What is the difference between decoding and verifying a JWT?

Decoding and verifying JWTs are two different operations with distinct purposes.

Decoding (Reading):

  • Extracts and displays the header and payload
  • Does NOT require the secret key or public key
  • Does NOT check if the token is valid or tampered with
  • Can be done by anyone who has the token
  • Useful for: Debugging, inspecting claims, checking expiration

Verifying (Validating):

  • Checks that the signature is valid and token hasn't been altered
  • REQUIRES the secret key (HMAC algorithms) or public key (RSA/ECDSA algorithms)
  • Confirms the token was issued by a trusted source
  • Should only be done by the server that issued the token
  • Essential for: Security, authentication, authorization

When to decode:

  • Debugging API authentication issues
  • Inspecting what data is in your token
  • Checking if a token is expired
  • Learning how JWTs work
  • Analyzing tokens from third-party services

When to verify:

  • Before trusting any data in the token
  • Before granting access to protected resources
  • On every API request that uses JWT authentication
  • In production applications (always verify server-side!)

Security Note:

Never trust decoded JWT data without verification. A decoded token tells you what the token claims to contain, but only verification proves it's legitimate. Always verify tokens on your server before using them for authentication or authorization.

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