encoding
JWT Decoder
Decode and verify JSON Web Tokens — header, payload, claims, and signature.
JWT Decoder
Decode and verify JSON Web Tokens. Signature verification runs in your browser — your secret never leaves.
{
"alg": "HS256",
"typ": "JWT"
}{
"sub": "1234567890",
"name": "John Doe",
"iat": 1516239022
}- Issued at
- 1/18/2018, 1:30:22 AM
- Subject
- 1234567890
SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c
What is a JWT?
A JSON Web Token (JWT) is a compact, URL-safe way to represent claims. It has three base64url-encoded parts separated by dots: a header (algorithm + token type), a payload (claims), and a signature (verification). This tool decodes the header and payload, and can optionally verify the signature using your secret or public key.
Is this secure?
Signature verification uses your browser's Web Crypto API — your secret or private key is sent to our server only for the verification call and is never logged or stored. The decoded header and payload are computed entirely in your browser, no server involved.
What is a JWT Decoder?
A JSON Web Token (JWT) is a compact, URL-safe string used to securely transmit claims between two parties. It consists of three base64url-encoded segments separated by dots: a header, a payload, and a signature. The header specifies the signing algorithm (such as HS256, RS256, or ES256). The payload contains claims — statements about an entity and additional metadata, such as a user ID (sub), an expiry time (exp), an issuer (iss), and any custom application-defined fields. The signature is computed over the header and payload using a secret or private key, allowing the recipient to verify that the token was issued by a trusted source and has not been modified. JWTs are the standard mechanism for stateless authentication in modern web applications: once issued, they travel with every API request (typically in the Authorization: Bearer header) and can be validated without a database lookup. This tool decodes the header and payload instantly without requiring a key, and can optionally verify the signature using your HMAC secret or RSA/ECDSA public key — all within your browser.
How to decode a JWT
- Paste your token: Copy your JWT string — it starts with 'eyJ' and contains two dots separating the three segments. Paste it into the input field.
- Inspect the claims: The decoded header and payload appear as formatted JSON instantly. Check the algorithm (alg) in the header, and look for exp (expiry as a Unix timestamp), sub (subject/user ID), iss (issuer), and any application-specific claims in the payload.
- Verify the signature (optional): To confirm the token has not been tampered with, enter your HMAC secret (for HS256/HS384/HS512) or PEM public key (for RS*/ES*) in the Verify section and click Verify. A green badge means the signature is valid.
Frequently asked questions
- Does this verify the signature?
- Yes — enter your HMAC secret or RSA/ECDSA public key in the Verify section. For HS* algorithms the secret is a plain text string; for RS*/ES* provide a PEM-formatted public key matching the private key that signed the token.
- Is my token sent to a server?
- The browser-side decoder never makes a network call — decoding is done entirely in JavaScript. The signature-verification call does send your token and key over HTTPS to a server-side endpoint, but neither is logged or stored.
- What is the difference between HS256 and RS256?
- HS256 (HMAC-SHA-256) uses a single shared secret that both the issuer and the verifier must know. RS256 (RSA-SHA-256) uses a private key to sign and a public key to verify — the public key can be shared openly via a JWKS endpoint without exposing the signing key. RS256 is preferred for public APIs and multi-tenant systems.
- How do I check if a JWT has expired?
- After decoding, look for the 'exp' claim in the payload — it is a Unix timestamp (seconds since 1970-01-01 UTC). If Date.now() / 1000 > exp, the token is expired. The decoder converts timestamp claims to human-readable dates automatically so you can see the expiry at a glance.
- Can I decode a JWT without the secret?
- Yes. Decoding only requires base64url-decoding the header and payload segments, which requires no key. Anyone with the token string can read the claims — JWTs are encoded, not encrypted. Never store sensitive data (passwords, payment details) in a JWT payload unless you also encrypt it (JWE).
- What should I do if I see 'Invalid signature'?
- Confirm you are using the correct key for the algorithm shown in the header. For HS256, check for extra whitespace or base64 encoding on the secret. For RS256/ES256, confirm you are using the public key (not the private key) in PEM format. A wrong key is always the cause of a signature mismatch.