Lesson 7 of 12 · 7 min read
Validating JWTs Safely
A JSON Web Token is a signed (sometimes encrypted) set of claims, whose safety depends on explicit verifier policy. Safe validation pins the allowed algorithm to each key, selects keys only from the configured issuer's key set, checks the token type, issuer, audience and time claims, treats header fields such as kid and jku as untrusted input, and keeps different token kinds mutually exclusive. Key rotation needs a schedule that accounts for cache lifetimes, token lifetimes and clock leeway.
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Practise it as an interviewer asks it
Explain encoding, encryption, hashing and signing, and when you would use each.Symmetric versus asymmetric cryptography: when is each used, and why do real protocols use both?How does the OAuth 2.0 authorization code flow with PKCE work, and what attacks does each part stop?Walk through a TLS 1.3 handshake. What does each step protect against?What is the difference between OpenID Connect and OAuth, and between an ID token and an access token?How is a certificate chain validated, and what goes wrong in practice?