Sessions or JWTs? Both log users in. The difference is where the state lives.
SESSIONS
The server keeps the authentication state. The browser holds only an opaque
session ID in a cookie — a lookup key, not the data. Change the session
server-side and it takes effect on the very next request.
session[:user_id] = user.id
JWTs
The server signs a token and hands it to the client. The client presents it
on later requests:
Authorization: Bearer token
THE THING MOST PEOPLE GET WRONG
A JWT is signed, not encrypted. Those three base64 segments are not
protection — anyone holding the token can decode the payload and read every
claim in it. Paste one into a decoder and it's all there in plain JSON.
The signature prevents *tampering*. It hides nothing.
So: no passwords, no API keys, no personal data you wouldn't put in a URL.
Treat the payload as public.
DECODING IS NOT VERIFYING
A token that decodes cleanly is just well-formed base64. Before you trust it,
check:
• signature — against the key you actually control
• algorithm — pin it; don't let the token choose (`alg: none` is a classic)
• issuer & audience — is this token even meant for you?
• expiry — and keep the window short
AND IF YOU USE COOKIES
`HttpOnly`, `Secure`, and a sensible `SameSite` policy. Cookies authenticate
automatically, so keep CSRF protection on.
THE HONEST COMPARISON
JWTs aren't faster, safer, or truly stateless in most real systems — the
moment you need logout-everywhere or instant revocation, you're maintaining
server-side state again (a denylist, short expiry plus refresh tokens,
something). Sessions handle revocation for free because the state was
already on the server.
Sessions suit server-rendered apps. JWTs suit token-based APIs and
cross-service auth. Storage, expiry and revocation matter far more than
which one you picked.
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