JWT Structure Explained – Java Backend Security Interview

Direct Answer

A JWT (JSON Web Token) is a compact format for carrying claims between parties. The common signed form is a JWS with three Base64URL segments: protected header, payload, and signature. The header identifies the signing algorithm and often a key ID; the payload contains claims such as iss, sub, aud, and exp; the signature lets a verifier detect changes and authenticate the issuer's key. Base64URL is encoding, not encryption. An encrypted JWT uses JWE and has a different five-part compact form.

Code Example

import io.jsonwebtoken.Jwts;
import javax.crypto.SecretKey;
import java.util.Date;

SecretKey key = Jwts.SIG.HS256.key().build(); // demo: persist securely in a real service
Date now = new Date();

String token = Jwts.builder()
    .issuer("https://auth.example")
    .subject("user-42")
    .audience().add("orders-api").and()
    .issuedAt(now)
    .expiration(new Date(now.getTime() + 10 * 60_000L))
    .signWith(key)
    .compact();

var claims = Jwts.parser()
    .verifyWith(key)
    .build()
    .parseSignedClaims(token)
    .getPayload();

Signature verification is only one check. A real verifier must also validate issuer, audience, time claims, and the expected algorithm for its trust boundary.

Common Mistakes & Pitfalls

• Treating the payload as encrypted: Freshers often think JWT hides the data because it looks encoded. The payload is only Base64URL-encoded — anyone can decode it and read the claims. Never put passwords, credit card numbers, or API keys in a JWT payload.
• Not validating the signature: Some implementations extract the payload without calling parseClaimsJws(), which skips signature verification. An attacker can then modify the token and the server will trust it. Always validate before using claims.
• Using weak or hardcoded secrets: Using "secret" or "12345" as the signing key makes JWTs trivially forgeable. In production, use a strong random key stored in environment variables, rotated regularly.