标签：#jwt

JWTs are one of those technologies that feel wonderful right up until you hit your first "log me out" requirement. Then you discover the awkward truth: the very property that makes JWTs attractive — statelessness — is also what makes logout hard. This post walks through what JWTs actually are, why "invalidating" one is a design problem rather than a one-liner, and the practical methods available to revoke an access token on logout, along with the bottleneck each one introduces. A quick refresher on JWTs A JSON Web Token (JWT) is a signed, self-contained token. It carries a JSON payload of claims — who the user is, when the token was issued, when it expires, and often a unique token id ( jti ) — and a cryptographic signature over that payload. Because the token is signed with a secret (or a private key), any server holding the corresponding key can verify it is authentic and untampered without calling a database . That last part is the entire point. When a request arrives with a JWT, the server checks the signature and the expiry, reads the claims, and proceeds. No lookup, no shared session store, no round trip. This is what people mean when they call JWT auth stateless : the server keeps no per-user session record. The token itself is the session, and it's valid until it expires. Access tokens and refresh tokens In practice you rarely use a single token. The common pattern splits responsibility across two: The access token is the short-lived workhorse. It's sent on every API request and typically expires in minutes (5–15 is common). Because it's checked statelessly on every call, you want its lifetime short — if it leaks, the damage window is small. The refresh token is long-lived (days or weeks) and does one job: obtain new access tokens when the current one expires. It is not sent on every request — only to a dedicated token endpoint. This lets the access token stay short and stateless while the user avoids logging in every ten minutes. The refresh token is easy —

A stack de autenticação em .NET fica sólida quando separamos duas responsabilidades: ✅ Argon2id para guardar senhas (hash irreversível, lento, memória-intensivo) ✅ JWT Bearer para provar identidade depois do login ✅ Validação de iss , aud , exp e assinatura em cada request ✅ Segredos fora do repositório (ambiente / Key Vault) Se o ecossistema .NET já oferece hosting, APIs e pacotes maduros, combinar Argon2 (referência da Password Hashing Competition , testável em argon2.online ) com JWT é o caminho natural para microsserviços e Web APIs. Neste artigo, mostro o fluxo registo → login → token → rotas protegidas com foco no que implementar no dia a dia. ⚠️ Observação importante JWT não substitui Argon2. Nunca coloque senha ou hash no payload do token. Argon2 protege a credencial na base de dados; JWT é sessão assinada com expiração. 🧠 Visão Geral Aspecto Argon2 (senha) JWT (sessão) Foco Resistir a offline cracking Autorizar requests após login Onde vive Coluna password_hash na BD Header Authorization: Bearer Algoritmo Argon2id (OWASP) HMAC-SHA256 ou RSA (config) Ferramenta de estudo argon2.online docs Microsoft JWT Bearer Runtime Biblioteca .NET (ex.: Konscious Argon2) Microsoft.AspNetCore.Authentication.JwtBearer Erro clássico MD5/SHA rápido na senha Token sem validar aud / iss 🧩 O que o Argon2 resolve (camada 1) O Argon2 é o vencedor da Password Hashing Competition — hoje a referência para novas passwords . 1️⃣ Hash irreversível com Argon2id var hash = hasher . Hash ( password ); await store . CreateAsync ( email , hash ); ✅ Salt único por utilizador ✅ Parâmetros m , t , p documentados no próprio hash ✅ Verificação com tempo constante ( FixedTimeEquals ) 2️⃣ Calibrar custo com consciência Em argon2.online podes experimentar memory cost e iterations — útil em laboratório. 📌 Em produção usa biblioteca auditada (.NET), não hashes de utilizadores reais em sites públicos. 3️⃣ O que não fazer na senha ✅ Não “criptografar” senha com AES reversível ✅ Não MD5 / SHA-1 / SHA-256

Most developers paste production JWTs into online decoders without thinking. Here's a 10-second DevTools check to see if your token is actually leaving your machine. A coworker was debugging an auth bug last month. Standard workflow: copy the JWT from the failing request, paste it into an online decoder, read the payload. I've done it a thousand times. You probably have too. Except the token he pasted belonged to a real customer. And the decoder he used is owned by an identity company that's had its share of security incidents. Nothing bad happened. Probably. But it made me think about something I'd never actually checked: when you paste a JWT into an online decoder, where does that token go? What a JWT actually contains Quick reminder of why this matters. A JWT isn't encrypted — it's just Base64URL-encoded. Anyone who has the token can read everything in it: header.payload.signature The payload routinely contains: User ID, email, and role Session identifiers Token expiry ( exp ) and issue time ( iat ) Sometimes — against best practice — far more And here's the part people forget: a valid, unexpired JWT is a live credential. If it hasn't expired, whoever holds it can often impersonate the user. Pasting it into a random website is functionally similar to pasting a password. The 10-second check Most online JWT decoders claim to be "secure" and "client-side." Some are. Some aren't. You don't have to trust the claim — you can verify it yourself in 10 seconds: Open the decoder in your browser Open DevTools → Network tab Clear the network log Paste a JWT and decode it Watch the Network tab If any request fires when you decode — your token left your machine. A truly client-side decoder fires zero network requests during decoding. The JavaScript does everything locally; nothing is sent anywhere. Try this on whatever decoder you currently use. You might be surprised. Why most "online" tools send data It's usually not malicious. Building decoding logic on the server is someti