ts-otp-aws

Implement OTP / passwordless authentication on AWS for TypeScript projects.

When to use

• Adding email or SMS verification codes to a TypeScript app on AWS Lambda.
• Wiring or debugging Cognito CUSTOM_AUTH triggers (DefineAuthChallenge, CreateAuthChallenge, VerifyAuthChallengeResponse, PreSignUp).
• Standing up a non-Cognito OTP flow on DynamoDB with TTL.

Default approach

Cognito User Pool with CUSTOM_AUTH flow + four Lambda triggers, SESv2 for email, SNS for SMS. The user pool stays the source of truth; the four triggers handle OTP generation, delivery, and verification. Drop-in handlers and Terraform live in assets/. The custom-DynamoDB alternate is documented in references/custom-dynamodb-otp.md — pick it only when you can't use Cognito.

Cognito vs custom — quick decision

Full matrix + threat model: references/architecture-decision.md.

Architecture at a glance

┌────────┐  InitiateAuth(CUSTOM_AUTH)   ┌──────────────┐  Define→Create   ┌──────────────────┐
│ client │ ───────────────────────────▶ │   Cognito    │ ───────────────▶ │ auth-triggers λ  │
│ (Next/ │ ◀── session + masked hint ── │  User Pool   │                  │ ─ create-chall   │
│ Astro) │                              │              │                  │ ─ define-chall   │
│        │  Respond(answer=otp)         │              │                  │ ─ verify-chall   │
│        │ ───────────────────────────▶ │              │                  │ ─ pre-signup     │
│        │ ◀── id/access/refresh ────── │              │                  └────┬──────┬──────┘
└────────┘                              └──────────────┘                       │      │
                                                                          SESv2│      │SNS
                                                                       ┌───────▼──┐ ┌─▼──────┐
                                                                       │  email   │ │  SMS   │
                                                                       └──────────┘ └────────┘

Custom path swaps Cognito + triggers for one Lambda + a DynamoDB OTP table (PK/SK + ttl).

Step 1 — Audit the project

scripts/audit-project.sh /path/to/repo

Reports which patterns the repo already has (Cognito CUSTOM_AUTH, custom DDB, client-only) and what's missing. Exits non-zero on missing rg. See scripts/README.md for sample output.

Step 2 — Pick the path

• Existing Cognito user pool, or you want managed JWT issuance → Cognito CUSTOM_AUTH (continue to Step 3).
• No user pool, single-purpose API, or you already issue your own JWTs → custom DynamoDB (jump to references/custom-dynamodb-otp.md).

Step 3 — Drop in handlers (Cognito path)

Copy the five trigger handlers and two notifiers verbatim:

assets/handlers/      → lambda/auth-triggers/src/handlers/
assets/notifiers/     → lambda/auth-triggers/src/notifiers/

Required env vars on the Lambda:

The dispatcher (handler.ts) routes by event.triggerSource. Walkthrough of each trigger and the phantom-user (prevent_user_existence_errors) path: references/cognito-custom-auth.md.

Step 4 — Wire infrastructure (Terraform)

Minimum viable set under assets/terraform/:

• cognito-userpool.tf — pool + 4 trigger ARN bindings, prevent_user_existence_errors=ENABLED, no MFA (OTP is the auth).
• auth-triggers-lambda.tf — Node 22 ESM Lambda, log retention, IAM role.
• ses-identity.tf — domain identity + DKIM + custom MAIL FROM.
• sns-sms.tf — MonthlySpendLimit, DefaultSMSType=Transactional, opt-out preferences.
• dynamodb-otp-table.tf — only for the custom path (PK/SK + ttl + PITR + deletion protection).

IAM least-privilege policies: assets/iam/auth-trigger-policy.json and assets/iam/custom-otp-policy.json.

Step 5 — Frontend integration

Server-side adapter calling InitiateAuthCommand + RespondToAuthChallengeCommand lives at assets/frontend/cognito-server-snippet.ts. Key points:

• AuthFlow: "CUSTOM_AUTH", pass ClientMetadata: { channel: "email" | "sms" }.
• For phone identifiers, ListUsers with phone_number = "..." filter; pass through unknown identifiers unchanged so Cognito's phantom-user path stays opaque.
• Error mapping: NotAuthorizedException → session expired; fresh challenge response → wrong code.

Step 6 — Security must-haves

• Constant-time OTP comparison (crypto.timingSafeEqual); reject early if not 6 ASCII digits.
• 3-attempt cap inside define-challenge.ts; surface as failAuthentication=true.
• 5-minute OTP TTL; never log the code.
• Phantom-user safety: never throw on userNotFound=true — silently return a synthetic challenge or you'll leak existence.
• SES out of sandbox, MAIL FROM domain set, DKIM aligned. SNS spend limit + transactional + sender-ID per region.
• Mask channel hint (***1234, not +5511915551234).

Full checklist: references/security-checklist.md.

Step 7 — Test

aws-sdk-client-mock v4, one mock per service. Inline minimum:

import { SESv2Client, SendEmailCommand } from "@aws-sdk/client-sesv2";
import { mockClient } from "aws-sdk-client-mock";
const ses = mockClient(SESv2Client);
ses.on(SendEmailCommand).resolves({});
// run handler, then:
expect(ses.commandCalls(SendEmailCommand)).toHaveLength(1);

State-machine and phantom-user assertions: references/testing-patterns.md.

Custom DynamoDB alternate + second factor

When Cognito is not an option, follow references/custom-dynamodb-otp.md. For layering TOTP authenticator apps or WebAuthn passkeys on top of OTP, see references/second-factor.md — kept brief on purpose.

Channel deliverability (SES sandbox/DKIM, SNS 10DLC/DLT/origination): references/messaging-channels.md. Source attributions: references/citations.md.

Validation

• skills-ref validate /home/kayaman/Projects/ts-otp-aws-skill (frontmatter + naming).
• scripts/audit-project.sh ~/Projects/ai-assisted-dev should say "Cognito CUSTOM_AUTH — complete".
• wc -l SKILL.md < 200; each references/*.md < 500.

Out of scope

• Pinpoint marketing campaigns (this is transactional auth).
• Cognito IdP federation (Google/Apple).
• Full WebAuthn/passkey or HOTP/TOTP authenticator-app implementations.
• UI component libraries (an OtpCodeInput is referenced but not bundled).
• Generic IAM/Lambda/Terraform fundamentals.