JSON Web Tokens

What is JSON Web Token (jot)

  • JWT is a standard for transmitting information.
  • JWT can either be signed (JWS) or encrypted (JWE)
  • JWT can be generated using a shared secret or a public and private key
  • JWT is an alternative to the server creating a session and SAML
  • JWT’s most common use is authentication
  • Stateless - no sessions are created from the server
  • Compact - Sent with every request transmission typically in Header Authorization
  • Self-contained - all the information about the user is in the encoded payload

How does JWT Work?
The server returns an authentication token after a user logs in from the client. The client stores that token inside localStorage (or a cookie). Every subsequent request made by the client includes the token. The server then verifies the correctness of the token before sending a response.

JWT Structure is dot-delimited - Header.payload.signature
Header - cryptographic information Payload - user claims. Built it claims are 3 characters long Signature - the hashed value of its parts along with the secret key

JWS vs JWT Comparison

JWS JWE
Shared Secret Holder Can verify and generate tokens Can encrypt and decrypt tokens
Public/Private Key
  • Private key holder can sign and verify tokens
  • Public key holder can only verify tokens
  • Data flows from private to public key holders
  • Private key holder can only decrypt
  • Public key holders can encrypt
  • Data flows from public to private key holders

When to sign (JWS) and when to encrypt (JWE) JWTs

  • Sign your tokens (JSON Web Signature - JWS) using hashing algorithms if there are no security nor privacy concerns when it is read by other parties, otherwise it should be encrypted (JSON Web Encryption - JWE).
  • Sign it if all you need is validation, encrypt it if you need protection

Best Practices

  • Never accept unsigned JWTs - Signature stripping is a form of attack where the signature is removed, and replaced as an unsigned token.
  • Don’t store tokens in cookies because they’re subject to CSRF attacks. Use localStorage whenever possible.
  • Perform origin checks
  • Add expiration