/
Direct answer
The UseCOS JWT Decoder parses Base64URL-encoded JSON Web Tokens directly within your browser memory sandbox. It parses Header, Payload, and Signature segments, humanizes UNIX expiration and issuance timestamps, verifies standard RFC 7519 registered claims, and guarantees zero network transmission of sensitive security tokens.
100% Client-side in-browser decoding. Tokens are never transmitted across the network or stored.
{
"alg": "RS256",
"typ": "JWT",
"kid": "key-2026-01"
}{
"iss": "https://auth.usecos.app/",
"sub": "usr_0a1b2c3d4e",
"aud": [
"https://api.usecos.app"
],
"iat": 1780000000,
"exp": 1880000000,
"scope": "read write openid profile",
"email": "developer@usecos.app"
}| Claim | Parsed Value | Interpreted Time / Offset | Specification Role |
|---|---|---|---|
| expExpiration Time | 1880000000 | Sun, 29 Jul 2029 06:13:20 GMT in 994d | Identifies the expiration time on or after which the JWT MUST NOT be accepted for processing. |
| iatIssued At | 1780000000 | Thu, 28 May 2026 20:26:40 GMT 162d ago | Identifies the time at which the JWT was issued. |
| issIssuer | "https://auth.usecos.app/" | — | Identifies the principal that issued the JWT (e.g. auth.example.com). |
| subSubject | "usr_0a1b2c3d4e" | — | Identifies the principal that is the subject of the JWT (user UUID or account ID). |
| audAudience | ["https://api.usecos.app"] | — | Identifies the recipients that the JWT is intended for. |
JWT signatures verify that the token sender is who it says it is and ensure that the payload has not been tampered with. For asymmetric keys (RS256, ES256), verification requires the issuer's public JWKS key. For symmetric keys (HS256), verification requires the shared secret. Because UseCOS runs 100% locally in your browser, your private secrets are never uploaded or exposed.
Standard RFC 7519 registered claims, their security validation rules, and common developer pitfalls.
| Claim | Standard Type | Validation Rule | Production Impact |
|---|---|---|---|
| exp | NumericDate (Seconds) | Current time < exp (accounting for clock skew) | Token must be rejected immediately once expired. |
| iat | NumericDate (Seconds) | Time token was generated | Used to calculate session age and invalidate prior tokens. |
| nbf | NumericDate (Seconds) | Current time >= nbf | Tokens scheduled for future activation cannot be processed before this time. |
| iss | StringOrURI | Must match expected IdP URL exactly | Protects against spoofed tokens issued by foreign identity providers. |
| aud | String / Array | Must include recipient's Client ID / API Audience | Prevents token substitution attacks across distinct backend microservices. |
Why In-Browser Client-Side Decoding Matters: Many online JWT debuggers send tokens to backend servers for decoding, creating serious compliance violations under SOC2, HIPAA, and GDPR. Pasting real production tokens into cloud-hosted tools can expose sensitive user emails, tenant IDs, authorization scopes, and internal microservice endpoints. The UseCOS engine runs entirely inside your browser's JavaScript runtime via client-side base64url parsing, meaning no token data ever leaves your computer.
Signature Verification Architecture: A JWT is cryptographically signed using either a symmetric shared secret (e.g., HMAC-SHA256) or an asymmetric private key (e.g., RSA or ECDSA). When inspecting tokens from providers such as Auth0, Supabase, Firebase, AWS Cognito, or Microsoft Entra ID, verification in production should be performed by querying the provider's JSON Web Key Set (/.well-known/jwks.json) to obtain the matching public key for the header's kid (Key ID).
Zero telemetry, zero remote requests, and no external tracking. Your tokens and claims are parsed in temporary memory and never stored in cookies or localStorage.