Passwords, MFA & Modern Authenticators Understand credential security without building a password system from scratch
Phase 16 - Authentication Fundamentals Volume 05 - Authentication & Security
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Phase 16 builds the security mental model before any Supabase Auth integration code is introduced.
Passwords are authenticators, not database values to store directly A password is evidence the user knows a secret. A secure identity system never needs the original password after enrollment. It stores a slow password hash designed to resist offline guessing if the credential database is stolen.
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The original password should not be recoverable from the stored verifier.
Why SHA-256 alone is not password storage General-purpose hashes are intentionally fast. Password storage needs a deliberately expensive password hashing function so each attacker guess costs meaningful CPU/memory. OWASP currently recommends Argon2id first, with alternatives such as scrypt, bcrypt, or PBKDF2 in appropriate environments.
We will use Supabase Auth instead of writing password hashing, reset flows, email verification, breach defenses, and credential lifecycle ourselves. Understanding these concepts tells you why delegating identity is a security architecture choice, not laziness.
MFA and stronger authenticators Multi-factor authentication combines independent categories of evidence. Passkeys/WebAuthn use public-key cryptography and are phishing-resistant compared with reusable shared secrets. TaskFlow does not implement passkeys in Phase 16, but a 2026 full-stack developer should recognize that passwords are only one authenticator option.
Password Widely supported; phishable; requires safe storage and recovery
TOTP app Adds possession factor; codes can still be phished
SMS OTP Better than password-only in some contexts, but vulnerable to SIM/telephony attacks
WebAuthn / passkey Public-key, origin-bound, phishing-resistant; modern preferred direction
Never log credentials • Do not log plaintext passwords.
• Do not log password reset tokens.
• Do not log MFA secrets or recovery codes.
• Do not send credentials in URL query strings.
• Use TLS/HTTPS for authentication traffic.
Authentication answers who the request represents. Treating decoded or client-supplied identity as trusted is a security boundary failure.
Restate the goal of “Passwords MFA and Modern Authenticators” as one observable behavior. Write the success signal before touching code.
Trace the boundary map: credentials/session → token → verification → trusted identity → protected operation. Mark the one boundary this lesson changes.
Inspect the current repository state and record the exact files, route, table, or runtime involved. Avoid creating an abstraction before you can name the existing owner.
Implement the smallest vertical slice. Add one coherent change, run the narrowest verification command, and read the output before continuing.
Connect the slice to TaskFlow’s phase artifact: Authentication threat/flow diagram explaining what the browser stores, what servers verify, and what happens when credentials expire.. Verify behavior from the outside as a user or API client would.
Create one controlled failure that is safe to reproduce. Predict the symptom first, capture evidence, locate the earliest broken boundary, then fix only that cause.
Close the reference and rebuild or explain the core change from memory. Record what you would monitor or test before trusting the same change in production.
const token = readBearerToken(request);
const claims = await verifier.verify(token);
request.user = {
id: claims.sub,
email: claims.email,
};Do not copy this as a finished answer. Predict what each boundary does, type the smallest useful slice, run it, then compare the result with your prediction.
Inspect whether a token exists, whether it is expired, which issuer/audience signed it, and what the API verifier expects.
Trust only verified server claims; never repair identity by accepting a client-sent user id.
Without copying the final source, create a second small scenario that applies “Passwords MFA and Modern Authenticators” to a different TaskFlow resource or input. Write the acceptance criteria first, implement it, and provide the evidence that each criterion is satisfied.
Compare opaque sessions and JWT access tokens by state, revocation, and verification model.
Use the smallest change that demonstrates this lesson. Stop after it works and capture evidence.Storing long-lived secrets in localStorage by default
Predict the symptom before causing the failure. Then locate the earliest boundary where reality diverges from expectation.Why does this responsibility live here, what input does it trust, what output proves success, and what would go wrong if another layer owned it?
If your explanation is vague, repeat the boundary trace before moving on.Close the reference. Reproduce the core idea from memory or explain the exact request/data flow aloud. Then check your diff or notes for accidental shortcuts that bypass validation, authorization, error handling, or verification earned in earlier phases.