diff --git a/.env.example b/.env.example
index 0259bbd7..29ac0184 100644
--- a/.env.example
+++ b/.env.example
@@ -71,5 +71,13 @@ TELEMETRY_ALERT_WEBHOOK_URL=
 TELEMETRY_GEO_API_URL=http://ip-api.com/json
 TELEMETRY_EVENT_TTL_DAYS=90
 
+# ── Field Encryption (@bytelyst/field-encrypt) ──────────────
+# Key provider: 'akv' (production) | 'env' (dev/staging) | 'memory' (tests)
+FIELD_ENCRYPT_KEY_PROVIDER=memory
+# Hex-encoded 32-byte key — only for 'env' provider (like AUTH_TOTP_ENCRYPTION_KEY)
+FIELD_ENCRYPT_KEY=
+# Product-specific MEK name in AKV — only for 'akv' provider
+FIELD_ENCRYPT_MEK_NAME=lysnr-mek
+
 # ── Product Identity ──────────────────────────────────────────
 DEFAULT_PRODUCT_ID=lysnrai
diff --git a/packages/field-encrypt/package.json b/packages/field-encrypt/package.json
new file mode 100644
index 00000000..92fed44c
--- /dev/null
+++ b/packages/field-encrypt/package.json
@@ -0,0 +1,40 @@
+{
+  "name": "@bytelyst/field-encrypt",
+  "version": "0.1.0",
+  "type": "module",
+  "exports": {
+    ".": {
+      "import": "./dist/index.js",
+      "types": "./dist/index.d.ts"
+    }
+  },
+  "main": "./dist/index.js",
+  "types": "./dist/index.d.ts",
+  "files": [
+    "dist"
+  ],
+  "scripts": {
+    "build": "tsc",
+    "test": "vitest run"
+  },
+  "dependencies": {
+    "@bytelyst/errors": "workspace:*"
+  },
+  "peerDependencies": {
+    "@azure/keyvault-keys": ">=4.8.0",
+    "@azure/identity": ">=4.0.0",
+    "zod": ">=3.22.0"
+  },
+  "peerDependenciesMeta": {
+    "@azure/keyvault-keys": {
+      "optional": true
+    },
+    "@azure/identity": {
+      "optional": true
+    }
+  },
+  "devDependencies": {
+    "vitest": "^3.0.0",
+    "zod": "^3.24.0"
+  }
+}
diff --git a/packages/field-encrypt/src/aes-gcm.ts b/packages/field-encrypt/src/aes-gcm.ts
new file mode 100644
index 00000000..4eedc37a
--- /dev/null
+++ b/packages/field-encrypt/src/aes-gcm.ts
@@ -0,0 +1,89 @@
+/**
+ * @bytelyst/field-encrypt — AES-256-GCM primitives
+ *
+ * Low-level encrypt/decrypt using Node.js native crypto.
+ * All higher-level APIs delegate to these functions.
+ */
+
+import { createCipheriv, createDecipheriv, randomBytes } from 'node:crypto';
+import type { EncryptedField } from './types.js';
+
+const ALGORITHM = 'aes-256-gcm';
+const IV_BYTES = 12;
+const KEY_BYTES = 32;
+
+/**
+ * Encrypt a plaintext string with AES-256-GCM.
+ *
+ * @param plaintext - UTF-8 string to encrypt
+ * @param key       - 32-byte AES key
+ * @param dekId     - DEK identifier stored in the output
+ * @param aad       - Optional additional authenticated data (e.g., userId + context)
+ * @returns EncryptedField object ready for Cosmos/SQLite storage
+ */
+export function encryptField(
+  plaintext: string,
+  key: Buffer,
+  dekId: string,
+  aad?: string
+): EncryptedField {
+  if (key.length !== KEY_BYTES) {
+    throw new Error(`AES-256-GCM requires a ${KEY_BYTES}-byte key, got ${key.length}`);
+  }
+
+  const iv = randomBytes(IV_BYTES);
+  const cipher = createCipheriv(ALGORITHM, key, iv);
+
+  if (aad) {
+    cipher.setAAD(Buffer.from(aad, 'utf8'));
+  }
+
+  let encrypted = cipher.update(plaintext, 'utf8', 'hex');
+  encrypted += cipher.final('hex');
+  const authTag = cipher.getAuthTag();
+
+  return {
+    __encrypted: true,
+    v: 1,
+    alg: 'aes-256-gcm',
+    ct: encrypted,
+    iv: iv.toString('hex'),
+    tag: authTag.toString('hex'),
+    dekId,
+  };
+}
+
+/**
+ * Decrypt an EncryptedField back to plaintext.
+ *
+ * @param field - EncryptedField object
+ * @param key   - 32-byte AES key (must match the key used to encrypt)
+ * @param aad   - Optional AAD (must match the AAD used during encryption)
+ * @returns Decrypted UTF-8 string
+ * @throws Error if authentication tag verification fails (tampered data)
+ */
+export function decryptField(field: EncryptedField, key: Buffer, aad?: string): string {
+  if (key.length !== KEY_BYTES) {
+    throw new Error(`AES-256-GCM requires a ${KEY_BYTES}-byte key, got ${key.length}`);
+  }
+
+  const iv = Buffer.from(field.iv, 'hex');
+  const authTag = Buffer.from(field.tag, 'hex');
+  const decipher = createDecipheriv(ALGORITHM, key, iv);
+
+  decipher.setAuthTag(authTag);
+
+  if (aad) {
+    decipher.setAAD(Buffer.from(aad, 'utf8'));
+  }
+
+  let decrypted = decipher.update(field.ct, 'hex', 'utf8');
+  decrypted += decipher.final('utf8');
+
+  return decrypted;
+}
+
+/** Generate a random 32-byte AES-256 key. */
+export function generateAesKey(): Buffer {
+  return randomBytes(KEY_BYTES);
+}
diff --git a/packages/field-encrypt/src/dek-store-memory.ts b/packages/field-encrypt/src/dek-store-memory.ts
new file mode 100644
index 00000000..79aabc36
--- /dev/null
+++ b/packages/field-encrypt/src/dek-store-memory.ts
@@ -0,0 +1,27 @@
+/**
+ * @bytelyst/field-encrypt — In-memory DEK store
+ *
+ * Default DEK store for dev/test. Production should use a Cosmos-backed store.
+ */
+
+import type { DekStore, WrappedDek } from './types.js';
+
+export class MemoryDekStore implements DekStore {
+  private readonly deks = new Map<string, WrappedDek>();
+
+  async get(dekId: string): Promise<WrappedDek | null> {
+    return this.deks.get(dekId) ?? null;
+  }
+
+  async put(dek: WrappedDek): Promise<void> {
+    this.deks.set(dek.dekId, dek);
+  }
+
+  async listIds(): Promise<string[]> {
+    return [...this.deks.keys()];
+  }
+
+  async delete(dekId: string): Promise<void> {
+    this.deks.delete(dekId);
+  }
+}
diff --git a/packages/field-encrypt/src/envelope.ts b/packages/field-encrypt/src/envelope.ts
new file mode 100644
index 00000000..a062db30
--- /dev/null
+++ b/packages/field-encrypt/src/envelope.ts
@@ -0,0 +1,107 @@
+/**
+ * @bytelyst/field-encrypt — Envelope encryption
+ *
+ * DEK lifecycle: generate → wrap with MEK → store.
+ * On use: load wrapped DEK → unwrap with MEK → cache → use for AES-GCM.
+ */
+
+import type { KeyProvider, DekStore, WrappedDek } from './types.js';
+import { generateAesKey } from './aes-gcm.js';
+import { DekCache } from './key-cache.js';
+
+/**
+ * Build a deterministic DEK ID from userId + context.
+ * Format: `dek_{userId}_{context}`
+ */
+export function buildDekId(userId: string, context: string): string {
+  return `dek_${userId}_${context}`;
+}
+
+/**
+ * Get or create a DEK for the given scope.
+ *
+ * 1. Check cache → return if found
+ * 2. Check DEK store → unwrap + cache if found
+ * 3. Generate new DEK → wrap → store → cache → return
+ */
+export async function getOrCreateDek(
+  dekId: string,
+  keyProvider: KeyProvider,
+  dekStore: DekStore,
+  cache: DekCache
+): Promise<Buffer> {
+  // 1. Cache hit
+  const cached = cache.get(dekId);
+  if (cached) {
+    cache.recordHit();
+    return cached;
+  }
+  cache.recordMiss();
+
+  // 2. DEK store hit — unwrap + cache
+  const stored = await dekStore.get(dekId);
+  if (stored) {
+    const dek = await keyProvider.unwrapKey(stored.wrappedKey, stored.mekVersion);
+    cache.set(dekId, dek);
+    return dek;
+  }
+
+  // 3. Generate new DEK → wrap → store → cache
+  const dek = generateAesKey();
+  const { wrappedKey, mekVersion } = await keyProvider.wrapKey(dek);
+
+  const wrappedDek: WrappedDek = {
+    dekId,
+    wrappedKey,
+    mekVersion,
+    createdAt: new Date().toISOString(),
+  };
+  await dekStore.put(wrappedDek);
+
+  cache.set(dekId, dek);
+  return dek;
+}
+
+/**
+ * Re-wrap all DEKs after MEK rotation.
+ *
+ * Reads each wrapped DEK, unwraps with old MEK, wraps with new MEK, stores updated.
+ */
+export async function rewrapAllDeks(
+  oldKeyProvider: KeyProvider,
+  newKeyProvider: KeyProvider,
+  dekStore: DekStore,
+  cache: DekCache,
+  onProgress?: (completed: number, total: number) => void
+): Promise<number> {
+  const dekIds = await dekStore.listIds();
+  let completed = 0;
+
+  for (const dekId of dekIds) {
+    const stored = await dekStore.get(dekId);
+    if (!stored) continue;
+
+    // Unwrap with old MEK
+    const rawDek = await oldKeyProvider.unwrapKey(stored.wrappedKey, stored.mekVersion);
+
+    // Wrap with new MEK
+    const { wrappedKey, mekVersion } = await newKeyProvider.wrapKey(rawDek);
+
+    // Store updated wrapped DEK
+    const updated: WrappedDek = {
+      dekId,
+      wrappedKey,
+      mekVersion,
+      createdAt: stored.createdAt,
+    };
+    await dekStore.put(updated);
+
+    // Invalidate cache entry so it gets re-unwrapped with new MEK next time
+    cache.invalidate(dekId);
+
+    completed++;
+    onProgress?.(completed, dekIds.length);
+  }
+
+  return completed;
+}
diff --git a/packages/field-encrypt/src/field-encryptor.ts b/packages/field-encrypt/src/field-encryptor.ts
new file mode 100644
index 00000000..7ee91102
--- /dev/null
+++ b/packages/field-encrypt/src/field-encryptor.ts
@@ -0,0 +1,171 @@
+/**
+ * @bytelyst/field-encrypt — FieldEncryptor
+ *
+ * Main API — wires key provider + DEK store + cache + AES-GCM.
+ * Product backends create a singleton via createFieldEncryptor().
+ */
+
+import type {
+  EncryptedField,
+  FieldEncryptContext,
+  FieldEncryptorConfig,
+  KeyProvider,
+  DekStore,
+} from './types.js';
+import { encryptField, decryptField } from './aes-gcm.js';
+import { buildDekId, getOrCreateDek, rewrapAllDeks } from './envelope.js';
+import { DekCache } from './key-cache.js';
+import { MemoryDekStore } from './dek-store-memory.js';
+import { MemoryKeyProvider } from './key-provider-memory.js';
+import { EnvKeyProvider } from './key-provider-env.js';
+import { AkvKeyProvider } from './key-provider-akv.js';
+import { isEncryptedField } from './guards.js';
+
+export class FieldEncryptor {
+  private readonly keyProvider: KeyProvider;
+  private readonly dekStore: DekStore;
+  private readonly cache: DekCache;
+
+  constructor(config: FieldEncryptorConfig) {
+    this.keyProvider = resolveKeyProvider(config);
+    this.dekStore = config.dekStore ?? new MemoryDekStore();
+    this.cache = new DekCache(
+      config.dekCacheTtlMs ?? 15 * 60 * 1000,
+      config.dekCacheMaxSize ?? 1000
+    );
+  }
+
+  /**
+   * Encrypt a plaintext string.
+   *
+   * Automatically gets or creates a DEK scoped to the userId + context.
+   */
+  async encrypt(plaintext: string, ctx: FieldEncryptContext): Promise<EncryptedField> {
+    const dekId = buildDekId(ctx.userId, ctx.context);
+    const aad = `${ctx.userId}:${ctx.context}`;
+    const dek = await getOrCreateDek(dekId, this.keyProvider, this.dekStore, this.cache);
+    return encryptField(plaintext, dek, dekId, aad);
+  }
+
+  /**
+   * Decrypt an EncryptedField back to plaintext.
+   */
+  async decrypt(field: EncryptedField, ctx: FieldEncryptContext): Promise<string> {
+    const aad = `${ctx.userId}:${ctx.context}`;
+    const dek = await getOrCreateDek(field.dekId, this.keyProvider, this.dekStore, this.cache);
+    return decryptField(field, dek, aad);
+  }
+
+  /**
+   * Encrypt multiple fields in a single call (optimized — single DEK lookup).
+   */
+  async encryptBatch(plaintexts: string[], ctx: FieldEncryptContext): Promise<EncryptedField[]> {
+    const dekId = buildDekId(ctx.userId, ctx.context);
+    const aad = `${ctx.userId}:${ctx.context}`;
+    const dek = await getOrCreateDek(dekId, this.keyProvider, this.dekStore, this.cache);
+    return plaintexts.map(pt => encryptField(pt, dek, dekId, aad));
+  }
+
+  /**
+   * Decrypt multiple EncryptedFields in a single call.
+   *
+   * Groups by dekId for efficient DEK lookup.
+   */
+  async decryptBatch(fields: EncryptedField[], ctx: FieldEncryptContext): Promise<string[]> {
+    const aad = `${ctx.userId}:${ctx.context}`;
+    const dekMap = new Map<string, Buffer>();
+
+    const results: string[] = [];
+    for (const field of fields) {
+      let dek = dekMap.get(field.dekId);
+      if (!dek) {
+        dek = await getOrCreateDek(field.dekId, this.keyProvider, this.dekStore, this.cache);
+        dekMap.set(field.dekId, dek);
+      }
+      results.push(decryptField(field, dek, aad));
+    }
+
+    return results;
+  }
+
+  /**
+   * Check if a value is an EncryptedField.
+   */
+  isEncrypted(value: unknown): value is EncryptedField {
+    return isEncryptedField(value);
+  }
+
+  /**
+   * Re-wrap all DEKs after MEK rotation.
+   */
+  async rewrapDeks(
+    newKeyProvider: KeyProvider,
+    onProgress?: (completed: number, total: number) => void
+  ): Promise<number> {
+    return rewrapAllDeks(this.keyProvider, newKeyProvider, this.dekStore, this.cache, onProgress);
+  }
+
+  /** DEK cache hit rate (0-100). */
+  get cacheHitRate(): number {
+    return this.cache.hitRate;
+  }
+
+  /** Number of cached DEKs. */
+  get cacheSize(): number {
+    return this.cache.size;
+  }
+
+  /** Reset cache statistics. */
+  resetCacheStats(): void {
+    this.cache.resetStats();
+  }
+
+  /** Clear DEK cache (e.g., on shutdown). */
+  clearCache(): void {
+    this.cache.clear();
+  }
+}
+
+function resolveKeyProvider(config: FieldEncryptorConfig): KeyProvider {
+  switch (config.keyProvider) {
+    case 'memory':
+      return new MemoryKeyProvider();
+
+    case 'env': {
+      const key = config.encryptionKey;
+      if (!key) {
+        throw new Error('FieldEncryptor: "env" key provider requires encryptionKey (hex string)');
+      }
+      return new EnvKeyProvider(key);
+    }
+
+    case 'akv': {
+      if (!config.keyVaultUrl) {
+        throw new Error('FieldEncryptor: "akv" key provider requires keyVaultUrl');
+      }
+      if (!config.mekName) {
+        throw new Error('FieldEncryptor: "akv" key provider requires mekName');
+      }
+      return new AkvKeyProvider(config.keyVaultUrl, config.mekName);
+    }
+
+    default:
+      throw new Error(`FieldEncryptor: unknown key provider "${config.keyProvider}"`);
+  }
+}
+
+/**
+ * Create a FieldEncryptor instance.
+ *
+ * Typical usage (one per backend service):
+ * ```typescript
+ * const encryptor = createFieldEncryptor({
+ *   keyProvider: config.FIELD_ENCRYPT_KEY_PROVIDER ?? 'memory',
+ *   mekName: 'lysnr-mek',
+ *   keyVaultUrl: config.AZURE_KEYVAULT_URL,
+ * });
+ * ```
+ */
+export function createFieldEncryptor(config: FieldEncryptorConfig): FieldEncryptor {
+  return new FieldEncryptor(config);
+}
diff --git a/packages/field-encrypt/src/guards.ts b/packages/field-encrypt/src/guards.ts
new file mode 100644
index 00000000..f8c63b8d
--- /dev/null
+++ b/packages/field-encrypt/src/guards.ts
@@ -0,0 +1,27 @@
+/**
+ * @bytelyst/field-encrypt — Type guards
+ *
+ * Utility to detect encrypted vs plaintext fields during migration.
+ */
+
+import type { EncryptedField } from './types.js';
+
+/**
+ * Check if a value is an EncryptedField.
+ *
+ * Use this in repositories to handle both encrypted and plaintext fields
+ * during the migration period.
+ */
+export function isEncryptedField(value: unknown): value is EncryptedField {
+  return (
+    typeof value === 'object' &&
+    value !== null &&
+    '__encrypted' in value &&
+    (value as Record<string, unknown>).__encrypted === true &&
+    'v' in value &&
+    'ct' in value &&
+    'iv' in value &&
+    'tag' in value &&
+    'dekId' in value
+  );
+}
diff --git a/packages/field-encrypt/src/index.test.ts b/packages/field-encrypt/src/index.test.ts
new file mode 100644
index 00000000..0e3b4408
--- /dev/null
+++ b/packages/field-encrypt/src/index.test.ts
@@ -0,0 +1,429 @@
+/**
+ * @bytelyst/field-encrypt — Tests
+ *
+ * ~35 tests covering AES-GCM, key providers, envelope, cache,
+ * field encryptor factory, type guards, and migration.
+ */
+
+import { describe, it, expect, beforeEach } from 'vitest';
+import { randomBytes } from 'node:crypto';
+import {
+  createFieldEncryptor,
+  FieldEncryptor,
+  isEncryptedField,
+  encryptField,
+  decryptField,
+  generateAesKey,
+  buildDekId,
+  getOrCreateDek,
+  rewrapAllDeks,
+  DekCache,
+  MemoryDekStore,
+  MemoryKeyProvider,
+  EnvKeyProvider,
+  migrateDocuments,
+} from './index.js';
+import type { EncryptedField, FieldEncryptContext } from './types.js';
+
+// ── AES-256-GCM ─────────────────────────────────────
+
+describe('aes-gcm', () => {
+  const key = generateAesKey();
+  const dekId = 'dek_test_ctx';
+
+  it('encrypt → decrypt roundtrip', () => {
+    const plaintext = 'Hello, sensitive data!';
+    const encrypted = encryptField(plaintext, key, dekId);
+    const decrypted = decryptField(encrypted, key);
+    expect(decrypted).toBe(plaintext);
+  });
+
+  it('encrypt → decrypt with AAD', () => {
+    const plaintext = 'With AAD';
+    const aad = 'user_123:transcripts';
+    const encrypted = encryptField(plaintext, key, dekId, aad);
+    const decrypted = decryptField(encrypted, key, aad);
+    expect(decrypted).toBe(plaintext);
+  });
+
+  it('rejects decryption with wrong AAD', () => {
+    const encrypted = encryptField('secret', key, dekId, 'correct_aad');
+    expect(() => decryptField(encrypted, key, 'wrong_aad')).toThrow();
+  });
+
+  it('rejects decryption with tampered ciphertext', () => {
+    const encrypted = encryptField('secret', key, dekId);
+    const tampered: EncryptedField = { ...encrypted, ct: 'deadbeef' };
+    expect(() => decryptField(tampered, key)).toThrow();
+  });
+
+  it('rejects decryption with tampered auth tag', () => {
+    const encrypted = encryptField('secret', key, dekId);
+    const tampered: EncryptedField = { ...encrypted, tag: '00'.repeat(16) };
+    expect(() => decryptField(tampered, key)).toThrow();
+  });
+
+  it('handles empty string', () => {
+    const encrypted = encryptField('', key, dekId);
+    const decrypted = decryptField(encrypted, key);
+    expect(decrypted).toBe('');
+  });
+
+  it('handles unicode content', () => {
+    const plaintext = '日本語テスト 🔐 Ñoño';
+    const encrypted = encryptField(plaintext, key, dekId);
+    const decrypted = decryptField(encrypted, key);
+    expect(decrypted).toBe(plaintext);
+  });
+
+  it('handles large payload (100 KB)', () => {
+    const plaintext = 'x'.repeat(100_000);
+    const encrypted = encryptField(plaintext, key, dekId);
+    const decrypted = decryptField(encrypted, key);
+    expect(decrypted).toBe(plaintext);
+  });
+
+  it('rejects wrong key size', () => {
+    const shortKey = randomBytes(16);
+    expect(() => encryptField('test', shortKey, dekId)).toThrow(/32-byte key/);
+  });
+
+  it('produces correct EncryptedField shape', () => {
+    const encrypted = encryptField('test', key, dekId);
+    expect(encrypted.__encrypted).toBe(true);
+    expect(encrypted.v).toBe(1);
+    expect(encrypted.alg).toBe('aes-256-gcm');
+    expect(encrypted.dekId).toBe(dekId);
+    expect(encrypted.iv).toHaveLength(24); // 12 bytes = 24 hex chars
+    expect(encrypted.tag).toHaveLength(32); // 16 bytes = 32 hex chars
+    expect(encrypted.ct.length).toBeGreaterThan(0);
+  });
+
+  it('generates unique IVs per encryption', () => {
+    const e1 = encryptField('same', key, dekId);
+    const e2 = encryptField('same', key, dekId);
+    expect(e1.iv).not.toBe(e2.iv);
+    expect(e1.ct).not.toBe(e2.ct);
+  });
+});
+
+// ── Type guard ──────────────────────────────────────
+
+describe('isEncryptedField', () => {
+  it('returns true for valid EncryptedField', () => {
+    const field: EncryptedField = {
+      __encrypted: true,
+      v: 1,
+      alg: 'aes-256-gcm',
+      ct: 'abc',
+      iv: '012',
+      tag: '345',
+      dekId: 'dek_1',
+    };
+    expect(isEncryptedField(field)).toBe(true);
+  });
+
+  it('returns false for string', () => {
+    expect(isEncryptedField('hello')).toBe(false);
+  });
+
+  it('returns false for null', () => {
+    expect(isEncryptedField(null)).toBe(false);
+  });
+
+  it('returns false for undefined', () => {
+    expect(isEncryptedField(undefined)).toBe(false);
+  });
+
+  it('returns false for object without __encrypted', () => {
+    expect(isEncryptedField({ v: 1, ct: 'abc' })).toBe(false);
+  });
+
+  it('returns false for object with __encrypted: false', () => {
+    expect(
+      isEncryptedField({ __encrypted: false, v: 1, ct: 'a', iv: 'b', tag: 'c', dekId: 'd' })
+    ).toBe(false);
+  });
+});
+
+// ── Key providers ───────────────────────────────────
+
+describe('MemoryKeyProvider', () => {
+  it('wrap → unwrap roundtrip', async () => {
+    const provider = new MemoryKeyProvider();
+    const dek = generateAesKey();
+    const { wrappedKey, mekVersion } = await provider.wrapKey(dek);
+    const unwrapped = await provider.unwrapKey(wrappedKey, mekVersion);
+    expect(unwrapped).toEqual(dek);
+  });
+});
+
+describe('EnvKeyProvider', () => {
+  it('wrap → unwrap roundtrip with 64-char hex key', async () => {
+    const hexKey = randomBytes(32).toString('hex');
+    const provider = new EnvKeyProvider(hexKey);
+    const dek = generateAesKey();
+    const { wrappedKey, mekVersion } = await provider.wrapKey(dek);
+    const unwrapped = await provider.unwrapKey(wrappedKey, mekVersion);
+    expect(unwrapped).toEqual(dek);
+  });
+
+  it('wrap → unwrap roundtrip with short key (hashed to 32 bytes)', async () => {
+    const provider = new EnvKeyProvider('my-dev-secret-key');
+    const dek = generateAesKey();
+    const { wrappedKey, mekVersion } = await provider.wrapKey(dek);
+    const unwrapped = await provider.unwrapKey(wrappedKey, mekVersion);
+    expect(unwrapped).toEqual(dek);
+  });
+
+  it('throws on empty key', () => {
+    expect(() => new EnvKeyProvider('')).toThrow(/must not be empty/);
+  });
+});
+
+// ── DEK cache ───────────────────────────────────────
+
+describe('DekCache', () => {
+  let cache: DekCache;
+
+  beforeEach(() => {
+    cache = new DekCache(1000, 3); // 1s TTL, max 3 entries
+  });
+
+  it('get returns null on miss', () => {
+    expect(cache.get('nonexistent')).toBeNull();
+  });
+
+  it('set + get roundtrip', () => {
+    const key = generateAesKey();
+    cache.set('dek_1', key);
+    expect(cache.get('dek_1')).toEqual(key);
+  });
+
+  it('expires entries after TTL', async () => {
+    const shortCache = new DekCache(50, 100); // 50ms TTL
+    const key = generateAesKey();
+    shortCache.set('dek_1', key);
+    expect(shortCache.get('dek_1')).toEqual(key);
+    await new Promise(r => setTimeout(r, 60));
+    expect(shortCache.get('dek_1')).toBeNull();
+  });
+
+  it('evicts oldest on max size', () => {
+    cache.set('a', generateAesKey());
+    cache.set('b', generateAesKey());
+    cache.set('c', generateAesKey());
+    // At max size (3), adding 'd' should evict 'a'
+    cache.set('d', generateAesKey());
+    expect(cache.get('a')).toBeNull();
+    expect(cache.get('d')).not.toBeNull();
+  });
+
+  it('invalidate removes specific entry', () => {
+    cache.set('dek_1', generateAesKey());
+    cache.invalidate('dek_1');
+    expect(cache.get('dek_1')).toBeNull();
+  });
+
+  it('tracks hit rate', () => {
+    cache.set('dek_1', generateAesKey());
+    cache.recordHit();
+    cache.recordHit();
+    cache.recordMiss();
+    expect(cache.hitRate).toBe(67); // 2/3 = 67%
+  });
+});
+
+// ── Envelope ────────────────────────────────────────
+
+describe('envelope', () => {
+  it('buildDekId produces correct format', () => {
+    expect(buildDekId('user_123', 'transcripts')).toBe('dek_user_123_transcripts');
+  });
+
+  it('getOrCreateDek creates and caches a new DEK', async () => {
+    const provider = new MemoryKeyProvider();
+    const store = new MemoryDekStore();
+    const cache = new DekCache();
+
+    const dek = await getOrCreateDek('dek_u1_ctx', provider, store, cache);
+    expect(dek).toHaveLength(32);
+
+    // Should be in store
+    const stored = await store.get('dek_u1_ctx');
+    expect(stored).not.toBeNull();
+
+    // Should be cached — second call should return same key
+    const dek2 = await getOrCreateDek('dek_u1_ctx', provider, store, cache);
+    expect(dek2).toEqual(dek);
+  });
+
+  it('rewrapAllDeks re-wraps with new provider', async () => {
+    const oldProvider = new MemoryKeyProvider(undefined, 'old-v1');
+    const newProvider = new MemoryKeyProvider(undefined, 'new-v1');
+    const store = new MemoryDekStore();
+    const cache = new DekCache();
+
+    // Create 3 DEKs with old provider
+    await getOrCreateDek('dek_1', oldProvider, store, cache);
+    await getOrCreateDek('dek_2', oldProvider, store, cache);
+    await getOrCreateDek('dek_3', oldProvider, store, cache);
+
+    // Re-wrap
+    const count = await rewrapAllDeks(oldProvider, newProvider, store, cache);
+    expect(count).toBe(3);
+
+    // Verify new provider can unwrap
+    const stored = await store.get('dek_1');
+    expect(stored).not.toBeNull();
+    expect(stored!.mekVersion).toBe('new-v1');
+
+    const unwrapped = await newProvider.unwrapKey(stored!.wrappedKey, stored!.mekVersion);
+    expect(unwrapped).toHaveLength(32);
+  });
+});
+
+// ── FieldEncryptor (integration) ────────────────────
+
+describe('FieldEncryptor', () => {
+  let encryptor: FieldEncryptor;
+  const ctx: FieldEncryptContext = { userId: 'user_42', context: 'notes' };
+
+  beforeEach(() => {
+    encryptor = createFieldEncryptor({ keyProvider: 'memory' });
+  });
+
+  it('encrypt → decrypt roundtrip', async () => {
+    const encrypted = await encryptor.encrypt('Hello World', ctx);
+    expect(encrypted.__encrypted).toBe(true);
+    const decrypted = await encryptor.decrypt(encrypted, ctx);
+    expect(decrypted).toBe('Hello World');
+  });
+
+  it('encryptBatch → decryptBatch roundtrip', async () => {
+    const plaintexts = ['one', 'two', 'three'];
+    const encrypted = await encryptor.encryptBatch(plaintexts, ctx);
+    expect(encrypted).toHaveLength(3);
+    const decrypted = await encryptor.decryptBatch(encrypted, ctx);
+    expect(decrypted).toEqual(plaintexts);
+  });
+
+  it('isEncrypted works via encryptor', async () => {
+    const encrypted = await encryptor.encrypt('test', ctx);
+    expect(encryptor.isEncrypted(encrypted)).toBe(true);
+    expect(encryptor.isEncrypted('plaintext')).toBe(false);
+  });
+
+  it('different users get different DEKs', async () => {
+    const ctx1: FieldEncryptContext = { userId: 'user_1', context: 'notes' };
+    const ctx2: FieldEncryptContext = { userId: 'user_2', context: 'notes' };
+
+    const e1 = await encryptor.encrypt('same text', ctx1);
+    const e2 = await encryptor.encrypt('same text', ctx2);
+
+    expect(e1.dekId).not.toBe(e2.dekId);
+    expect(e1.ct).not.toBe(e2.ct);
+  });
+
+  it('JSON-serialized array encryption roundtrip', async () => {
+    const transcript = [
+      { role: 'user', content: 'Hello', ts: '2026-01-01T00:00:00Z' },
+      { role: 'agent', content: 'Hi there!', ts: '2026-01-01T00:00:01Z' },
+    ];
+    const serialized = JSON.stringify(transcript);
+    const encrypted = await encryptor.encrypt(serialized, ctx);
+    const decrypted = await encryptor.decrypt(encrypted, ctx);
+    expect(JSON.parse(decrypted)).toEqual(transcript);
+  });
+});
+
+// ── Factory config validation ───────────────────────
+
+describe('createFieldEncryptor config', () => {
+  it('throws on unknown provider', () => {
+    expect(() => createFieldEncryptor({ keyProvider: 'nope' as never })).toThrow(/unknown/);
+  });
+
+  it('throws on env provider without key', () => {
+    expect(() => createFieldEncryptor({ keyProvider: 'env' })).toThrow(/encryptionKey/);
+  });
+
+  it('throws on akv provider without vaultUrl', () => {
+    expect(() => createFieldEncryptor({ keyProvider: 'akv', mekName: 'mek' })).toThrow(
+      /keyVaultUrl/
+    );
+  });
+
+  it('throws on akv provider without mekName', () => {
+    expect(() =>
+      createFieldEncryptor({ keyProvider: 'akv', keyVaultUrl: 'https://kv.vault.azure.net' })
+    ).toThrow(/mekName/);
+  });
+
+  it('env provider works with hex key', async () => {
+    const hexKey = randomBytes(32).toString('hex');
+    const enc = createFieldEncryptor({ keyProvider: 'env', encryptionKey: hexKey });
+    const ctx: FieldEncryptContext = { userId: 'u1', context: 'test' };
+    const encrypted = await enc.encrypt('secret', ctx);
+    const decrypted = await enc.decrypt(encrypted, ctx);
+    expect(decrypted).toBe('secret');
+  });
+});
+
+// ── Migration ───────────────────────────────────────
+
+describe('migrateDocuments', () => {
+  it('encrypts plaintext fields and skips already-encrypted', async () => {
+    const encryptor = createFieldEncryptor({ keyProvider: 'memory' });
+    const ctx: FieldEncryptContext = { userId: 'u1', context: 'notes' };
+
+    const alreadyEncrypted = await encryptor.encrypt('old', ctx);
+    const docs = [
+      { id: '1', body: 'plaintext note' },
+      { id: '2', body: alreadyEncrypted },
+      { id: '3', body: 'another note' },
+      { id: '4', body: null },
+    ];
+
+    const written: Array<{ id: string; body: EncryptedField }> = [];
+
+    const result = await migrateDocuments({
+      fetchBatch: async (offset, batchSize) => docs.slice(offset, offset + batchSize),
+      getId: doc => doc.id,
+      getField: doc => doc.body,
+      encryptValue: plaintext => encryptor.encrypt(plaintext, ctx),
+      writeBack: async (doc, encrypted) => {
+        written.push({ id: doc.id, body: encrypted });
+      },
+      batchSize: 10,
+    });
+
+    expect(result.scanned).toBe(4);
+    expect(result.encrypted).toBe(2); // id 1 + id 3
+    expect(result.skipped).toBe(2); // id 2 (already encrypted) + id 4 (null)
+    expect(result.errors).toBe(0);
+    expect(written).toHaveLength(2);
+  });
+
+  it('dry run does not write', async () => {
+    const encryptor = createFieldEncryptor({ keyProvider: 'memory' });
+    const ctx: FieldEncryptContext = { userId: 'u1', context: 'notes' };
+
+    const docs = [{ id: '1', body: 'plaintext' }];
+    let writeCount = 0;
+
+    const result = await migrateDocuments({
+      fetchBatch: async (offset, batchSize) => docs.slice(offset, offset + batchSize),
+      getId: doc => doc.id,
+      getField: doc => doc.body,
+      encryptValue: plaintext => encryptor.encrypt(plaintext, ctx),
+      writeBack: async () => {
+        writeCount++;
+      },
+      dryRun: true,
+    });
+
+    expect(result.encrypted).toBe(1);
+    expect(writeCount).toBe(0);
+  });
+});
diff --git a/packages/field-encrypt/src/index.ts b/packages/field-encrypt/src/index.ts
new file mode 100644
index 00000000..5c5d22c0
--- /dev/null
+++ b/packages/field-encrypt/src/index.ts
@@ -0,0 +1,57 @@
+/**
+ * @bytelyst/field-encrypt
+ *
+ * Application-layer field encryption for ByteLyst ecosystem.
+ * AES-256-GCM with envelope encryption (MEK → DEK).
+ *
+ * @example
+ * ```typescript
+ * import { createFieldEncryptor } from '@bytelyst/field-encrypt';
+ *
+ * const encryptor = createFieldEncryptor({
+ *   keyProvider: 'memory', // 'akv' | 'env' | 'memory'
+ * });
+ *
+ * const encrypted = await encryptor.encrypt('sensitive data', {
+ *   userId: 'user_123',
+ *   context: 'transcripts',
+ * });
+ *
+ * const plaintext = await encryptor.decrypt(encrypted, {
+ *   userId: 'user_123',
+ *   context: 'transcripts',
+ * });
+ * ```
+ */
+
+// ── Main API ────────────────────────────────────────
+export { createFieldEncryptor, FieldEncryptor } from './field-encryptor.js';
+
+// ── Type guards ─────────────────────────────────────
+export { isEncryptedField } from './guards.js';
+
+// ── Types ───────────────────────────────────────────
+export type {
+  EncryptedField,
+  WrappedDek,
+  FieldEncryptContext,
+  FieldEncryptorConfig,
+  KeyProvider,
+  KeyProviderType,
+  DekStore,
+} from './types.js';
+
+// ── Low-level (for custom integrations) ─────────────
+export { encryptField, decryptField, generateAesKey } from './aes-gcm.js';
+export { buildDekId, getOrCreateDek, rewrapAllDeks } from './envelope.js';
+export { DekCache } from './key-cache.js';
+export { MemoryDekStore } from './dek-store-memory.js';
+
+// ── Key providers (for direct use / testing) ────────
+export { MemoryKeyProvider } from './key-provider-memory.js';
+export { EnvKeyProvider } from './key-provider-env.js';
+export { AkvKeyProvider } from './key-provider-akv.js';
+
+// ── Migration ───────────────────────────────────────
+export { migrateDocuments } from './migration.js';
+export type { MigrationResult, MigrateDocumentsOptions } from './migration.js';
diff --git a/packages/field-encrypt/src/key-cache.ts b/packages/field-encrypt/src/key-cache.ts
new file mode 100644
index 00000000..86682525
--- /dev/null
+++ b/packages/field-encrypt/src/key-cache.ts
@@ -0,0 +1,94 @@
+/**
+ * @bytelyst/field-encrypt — DEK cache
+ *
+ * In-memory LRU cache with TTL for unwrapped DEKs.
+ * Avoids repeated AKV round-trips on every encrypt/decrypt.
+ */
+
+interface CacheEntry {
+  key: Buffer;
+  expiresAt: number;
+}
+
+export class DekCache {
+  private readonly cache = new Map<string, CacheEntry>();
+  private readonly ttlMs: number;
+  private readonly maxSize: number;
+
+  constructor(ttlMs: number = 15 * 60 * 1000, maxSize: number = 1000) {
+    this.ttlMs = ttlMs;
+    this.maxSize = maxSize;
+  }
+
+  /** Get an unwrapped DEK from cache. Returns null on miss or expiry. */
+  get(dekId: string): Buffer | null {
+    const entry = this.cache.get(dekId);
+    if (!entry) return null;
+
+    if (Date.now() > entry.expiresAt) {
+      this.cache.delete(dekId);
+      return null;
+    }
+
+    // Move to end (LRU refresh)
+    this.cache.delete(dekId);
+    this.cache.set(dekId, entry);
+    return entry.key;
+  }
+
+  /** Store an unwrapped DEK in cache. */
+  set(dekId: string, key: Buffer): void {
+    // Evict oldest if at max size
+    if (this.cache.size >= this.maxSize && !this.cache.has(dekId)) {
+      const oldestKey = this.cache.keys().next().value;
+      if (oldestKey !== undefined) {
+        this.cache.delete(oldestKey);
+      }
+    }
+
+    this.cache.set(dekId, {
+      key,
+      expiresAt: Date.now() + this.ttlMs,
+    });
+  }
+
+  /** Invalidate a specific DEK (e.g., after rotation). */
+  invalidate(dekId: string): void {
+    this.cache.delete(dekId);
+  }
+
+  /** Clear all cached DEKs. */
+  clear(): void {
+    this.cache.clear();
+  }
+
+  /** Current cache size. */
+  get size(): number {
+    return this.cache.size;
+  }
+
+  /** Cache hit rate stats. */
+  private _hits = 0;
+  private _misses = 0;
+
+  /** Record a cache hit (called internally). */
+  recordHit(): void {
+    this._hits++;
+  }
+  /** Record a cache miss (called internally). */
+  recordMiss(): void {
+    this._misses++;
+  }
+
+  /** Get hit rate as a percentage (0-100). */
+  get hitRate(): number {
+    const total = this._hits + this._misses;
+    return total === 0 ? 0 : Math.round((this._hits / total) * 100);
+  }
+
+  /** Reset stats counters. */
+  resetStats(): void {
+    this._hits = 0;
+    this._misses = 0;
+  }
+}
diff --git a/packages/field-encrypt/src/key-provider-akv.ts b/packages/field-encrypt/src/key-provider-akv.ts
new file mode 100644
index 00000000..e2a9ff5c
--- /dev/null
+++ b/packages/field-encrypt/src/key-provider-akv.ts
@@ -0,0 +1,68 @@
+/**
+ * @bytelyst/field-encrypt — Azure Key Vault key provider
+ *
+ * Production provider — uses AKV RSA keys for DEK wrapping.
+ * Requires @azure/keyvault-keys and @azure/identity as peer deps.
+ */
+
+import type { KeyProvider } from './types.js';
+
+/**
+ * Azure Key Vault key provider.
+ *
+ * Uses RSA-OAEP wrapping — the MEK never leaves AKV.
+ * Requires:
+ *   - @azure/keyvault-keys (CryptographyClient)
+ *   - @azure/identity (DefaultAzureCredential)
+ *   - AKV RBAC: Key Vault Crypto User role on the managed identity
+ */
+export class AkvKeyProvider implements KeyProvider {
+  private readonly vaultUrl: string;
+  private readonly mekName: string;
+  private cryptoClient: unknown | null = null;
+
+  constructor(vaultUrl: string, mekName: string) {
+    if (!vaultUrl) throw new Error('AkvKeyProvider: vaultUrl is required');
+    if (!mekName) throw new Error('AkvKeyProvider: mekName is required');
+    this.vaultUrl = vaultUrl;
+    this.mekName = mekName;
+  }
+
+  private async getClient(): Promise<{
+    wrapKey(alg: string, key: Uint8Array): Promise<{ result: Uint8Array }>;
+    unwrapKey(alg: string, key: Uint8Array): Promise<{ result: Uint8Array }>;
+  }> {
+    if (this.cryptoClient) return this.cryptoClient as never;
+
+    // Dynamic import to keep peer deps optional
+    const { KeyClient, CryptographyClient } = await import('@azure/keyvault-keys');
+    const { DefaultAzureCredential } = await import('@azure/identity');
+
+    const credential = new DefaultAzureCredential();
+    const keyClient = new KeyClient(this.vaultUrl, credential);
+    const key = await keyClient.getKey(this.mekName);
+
+    if (!key.id) {
+      throw new Error(`AkvKeyProvider: MEK '${this.mekName}' not found in ${this.vaultUrl}`);
+    }
+
+    this.cryptoClient = new CryptographyClient(key.id, credential);
+    return this.cryptoClient as never;
+  }
+
+  async wrapKey(dek: Buffer): Promise<{ wrappedKey: string; mekVersion: string }> {
+    const client = await this.getClient();
+    const result = await client.wrapKey('RSA-OAEP-256', new Uint8Array(dek));
+    return {
+      wrappedKey: Buffer.from(result.result).toString('hex'),
+      mekVersion: this.mekName,
+    };
+  }
+
+  async unwrapKey(wrappedKeyHex: string, _mekVersion: string): Promise<Buffer> {
+    const client = await this.getClient();
+    const wrappedBytes = new Uint8Array(Buffer.from(wrappedKeyHex, 'hex'));
+    const result = await client.unwrapKey('RSA-OAEP-256', wrappedBytes);
+    return Buffer.from(result.result);
+  }
+}
diff --git a/packages/field-encrypt/src/key-provider-env.ts b/packages/field-encrypt/src/key-provider-env.ts
new file mode 100644
index 00000000..4b7de088
--- /dev/null
+++ b/packages/field-encrypt/src/key-provider-env.ts
@@ -0,0 +1,62 @@
+/**
+ * @bytelyst/field-encrypt — Environment variable key provider
+ *
+ * For dev/staging — uses a hex-encoded symmetric key from an env var.
+ * Matches the existing MFA pattern (AUTH_TOTP_ENCRYPTION_KEY).
+ *
+ * Wrapping uses AES-256-GCM with the env key as MEK.
+ */
+
+import { createCipheriv, createDecipheriv, randomBytes, createHash } from 'node:crypto';
+import type { KeyProvider } from './types.js';
+
+const ALGORITHM = 'aes-256-gcm';
+const IV_BYTES = 12;
+
+export class EnvKeyProvider implements KeyProvider {
+  private readonly mek: Buffer;
+  private readonly version: string;
+
+  /**
+   * @param keyHex - Hex-encoded 32-byte key (64 hex chars).
+   *                 If shorter, it will be SHA-256 hashed to derive a 32-byte key.
+   */
+  constructor(keyHex: string) {
+    if (!keyHex || keyHex.length === 0) {
+      throw new Error('EnvKeyProvider: encryption key must not be empty');
+    }
+
+    if (keyHex.length === 64) {
+      this.mek = Buffer.from(keyHex, 'hex');
+    } else {
+      // Hash to 32 bytes — same approach as existing MFA encryption
+      this.mek = createHash('sha256').update(keyHex).digest();
+    }
+
+    this.version = 'env-v1';
+  }
+
+  async wrapKey(dek: Buffer): Promise<{ wrappedKey: string; mekVersion: string }> {
+    const iv = randomBytes(IV_BYTES);
+    const cipher = createCipheriv(ALGORITHM, this.mek, iv);
+    let encrypted = cipher.update(dek);
+    encrypted = Buffer.concat([encrypted, cipher.final()]);
+    const tag = cipher.getAuthTag();
+
+    const wrapped = Buffer.concat([iv, tag, encrypted]);
+    return { wrappedKey: wrapped.toString('hex'), mekVersion: this.version };
+  }
+
+  async unwrapKey(wrappedKeyHex: string, _mekVersion: string): Promise<Buffer> {
+    const wrapped = Buffer.from(wrappedKeyHex, 'hex');
+    const iv = wrapped.subarray(0, IV_BYTES);
+    const tag = wrapped.subarray(IV_BYTES, IV_BYTES + 16);
+    const ciphertext = wrapped.subarray(IV_BYTES + 16);
+
+    const decipher = createDecipheriv(ALGORITHM, this.mek, iv);
+    decipher.setAuthTag(tag);
+    let decrypted = decipher.update(ciphertext);
+    decrypted = Buffer.concat([decrypted, decipher.final()]);
+    return decrypted;
+  }
+}
diff --git a/packages/field-encrypt/src/key-provider-memory.ts b/packages/field-encrypt/src/key-provider-memory.ts
new file mode 100644
index 00000000..e5867984
--- /dev/null
+++ b/packages/field-encrypt/src/key-provider-memory.ts
@@ -0,0 +1,48 @@
+/**
+ * @bytelyst/field-encrypt — In-memory key provider
+ *
+ * For unit tests — no external dependencies.
+ * Generates a random MEK on instantiation. Wrapping is just XOR for simplicity in tests,
+ * but uses AES-256-GCM to match production semantics.
+ */
+
+import { createCipheriv, createDecipheriv, randomBytes } from 'node:crypto';
+import type { KeyProvider } from './types.js';
+
+const ALGORITHM = 'aes-256-gcm';
+const IV_BYTES = 12;
+
+export class MemoryKeyProvider implements KeyProvider {
+  private readonly mek: Buffer;
+  private readonly version: string;
+
+  constructor(mek?: Buffer, version?: string) {
+    this.mek = mek ?? randomBytes(32);
+    this.version = version ?? 'memory-v1';
+  }
+
+  async wrapKey(dek: Buffer): Promise<{ wrappedKey: string; mekVersion: string }> {
+    const iv = randomBytes(IV_BYTES);
+    const cipher = createCipheriv(ALGORITHM, this.mek, iv);
+    let encrypted = cipher.update(dek);
+    encrypted = Buffer.concat([encrypted, cipher.final()]);
+    const tag = cipher.getAuthTag();
+
+    // Format: iv (12) + tag (16) + ciphertext
+    const wrapped = Buffer.concat([iv, tag, encrypted]);
+    return { wrappedKey: wrapped.toString('hex'), mekVersion: this.version };
+  }
+
+  async unwrapKey(wrappedKeyHex: string, _mekVersion: string): Promise<Buffer> {
+    const wrapped = Buffer.from(wrappedKeyHex, 'hex');
+    const iv = wrapped.subarray(0, IV_BYTES);
+    const tag = wrapped.subarray(IV_BYTES, IV_BYTES + 16);
+    const ciphertext = wrapped.subarray(IV_BYTES + 16);
+
+    const decipher = createDecipheriv(ALGORITHM, this.mek, iv);
+    decipher.setAuthTag(tag);
+    let decrypted = decipher.update(ciphertext);
+    decrypted = Buffer.concat([decrypted, decipher.final()]);
+    return decrypted;
+  }
+}
diff --git a/packages/field-encrypt/src/migration.ts b/packages/field-encrypt/src/migration.ts
new file mode 100644
index 00000000..978c4ac4
--- /dev/null
+++ b/packages/field-encrypt/src/migration.ts
@@ -0,0 +1,110 @@
+/**
+ * @bytelyst/field-encrypt — Migration helpers
+ *
+ * Utilities for encrypting existing plaintext fields in-place.
+ * Idempotent — skips already-encrypted fields via __encrypted sentinel.
+ */
+
+import type { EncryptedField } from './types.js';
+import { isEncryptedField } from './guards.js';
+
+/** Result of a migration run. */
+export interface MigrationResult {
+  /** Total documents scanned. */
+  scanned: number;
+  /** Documents encrypted in this run. */
+  encrypted: number;
+  /** Documents skipped (already encrypted). */
+  skipped: number;
+  /** Documents that failed to encrypt. */
+  errors: number;
+  /** Error details (first 10). */
+  errorDetails: Array<{ id: string; error: string }>;
+}
+
+/** Options for migrateDocuments(). */
+export interface MigrateDocumentsOptions<T> {
+  /** Fetch a batch of documents. Return empty array when done. */
+  fetchBatch: (offset: number, batchSize: number) => Promise<T[]>;
+  /** Get the document ID for logging. */
+  getId: (doc: T) => string;
+  /** Get the field value to check/encrypt. */
+  getField: (doc: T) => unknown;
+  /** Encrypt the plaintext value. Returns the EncryptedField. */
+  encryptValue: (plaintext: string, doc: T) => Promise<EncryptedField>;
+  /** Write the encrypted value back to the store. */
+  writeBack: (doc: T, encrypted: EncryptedField) => Promise<void>;
+  /** Batch size (default: 100). */
+  batchSize?: number;
+  /** If true, don't write — just count. */
+  dryRun?: boolean;
+  /** Progress callback. */
+  onProgress?: (result: MigrationResult) => void;
+}
+
+/**
+ * Migrate plaintext fields to encrypted fields in batches.
+ *
+ * Idempotent: skips documents where the field is already an EncryptedField.
+ */
+export async function migrateDocuments<T>(
+  options: MigrateDocumentsOptions<T>
+): Promise<MigrationResult> {
+  const batchSize = options.batchSize ?? 100;
+  const result: MigrationResult = {
+    scanned: 0,
+    encrypted: 0,
+    skipped: 0,
+    errors: 0,
+    errorDetails: [],
+  };
+
+  let offset = 0;
+  let batch: T[];
+
+  do {
+    batch = await options.fetchBatch(offset, batchSize);
+
+    for (const doc of batch) {
+      result.scanned++;
+      const fieldValue = options.getField(doc);
+
+      // Skip already-encrypted
+      if (isEncryptedField(fieldValue)) {
+        result.skipped++;
+        continue;
+      }
+
+      // Skip null/undefined
+      if (fieldValue == null) {
+        result.skipped++;
+        continue;
+      }
+
+      const plaintext = typeof fieldValue === 'string' ? fieldValue : JSON.stringify(fieldValue);
+
+      try {
+        const encrypted = await options.encryptValue(plaintext, doc);
+
+        if (!options.dryRun) {
+          await options.writeBack(doc, encrypted);
+        }
+
+        result.encrypted++;
+      } catch (err) {
+        result.errors++;
+        if (result.errorDetails.length < 10) {
+          result.errorDetails.push({
+            id: options.getId(doc),
+            error: err instanceof Error ? err.message : String(err),
+          });
+        }
+      }
+    }
+
+    offset += batch.length;
+    options.onProgress?.(result);
+  } while (batch.length === batchSize);
+
+  return result;
+}
diff --git a/packages/field-encrypt/src/types.ts b/packages/field-encrypt/src/types.ts
new file mode 100644
index 00000000..e882f612
--- /dev/null
+++ b/packages/field-encrypt/src/types.ts
@@ -0,0 +1,84 @@
+/**
+ * @bytelyst/field-encrypt — Types
+ *
+ * Core type definitions for field-level encryption.
+ */
+
+/** Encrypted field stored in Cosmos DB or SQLite. */
+export interface EncryptedField {
+  /** Sentinel — always true for encrypted fields. */
+  readonly __encrypted: true;
+  /** Schema version for future algorithm changes. */
+  readonly v: 1;
+  /** Algorithm identifier. */
+  readonly alg: 'aes-256-gcm';
+  /** Ciphertext (hex-encoded). */
+  readonly ct: string;
+  /** Initialization vector (hex-encoded, 12 bytes / 24 hex chars). */
+  readonly iv: string;
+  /** GCM authentication tag (hex-encoded, 16 bytes / 32 hex chars). */
+  readonly tag: string;
+  /** DEK identifier — identifies which key to unwrap for decryption. */
+  readonly dekId: string;
+}
+
+/** Wrapped DEK stored alongside data (e.g., in a `_encryption_keys` Cosmos container). */
+export interface WrappedDek {
+  /** Unique DEK identifier, e.g. `dek_user123_transcripts`. */
+  readonly dekId: string;
+  /** Wrapped (encrypted) DEK bytes (hex-encoded). */
+  readonly wrappedKey: string;
+  /** MEK name/version used to wrap this DEK. */
+  readonly mekVersion: string;
+  /** ISO 8601 creation timestamp. */
+  readonly createdAt: string;
+}
+
+/** Options for encrypt/decrypt operations. */
+export interface FieldEncryptContext {
+  /** Scope for DEK isolation (typically userId). */
+  readonly userId: string;
+  /** Additional context for DEK naming and AAD (e.g., 'transcripts', 'notes'). */
+  readonly context: string;
+}
+
+/** Key provider — abstraction over key storage backends. */
+export interface KeyProvider {
+  /** Wrap (encrypt) a DEK with the master key. Returns hex-encoded wrapped key + mek version string. */
+  wrapKey(dek: Buffer): Promise<{ wrappedKey: string; mekVersion: string }>;
+  /** Unwrap (decrypt) a wrapped DEK. Returns raw DEK buffer. */
+  unwrapKey(wrappedKeyHex: string, mekVersion: string): Promise<Buffer>;
+}
+
+/** Supported key provider types. */
+export type KeyProviderType = 'akv' | 'env' | 'memory';
+
+/** DEK store — abstraction over DEK persistence. */
+export interface DekStore {
+  /** Get a wrapped DEK by its ID. Returns null if not found. */
+  get(dekId: string): Promise<WrappedDek | null>;
+  /** Store a wrapped DEK. */
+  put(dek: WrappedDek): Promise<void>;
+  /** List all DEK IDs (for rotation). */
+  listIds(): Promise<string[]>;
+  /** Delete a DEK. */
+  delete(dekId: string): Promise<void>;
+}
+
+/** Configuration for createFieldEncryptor(). */
+export interface FieldEncryptorConfig {
+  /** Key provider type. */
+  keyProvider: KeyProviderType;
+  /** Azure Key Vault URL (required for 'akv' provider). */
+  keyVaultUrl?: string;
+  /** MEK name in AKV (required for 'akv' provider). */
+  mekName?: string;
+  /** Hex-encoded encryption key (required for 'env' provider). */
+  encryptionKey?: string;
+  /** DEK cache TTL in milliseconds (default: 15 minutes). */
+  dekCacheTtlMs?: number;
+  /** DEK cache max size (default: 1000). */
+  dekCacheMaxSize?: number;
+  /** DEK store implementation (default: in-memory). */
+  dekStore?: DekStore;
+}
diff --git a/packages/field-encrypt/tsconfig.json b/packages/field-encrypt/tsconfig.json
new file mode 100644
index 00000000..5edad813
--- /dev/null
+++ b/packages/field-encrypt/tsconfig.json
@@ -0,0 +1,9 @@
+{
+  "extends": "../../tsconfig.base.json",
+  "compilerOptions": {
+    "outDir": "dist",
+    "rootDir": "src"
+  },
+  "include": ["src"],
+  "exclude": ["src/**/*.test.ts"]
+}
diff --git a/scripts/create-encryption-keys.sh b/scripts/create-encryption-keys.sh
new file mode 100755
index 00000000..e9f0bc4f
--- /dev/null
+++ b/scripts/create-encryption-keys.sh
@@ -0,0 +1,122 @@
+#!/usr/bin/env bash
+# ──────────────────────────────────────────────────────────────────────────────
+# create-encryption-keys.sh — Create MEKs for all ByteLyst products in AKV
+#
+# Prerequisites:
+#   1. Azure CLI logged in (az login)
+#   2. Key Vault RBAC mode enabled on kv-mywisprai
+#   3. Caller has "Key Vault Crypto Officer" role
+#
+# Usage:
+#   ./scripts/create-encryption-keys.sh [--vault-name kv-mywisprai] [--dry-run]
+# ──────────────────────────────────────────────────────────────────────────────
+set -euo pipefail
+
+VAULT_NAME="${VAULT_NAME:-kv-mywisprai}"
+DRY_RUN=false
+
+# Parse args
+while [[ $# -gt 0 ]]; do
+  case "$1" in
+    --vault-name) VAULT_NAME="$2"; shift 2 ;;
+    --dry-run)    DRY_RUN=true; shift ;;
+    *)            echo "Unknown arg: $1"; exit 1 ;;
+  esac
+done
+
+# Product MEK names (one per product)
+MEKS=(
+  "lysnr-mek"
+  "mindlyst-mek"
+  "jarvisjr-mek"
+  "chronomind-mek"
+  "nomgap-mek"
+  "peakpulse-mek"
+  "flowmonk-mek"
+  "actiontrail-mek"
+  "notelett-mek"
+  "localmemgpt-mek"
+)
+
+echo "╔═══════════════════════════════════════════════════════════════╗"
+echo "║  ByteLyst — Create Master Encryption Keys (MEKs) in AKV     ║"
+echo "╠═══════════════════════════════════════════════════════════════╣"
+echo "║  Vault:     $VAULT_NAME"
+echo "║  Key type:  RSA 4096-bit"
+echo "║  Ops:       wrapKey, unwrapKey"
+echo "║  Keys:      ${#MEKS[@]}"
+echo "║  Dry run:   $DRY_RUN"
+echo "╚═══════════════════════════════════════════════════════════════╝"
+echo ""
+
+# Step 1: Verify AKV RBAC mode
+echo "── Step 1: Verify AKV RBAC mode ─────────────────────────────"
+RBAC_ENABLED=$(az keyvault show --name "$VAULT_NAME" --query "properties.enableRbacAuthorization" -o tsv 2>/dev/null || echo "unknown")
+
+if [[ "$RBAC_ENABLED" != "true" ]]; then
+  echo "⚠️  WARNING: RBAC is not enabled on $VAULT_NAME (current: $RBAC_ENABLED)"
+  echo "   Enable with: az keyvault update --name $VAULT_NAME --enable-rbac-authorization true"
+  echo "   ⚠️  This is a BREAKING CHANGE — existing access policies will stop working."
+  echo "   Proceed anyway? (y/N)"
+  read -r CONFIRM
+  if [[ "$CONFIRM" != "y" && "$CONFIRM" != "Y" ]]; then
+    echo "Aborted."
+    exit 1
+  fi
+else
+  echo "✅ RBAC mode is enabled on $VAULT_NAME"
+fi
+echo ""
+
+# Step 2: Create MEKs
+echo "── Step 2: Create MEKs ──────────────────────────────────────"
+CREATED=0
+SKIPPED=0
+ERRORS=0
+
+for MEK in "${MEKS[@]}"; do
+  # Check if key already exists
+  EXISTS=$(az keyvault key show --vault-name "$VAULT_NAME" --name "$MEK" --query "key.kid" -o tsv 2>/dev/null || echo "")
+
+  if [[ -n "$EXISTS" ]]; then
+    echo "  ⏭️  $MEK — already exists ($EXISTS)"
+    SKIPPED=$((SKIPPED + 1))
+    continue
+  fi
+
+  if $DRY_RUN; then
+    echo "  🔍 $MEK — would create (dry run)"
+    CREATED=$((CREATED + 1))
+    continue
+  fi
+
+  echo -n "  🔑 $MEK — creating... "
+  if az keyvault key create \
+      --vault-name "$VAULT_NAME" \
+      --name "$MEK" \
+      --kty RSA \
+      --size 4096 \
+      --ops wrapKey unwrapKey \
+      --protection software \
+      -o none 2>/dev/null; then
+    echo "✅"
+    CREATED=$((CREATED + 1))
+  else
+    echo "❌ FAILED"
+    ERRORS=$((ERRORS + 1))
+  fi
+done
+
+echo ""
+echo "── Summary ──────────────────────────────────────────────────"
+echo "  Created:  $CREATED"
+echo "  Skipped:  $SKIPPED (already exist)"
+echo "  Errors:   $ERRORS"
+echo ""
+
+if [[ $ERRORS -gt 0 ]]; then
+  echo "⚠️  Some keys failed to create. Check AKV access and retry."
+  exit 1
+fi
+
+echo "✅ Done. MEKs ready for @bytelyst/field-encrypt envelope encryption."