JWT Tester

JWT Tester decodes a compact JSON Web Token into its header and payload and, when a secret or public key is provided, verifies the signature. Conceptually it treats the token as base64url-encoded JSON separated by dots; the header and payload are parsed as JSON and displayed for inspection. For verification, the tool computes the signature using the selected algorithm and compares it against the token's signature portion, returning pass/fail results and detected issues. This is valuable for API developers, security engineers, and QA teams who need to confirm token structure, validate claims such as iss, aud, exp, and iat, and troubleshoot signing problems. The tool supports common algorithms (HS256, RS256, ES256) when keys are supplied and can flag algorithm mismatches. Advanced users can test edge cases like expired tokens, future-dated tokens, or tokens with missing fields, and receive guidance on remediation. Unique value comes from combining immediate decoding with optional cryptographic verification in a single workflow, without requiring local tooling beyond the browser. Typical use cases include API client debugging, JWT-based authentication audits, and validating tokens captured from live traffic. Core features include header/payload JSON visualization, base64url decoding without padding, optional signature verification, algorithm hints, and concise threat signals for invalid tokens. This tool fits DevOps, backend developers, and security reviewers who need fast, deterministic token analysis. Outputs are designed for programmatic consumption and for human review in a token analysis report. Outputs are structured to aid downstream tooling and documentation workflows for token-based authentication. The approach emphasizes deterministic behavior and reproducible results across token sets.