JWT Decoder

What is a JWT?

A JSON Web Token (JWT) is a compact, URL-safe method for representing claims between two parties. JWTs are the standard format for authentication tokens in modern web applications. A JWT consists of three Base64URL-encoded segments separated by dots: the header (specifying the algorithm and token type), the payload (containing the claims or data), and the signature (used to verify the token's integrity). The entire structure looks like: eyJhbGci... .eyJzdWIi... .SflKxwR...

JWTs are used extensively in OAuth 2.0 flows, single sign-on systems, API authentication, and session management. They allow stateless authentication — the server can verify a token's validity by checking its signature without consulting a database. Understanding JWT structure is essential for debugging authentication issues, implementing secure APIs, and building identity systems.

When Should You Decode a JWT?

JWT decoding is essential when debugging authentication problems. When a login fails, decoding the token reveals whether the issue is with expired timestamps, incorrect claims, or wrong user identifiers. When implementing API integrations that use Bearer tokens, decoding verifies the token contains expected data. Security audits require inspecting JWT payloads to ensure no sensitive data is exposed in the unencrypted payload.

Developers debugging OAuth flows decode tokens to verify correct audience, issuer, and scope claims. DevOps engineers decode tokens from logs to understand authentication failures. API developers decode tokens to verify their JWT generation logic is producing correct structures.

How It Works

The decoder splits the JWT string on the dot separator into three segments. Each segment is Base64URL-decoded (Base64 with URL-safe character substitutions). The header and payload are parsed as JSON objects and displayed with syntax highlighting. The signature segment is displayed as raw bytes. The decoder identifies standard claims like exp (expiration), iat (issued at), sub (subject), and aud (audience) and displays them in human-readable format.

Tips for Best Results

Remember that the JWT payload is Base64-encoded, not encrypted — anyone can decode it. Never put sensitive information like passwords or API keys in JWT payloads. When debugging, pay special attention to the exp claim to verify token expiration. For signature verification, you need the secret key, which this tool does not provide (nor should it — that would be a security risk).