# JWT Structure and Signing — Authentication & Authorization

Source: https://www.skillbyai.com/en/authentication/j-structure

> Header, payload, signature; HS256 versus RS256 and ES256.

## Three base64url parts

A JWT (RFC 7519) in its common signed form (JWS) is `header.payload.signature`, each part base64url-encoded. The **header** names the algorithm (`alg`) and often a key ID (`kid`). The **payload** holds **claims**: registered ones such as `iss` (issuer), `sub` (subject), `aud` (audience), `exp` (expiry), `iat` (issued at) and `nbf` (not before), plus your own. The payload is **encoded, not encrypted**, so anyone holding the token can read it; never put secrets in it. **HS256** uses one shared secret for signing and verifying, so every verifier could also mint tokens. **RS256** (RSA) and **ES256** (ECDSA P-256) use a private key to sign and a public key to verify, which suits many services verifying tokens from one issuer.

## Self-contained, signed claims

JSON Web Tokens carry signed claims that a server can verify without a database lookup, which brings both convenience and pitfalls.

![Three ideas: JWT structure and signing, verifying correctly, access and refresh tokens in browsers.](assets/figures/authentication/section-4-map.svg) — Figure 4.1 — Sign, verify, refresh.

## A decoded JWT

Illustrative values.

```json
// header
{ "alg": "ES256", "typ": "JWT", "kid": "2026-09-key-1" }

// payload (claims)
{
  "iss": "https://auth.example.com",
  "sub": "user_8f3a",
  "aud": "https://api.example.com",
  "iat": 1790000000,
  "exp": 1790000600,
  "scope": "orders:read"
}

// signature = ECDSA-P256-SHA256(base64url(header) + "." + base64url(payload), privateKey)
```

## A sealed, transparent envelope

A JWT is like a letter in a clear envelope with a wax seal. Everyone can read the letter; the seal only proves who sent it and that nobody changed it.

**Quiz:** Why might several microservices prefer RS256 or ES256 over HS256?

- [x] They can verify with a public key without being able to mint tokens
- [ ] Asymmetric signatures make the payload encrypted
- [ ] HS256 tokens cannot carry an exp claim
- [ ] RS256 tokens never expire

*Answer:* They can verify with a public key without being able to mint tokens. With HS256 every verifier holds the signing secret.