diff --git a/apps/api/tests/broker/crypto.test.ts b/apps/api/tests/broker/crypto.test.ts
index e79a8d0..c31932c 100644
--- a/apps/api/tests/broker/crypto.test.ts
+++ b/apps/api/tests/broker/crypto.test.ts
@@ -1,22 +1,80 @@
 /**
- * Wave 0 stub — Broker: AES-GCM app-password encryption
+ * Broker: AES-GCM app-password encryption
  *
- * These tests are RED stubs. Implementation lives in:
- *   apps/api/src/broker/crypto.ts  (Plan 03)
- *
- * Tests will be filled GREEN in Plan 03 when crypto helpers are implemented.
+ * Tests the encryptPassword / decryptPassword helpers in src/broker/crypto.ts.
+ * Key behaviors verified (T-03-01, T-03-03):
+ *   - Lossless roundtrip
+ *   - Unique IVs per encryption (never reuse IV)
+ *   - GCM auth tag tamper detection
+ *   - Stored payload shape
  */
 
-import { describe, it } from 'vitest'
+import { describe, it, expect, beforeAll } from 'vitest'
+
+// Set a fixed 32-byte (64-char hex) key before importing the module
+const TEST_KEY = 'a1b2c3d4e5f6a7b8c9d0e1f2a3b4c5d6a7b8c9d0e1f2a3b4c5d6a7b8c9d0e1f2'
+
+beforeAll(() => {
+  process.env.APP_PASSWORD_ENCRYPTION_KEY = TEST_KEY
+})
+
+// Dynamic import so env is set before module-level KEY evaluation
+async function getCrypto() {
+  return import('../../src/broker/crypto.js')
+}
 
 describe('encryptPassword / decryptPassword', () => {
-  it.todo('roundtrip: decrypt(encrypt(plaintext)) === plaintext (Plan 03)')
+  it('roundtrip: decrypt(encrypt(plaintext)) === plaintext', async () => {
+    const { encryptPassword, decryptPassword } = await getCrypto()
+    const plaintext = 'my-fastmail-app-password-abc123'
+    const encrypted = encryptPassword(plaintext)
+    expect(decryptPassword(encrypted)).toBe(plaintext)
+  })
 
-  it.todo('different IVs produce different ciphertext for the same plaintext (Plan 03)')
+  it('different IVs produce different ciphertext for the same plaintext', async () => {
+    const { encryptPassword } = await getCrypto()
+    const plaintext = 'same-password'
+    const enc1 = encryptPassword(plaintext)
+    const enc2 = encryptPassword(plaintext)
+    // The JSON payloads must differ (different IVs → different ciphertext)
+    expect(enc1).not.toBe(enc2)
+    // And the IVs themselves must differ
+    const p1 = JSON.parse(enc1)
+    const p2 = JSON.parse(enc2)
+    expect(p1.iv).not.toBe(p2.iv)
+  })
 
-  it.todo('decrypting with a tampered authTag throws (Plan 03)')
+  it('decrypting with a tampered authTag throws', async () => {
+    const { encryptPassword, decryptPassword } = await getCrypto()
+    const encrypted = encryptPassword('secret')
+    const payload = JSON.parse(encrypted)
+    // Flip first byte of authTag
+    payload.authTag = ('ff' + payload.authTag.slice(2))
+    expect(() => decryptPassword(JSON.stringify(payload))).toThrow()
+  })
 
-  it.todo('decrypting with a tampered ciphertext throws (Plan 03)')
+  it('decrypting with a tampered ciphertext throws', async () => {
+    const { encryptPassword, decryptPassword } = await getCrypto()
+    const encrypted = encryptPassword('secret')
+    const payload = JSON.parse(encrypted)
+    // Flip first byte of ciphertext
+    payload.ciphertext = ('ff' + payload.ciphertext.slice(2))
+    expect(() => decryptPassword(JSON.stringify(payload))).toThrow()
+  })
 
-  it.todo('stored payload is valid JSON with iv, authTag, ciphertext fields (Plan 03)')
+  it('stored payload is valid JSON with iv, authTag, ciphertext fields', async () => {
+    const { encryptPassword } = await getCrypto()
+    const encrypted = encryptPassword('test-password')
+    const payload = JSON.parse(encrypted)
+    expect(payload).toHaveProperty('iv')
+    expect(payload).toHaveProperty('authTag')
+    expect(payload).toHaveProperty('ciphertext')
+    // All values are non-empty hex strings
+    expect(typeof payload.iv).toBe('string')
+    expect(payload.iv.length).toBeGreaterThan(0)
+    expect(typeof payload.authTag).toBe('string')
+    expect(payload.authTag.length).toBeGreaterThan(0)
+    expect(typeof payload.ciphertext).toBe('string')
+    expect(payload.ciphertext.length).toBeGreaterThan(0)
+  })
 })