JWT decoding is used to view token contents, assisting developers in debugging authentication issues, analyzing user permissions, checking expiration times, and other critical information.
A JWT consists of three parts: Header, Payload, and Signature, separated by dots. The first two parts can be directly Base64 decoded for viewing.
The Header contains the token type (typ) and signature algorithm (alg), such as '{"typ":"JWT","alg":"HS256"}', indicating how to verify the token.
The Payload contains claim information, such as user ID (sub), expiration time (exp), issuance time (iat), and other standard claims and custom data.
The exp field represents the expiration timestamp. Decoding can quickly determine if the token has expired, avoiding the use of invalid tokens for requests.
Decoding only displays token contents and does not verify signature validity. In production environments, signature verification is also required to ensure the token has not been tampered with.