytes of salt -- roughly 1,580 bytes total, an overhead of under 3%.

Layer 3: Passphrase Verification

Encryption protects data at rest, but users need a way to know whether they entered the correct password before attempting to decrypt an entire database. The crypto module includes constant-time passphrase verification:

import { hashPassphrase, verifyPassphrase } from '@localmode/core';

// On vault creation: hash and store
const hash = await hashPassphrase('user-password');
// SHA-256 hash, base64-encoded

// On unlock: verify before attempting decryption
const isValid = await verifyPassphrase('user-password', storedHash);

The verifyPassphrase() function uses a constant-time comparison loop to prevent timing attacks -- each character is compared via XOR accumulation regardless of whether an earlier character mismatched:

// Constant-time comparison (from crypto.ts)
let result = 0;
for (let i = 0; i < newHash.length; i++) {
  result |= newHash.charCodeAt(i) ^ hash.charCodeAt(i);
}
return result === 0;

This is not a full timing-attack-proof implementation (JavaScript's runtime characteristics make true constant-time guarantees difficult), but it eliminates the most obvious timing leaks from early-exit string comparison.

Layer 4: The Keystore -- Persistent Key Management

Raw encryption functions are low-level primitives. The Keystore class provides the session management layer that applications actually interact with:

// Keystore is used internally by the encrypted-vault showcase app.
// For production use, the encryptionMiddleware + deriveEncryptionKey pattern is recommended.
import { deriveEncryptionKey, hashPassphrase, verifyPassphrase } from '@localmode/core';

const keystore = createKeystore();

// First time: initialize encryption for a database
await keystore.initialize('my-vector-db', 'master-password', 100_000);

// Later sessions: unlock with the password
const isValid = await keystore.unlock('my-vector-db', 'master-password');

// Check status
keystore.isUnlocked(); // true

// When done: clear the password from memory
keystore.lock();

The Keystore persists key metadata in a dedicated IndexedDB database (vectordb_keystore), separate from application data. Each entry stores:

The Keystore never stores the password or the derived key in IndexedDB. The password lives only in JavaScript memory while the vault is unlocked, and is cleared to null on lock(). The derived CryptoKey is non-extractable by the Web Crypto API's design.

Key Lifecycle Operations

The Keystore supports the full lifecycle of an encryption key:

// Change password (verifies old password first)
await keystore.changePassphrase('my-db', 'old-password', 'new-password');

// Disable encryption (requires password verification)
await keystore.disable('my-db', 'master-password');

// Delete all key metadata
await keystore.delete('my-db');

// Check if a database has encryption configured
const hasEncryption = await keystore.hasEncryption('my-db');

Layer 5: The Encryption Middleware -- Transparent VectorDB Encryption

The layers above are building blocks. The encryptionMiddleware ties everything together into a single VectorDB middleware that encrypts documents on write and decrypts them on read -- completely transparently to application code.

import {
  createVectorDB,
  wrapVectorDB,
  encryptionMiddleware,
  deriveEncryptionKey,
  embed,
} from '@localmode/core';
import { transformers } from '@localmode/transformers';

// 1. Derive encryption key from user password
const { key } = await deriveEncryptionKey('master-password', storedSalt);

// 2. Create a standard VectorDB
const db = await createVectorDB({ name: 'encrypted-notes', dimensions: 384 });

// 3. Wrap it with encryption middleware
const encryptedDb = wrapVectorDB({
  db,
  middleware: encryptionMiddleware({ key }),
});

// 4. Use normally -- encryption is transparent
const model = transformers.embedding('Xenova/bge-small-en-v1.5');
const { embedding } = await embed({ model, value: 'Patient diagnosed with...' });

await encryptedDb.add({
  id: 'note-1',
  vector: embedding,
  metadata: { title: 'Medical Note', content: 'Patient diagnosed with...' },
});

// 5. Search works normally -- results are decrypted automatically
const results = await encryptedDb.search(queryVector, { topK: 5 });
// results[0].metadata.content === 'Patient diagnosed with...' (decrypted)

What Gets Encrypted

The middleware encrypts three categories of data independently, each controlled by a configuration flag:

You can exclude specific metadata fields from encryption -- useful for fields that need to remain filterable:

const encryptedDb = wrapVectorDB({
  db,
  middleware: encryptionMiddleware({
    key,
    excludeFields: ['category', 'timestamp'],
    // 'category' stays as plaintext for filtered search
    // 'timestamp' stays readable for sorting
  }),
});

How Vector Encryption Works

Vector encryption deserves special attention because embedding vectors are Float32Array values that need to remain storable in IndexedDB. The middleware handles this by:

1. Generating a fresh 12-byte IV via crypto.getRandomValues()
2. Encrypting the raw Float32Array buffer with AES-GCM
3. Prepending the IV to the ciphertext
4. Padding the combined bytes to a multiple of 4 bytes
5. Wrapping the result as a new Float32Array for storage compatibility

On retrieval, the process reverses: the first 12 bytes are extracted as the IV, the remainder is decrypted, and the original Float32Array is reconstructed.

The Practical Pattern: Encrypt at Rest, Search in Memory

The most common architecture for encrypted semantic search in LocalMode is:

1. On unlock: derive the key, load encrypted documents, decrypt vectors into memory
2. During session: search against in-memory plaintext vectors; all results pass through afterSearch for metadata decryption
3. On lock: clear in-memory vectors and the derived key

This gives you zero-knowledge storage (nothing readable on disk without the password) while maintaining full-speed HNSW search during active sessions. The encrypted-vault showcase app demonstrates this pattern.

Layer 6: Combining Encryption with Other Middleware

The encryption middleware composes with LocalMode's other middleware through wrapVectorDB:

import {
  wrapVectorDB,
  encryptionMiddleware,
  loggingMiddleware,
  validationMiddleware,
} from '@localmode/core';

const secureDb = wrapVectorDB({
  db,
  middleware: [
    validationMiddleware({ dimensions: 384 }),   // Validate first
    loggingMiddleware({ level: 'info' }),         // Log operations
    encryptionMiddleware({ key }),                // Encrypt last (before storage)
  ],
});

You can also combine it with PII redaction on the embedding model side for defense in depth:

import { wrapEmbeddingModel, piiRedactionMiddleware } from '@localmode/core';

// Redact PII before embedding, encrypt vectors after embedding
const safeModel = wrapEmbeddingModel({
  model: transformers.embedding('Xenova/bge-small-en-v1.5'),
  middleware: piiRedactionMiddleware({ emails: true, phones: true, ssn: true }),
});

// PII is stripped before it reaches the model
// Vectors are encrypted before they reach storage

This two-layer approach means that even if the encryption is somehow bypassed, the stored embeddings do not encode any PII -- because it was redacted before the embedding model ever saw it.

Browser Compatibility

The entire encryption stack depends on the Web Crypto API, specifically crypto.subtle. Browser support is effectively universal across modern browsers:

One important constraint: crypto.subtle is only available in secure contexts (HTTPS or localhost). LocalMode's isCryptoSupported() function checks for availability before any encryption operation:

// Check for Web Crypto API availability (HTTPS or localhost required)
const isCryptoAvailable = typeof globalThis.crypto?.subtle !== 'undefined';

if (!isCryptoSupported()) {
  // Fallback: warn the user or disable encryption features
  console.warn('Web Crypto API not available -- encryption disabled');
}

Security Properties Summary

What This Does Not Protect Against

Transparency requires acknowledging limitations:

• Weak passwords: PBKDF2 slows brute-force attacks but cannot compensate for a four-character password. Enforce minimum length and complexity at the application layer.
• Memory inspection: While the CryptoKey is non-extractable, decrypted plaintext exists in JavaScript memory during processing. A compromised browser process could read it.
• Side-channel attacks: JavaScript's runtime does not guarantee constant-time execution. The comparison loop in verifyPassphrase() mitigates but does not eliminate timing leaks.
• Key management: If the user loses their password, the data is unrecoverable by design. This is a feature, not a bug -- but users must understand it.

Getting Started

Install the core package and a provider:

npm install @localmode/core @localmode/transformers

Set up encrypted storage in under 20 lines:

import { createVectorDB, wrapVectorDB, encryptionMiddleware, deriveEncryptionKey, embed } from '@localmode/core';
import { transformers } from '@localmode/transformers';

// Derive key from password
const { key, salt } = await deriveEncryptionKey('master-password');

// Create encrypted VectorDB
const db = await createVectorDB({ name: 'private-notes', dimensions: 384 });
const encryptedDb = wrapVectorDB({
  db,
  middleware: encryptionMiddleware({ key }),
});

// Embed and store -- encryption is automatic
const model = transformers.embedding('Xenova/bge-small-en-v1.5');
const { embedding } = await embed({ model, value: 'Confidential document text' });
await encryptedDb.add({ id: '1', vector: embedding, metadata: { text: 'Confidential document text' } });

// Search results are decrypted transparently
const results = await encryptedDb.search(embedding, { topK: 5 });

For a complete working application, explore the Encrypted Vault showcase app, which demonstrates vault creation, password-based unlock, entry encryption/decryption, and locking -- all built on the stack described in this post.