What is a JWT Decoder?
A JWT (JSON Web Token) Decoder is a specialized tool that parses and displays the contents of JWT tokens without requiring server-side processing. JWTs are compact, URL-safe tokens used for authentication and information exchange in modern web applications.
They consist of three Base64Url-encoded parts separated by dots: the header, the payload (carrying claims and user data), and the signature. Developers frequently need to inspect these tokens during debugging or security audits to verify token structure and claims.
How it works locally (WASM/JS)
Our JWT Decoder operates entirely within your browser using JavaScript, ensuring zero data transmission to external servers. When you paste a token, the browser parses the string and applies Base64Url decoding in-memory.
All processing occurs in a sandboxed environment. Your sensitive authentication tokens never leave your machine, making this tool suitable for examining production tokens containing proprietary claims or personally identifiable information (PII).
Smart Timestamp Parsing
Common JWT claims like exp, iat, and nbf are Unix timestamps — raw numbers that are hard to read at a glance. Our decoder automatically converts them to human-readable dates in both UTC and your local timezone, plus a relative description like "3 hours ago" or "expired 2 days ago".
Security Risks of Cloud-based Alternatives
Most online JWT tools transmit your token to their servers. This means your potentially sensitive session data, user IDs, and role permissions are now sitting in someone else's log files.
オンラインのデコーダーは、中間者攻撃やサーバー側の侵害の影響を受けやすく、重大なコンプライアンス違反（GDPR/SOC2）につながる可能性があります。
Security Analysis
This decoder automatically flags common JWT security issues: alg: nonetokens (which are trivially forgeable), expired tokens, and missing expiration claims. It doesn't replace a full audit tool, but it gives you immediate visibility into token health during debugging.
FAQ
Can this tool validate JWT signatures?
No. Signature verification requires access to your secret key. To maintain security, we never ask for your keys. This tool is for inspection only.
Does it support encrypted JWTs (JWE)?
Currently, we support signed tokens (JWS). Encrypted tokens require decryption keys that should not be handled in a browser for security reasons.
What timestamps are automatically converted?
The decoder recognizes exp (expiration), iat (issued at), nbf (not before), and auth_time (authentication time) claims and displays them as human-readable dates.