🔑 JWT Token Decoder & Inspector

A JWT decoder that also verifies the signature in your browser: HS256 with a secret, RS256 and ES256 with a public key. Claims, expiry and alg:none flagged.

✓ Free✓ No Signup Required✓ Browser-Based

Paste Encoded JSON Web Token (JWT)

Shared secret for HS256 (optional — to verify the signature)

Verification runs in your browser through the Web Crypto API and nothing is transmitted. Even so, a production signing secret pasted into any web page has been typed into a browser and may sit in your clipboard and form history — use a test key.

HEADER: ALGORITHM & TOKEN TYPE

{
  "alg": "HS256",
  "typ": "JWT"
}

HS256: HMAC using SHA-256.

PAYLOAD: DATA CLAIMS

{
  "sub": "1234567890",
  "name": "Alex Turning",
  "iat": 1516239022,
  "exp": 1799999999,
  "role": "admin"
}

sub — Subject — who the token is about
iat — Issued At — when it was created
exp — Expiration Time — must not be accepted at or after this

What JWT Token Decoder & Inspector Does

A JSON Web Token is three Base64url-encoded segments separated by dots: a header describing the signing algorithm, a payload of claims, and a signature over the first two. Decoding the first two segments requires no key at all — they are encoded, not encrypted.

That property is the source of the most common and most costly JWT misunderstanding. Anyone holding a token can read every claim in it. Putting an email address, a role, an internal user ID or anything else sensitive in a JWT payload is equivalent to printing it on the outside of the envelope.

This decoder runs entirely in your browser and does not transmit the token. It shows the header, the decoded claims and the expiry status. It deliberately does not verify the signature, because verification requires the secret or public key — and pasting a signing secret into any web page is a mistake.

How to Use JWT Token Decoder & Inspector

Paste your encoded JSON Web Token (JWT) into the text area
Instantly inspect the decoded Header and Payload JSON structures
Check token expiration status and timestamp validity
Optionally paste the shared secret (HS256) or PEM public key (RS256, ES256) to verify the signature in your browser

JWT Structure

Example: eyJhbGciOiJIUzI1NiJ9.eyJzdWIiOiIxMjMifQ.dBjftJeZ4CVP… — three dot-separated segments.

Segment	Contents	Encrypted?
Header	Algorithm (alg) and token type (typ)	No — Base64url only
Payload	Claims: subject, expiry, issuer, custom fields	No — Base64url only, readable by anyone
Signature	HMAC or digital signature over header.payload	Not encrypted; proves integrity, not confidentiality

Registered Claims (RFC 7519 §4.1)

All are optional, but each has a defined meaning that libraries rely on.

Claim	Name	Meaning
iss	Issuer	Who created and signed the token
sub	Subject	Who the token is about — usually a user ID
aud	Audience	Who the token is intended for; reject if it is not you
exp	Expiration Time	Unix seconds after which the token must be rejected
nbf	Not Before	Unix seconds before which the token must be rejected
iat	Issued At	Unix seconds when the token was created
jti	JWT ID	Unique identifier, used to prevent replay

Source: RFC 7519 — JSON Web Token

Common Signing Algorithms

alg	Type	Key	Notes
HS256	HMAC + SHA-256	Shared secret	Verifier can also forge tokens — same key both ways
RS256	RSA + SHA-256	Private/public pair	Verifiers only need the public key; standard for OIDC
ES256	ECDSA P-256 + SHA-256	Private/public pair	Much shorter signatures than RS256
none	Unsecured	None	Must be rejected. Accepting it means accepting forged tokens

Every JWS Algorithm and What the Spec Requires of It

RFC 7518 §3.1, in full. The requirement column is the specification's own — only HS256 is Required of a conforming implementation, which is why it is the one you meet everywhere. "Recommended+" means ES256 is expected to become Required in a future revision.

alg	Algorithm	Implementation requirement
HS256	HMAC using SHA-256	Required
HS384	HMAC using SHA-384	Optional
HS512	HMAC using SHA-512	Optional
RS256	RSASSA-PKCS1-v1_5 using SHA-256	Recommended
RS384	RSASSA-PKCS1-v1_5 using SHA-384	Optional
RS512	RSASSA-PKCS1-v1_5 using SHA-512	Optional
ES256	ECDSA using P-256 and SHA-256	Recommended+
ES384	ECDSA using P-384 and SHA-384	Optional
ES512	ECDSA using P-521 and SHA-512	Optional
PS256	RSASSA-PSS using SHA-256 and MGF1 with SHA-256	Optional
PS384	RSASSA-PSS using SHA-384 and MGF1 with SHA-384	Optional
PS512	RSASSA-PSS using SHA-512 and MGF1 with SHA-512	Optional
none	No digital signature or MAC performed	Optional

Source: RFC 7518 — JSON Web Algorithms §3.1

Decoding and Verifying in Your Language

The libraries differ; the rule does not. Pass the algorithms you expect as a fixed list. A verifier that takes the algorithm from the token's own header is letting the attacker choose how their forgery gets checked.

Language	Usual library	The thing to get right
JavaScript / Node	jsonwebtoken, or jose for JWK and JWKS	Pass an explicit algorithms list; do not rely on the default
Python	PyJWT	algorithms= is documented as "do not compute from the alg in the token itself" — hard-code it or configure it beside the key
Java	java-jwt (Auth0) or jjwt	Build the verifier around one algorithm object rather than reading the header
Go	golang-jwt/jwt	Check the method inside the keyfunc — the classic mistake is returning a key without looking at it
PHP	firebase/php-jwt	Pass a Key with its algorithm, not a bare string
.NET	System.IdentityModel.Tokens.Jwt	Set ValidAlgorithms in the validation parameters
Browser	Web Crypto API	What this page uses — importKey then verify, no dependency
Shell	base64 -d and jq for reading	Reading is trivial; verifying in shell is not — use a library

How to Read Your Result

What a tampered token actually looks like

This page shipped one for months without anyone noticing, which is the point. Its payload read {"sub":"1234567890","name":"Alex Turning","iat":1516239022,"exp":1799999999,"role":"admin"} and its signature was SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c — the famous jwt.io signature, which is computed over a completely different payload, {"sub":"1234567890","name":"John Doe","iat":1516239022}. Somebody had edited the claims, granted themselves an admin role and an expiry three years out, and kept the old signature. It decodes perfectly. Every field displays. Nothing about reading it reveals the problem: only recomputing the HMAC over header.payload and comparing does, which is why "it decoded fine" is never evidence that a token is genuine.

Verifying HS256 needs the secret; verifying RS256 does not

This is the practical difference between the families and it decides which one you should be using. HMAC signs and verifies with the same shared secret, so anyone who can check a token can also mint one — fine inside a single service, dangerous the moment a second party needs to validate. The RSA and ECDSA families split it: the issuer holds the private key, everyone else verifies with a public key that grants no signing power. That is why OpenID Connect providers publish a JWKS endpoint of public keys and sign with RS256 or ES256. RFC 7518 also sets a floor on the HMAC side: "a key of the same size as the hash output (for instance, 256 bits for HS256) or larger MUST be used" — a short passphrase does not meet it, however strong it looks.

The "alg: none" attack

The specification permits an unsecured JWT with alg set to "none" and an empty signature. Libraries that honored the header's own claim about which algorithm to use would accept an attacker-modified token with the signature stripped. Any verifier must decide the expected algorithm itself and refuse anything else — never trust the token to tell you how to check the token.

Algorithm confusion (RS256 → HS256)

A related flaw: an attacker changes alg from RS256 to HS256 and signs with the server's public key as if it were an HMAC secret. If the library selects the verification method from the header, it validates successfully. The public key is, by definition, public. Pin the algorithm server-side.

Expiry and revocation

exp is a Unix timestamp in seconds, not milliseconds — a common bug that produces tokens valid until the year 56000. More fundamentally, a signed JWT cannot be revoked: it is valid until it expires, because verification is offline. Short lifetimes plus a refresh token, or a server-side deny list, are the usual mitigations.

Limitations & Accuracy Notes

Verification here proves the signature matches the key you supplied. It says nothing about whether that is the right key, whether the issuer is one you trust, or whether the token has been revoked — all of which your server still has to decide.
Signature checking covers the HS, RS, PS and ES families through the Web Crypto API. EdDSA (Ed25519) is not covered because browser support for it is still uneven, and JWE encrypted tokens are a different format entirely.
Never paste a production token containing live session credentials into any online decoder, including this one. Although decoding here happens locally in your browser, the safe habit is to use tokens from a development environment.
JWE (encrypted JWTs) have five segments rather than three and cannot be read without the decryption key. This decoder handles JWS, the signed three-segment form.
Claim names beyond the registered set are application-defined. A "role" or "scope" claim means whatever the issuing system decided it means.

Frequently Asked Questions

What are the three parts of a JWT token?

A JWT consists of 3 dot-separated Base64URL-encoded strings: Header (algorithm & token type), Payload (data claims & expiration), and Signature (tamper-verification).

Is my secret key exposed when decoding a JWT here?

No, decoding only reads the public Base64URL payload. All decoding executes strictly inside your browser, keeping your tokens confidential.

Can decoding a JWT verify that it is genuine?

No — decoding only reads the header and payload, which anyone can do since they are just Base64URL-encoded, not encrypted. Verifying authenticity requires checking the signature, which this decoder does not do.

Why shouldn’t I put sensitive data in a JWT payload?

Because the payload is only encoded, not encrypted, anyone who obtains the token can read its contents directly. JWTs prove a claim’s origin and integrity, not secrecy of its contents.

Can this verify the JWT signature, not just decode it?

Yes. Paste a shared secret for the HS256, HS384 and HS512 tokens, or a PEM public key for the RS, PS and ES families, and the check runs in your browser through the Web Crypto API. A valid result means the token has not been altered since it was signed with that key — it does not mean the key is the right one, the issuer is trusted, or the token has not been revoked. Those decisions still belong to your server.

Why does my token show "invalid signature"?

Three usual causes. The key is wrong — for HMAC the secret must match byte for byte, and a trailing newline counts. The algorithm is not the one you think: an RS256 token needs the public key, not the secret. Or the token really was modified, which is the case worth taking seriously. Note that a token can decode perfectly and still be forged; this page previously shipped a sample that did exactly that.

How do I decode a JWT with the secret to check it is genuine?

You recompute the signature rather than decrypt anything. Take the first two segments exactly as they appear, join them with a dot, run HMAC-SHA256 over that string with your secret, base64url-encode the result and compare it to the third segment. If they match, the claims are unchanged. That is all verification is for HS256 — which is also why the same secret can forge a token as easily as check one.

What key size does HS256 need?

At least 256 bits. RFC 7518 §3.2 states that "a key of the same size as the hash output (for instance, 256 bits for HS256) or larger MUST be used". A memorable passphrase is usually far short of 32 bytes, which is how HMAC-signed tokens end up brute-forceable regardless of how good the algorithm is. Generate random bytes instead.

Is my token sent anywhere?

No. Decoding happens entirely in your browser, which is the point — pasting a live token into a server-side decoder means handing a working credential to someone else.

Does decoding a JWT verify it?

No, and this is the critical distinction. Decoding reads the header and payload, which are only Base64url-encoded. Verifying means checking the signature against a secret or public key, which proves the token has not been altered. A decoded token tells you what it claims, not whether those claims are trustworthy.

Is the payload encrypted?

No. Anyone holding the token can read every claim in it. That means a JWT should never carry anything you would not be willing to show the person holding it — no passwords, no internal identifiers you consider sensitive, no personal data beyond what is necessary.

What do exp, iat and nbf mean?

They are standard registered claims: exp is the expiry time, iat is when the token was issued, and nbf is the time before which it must not be accepted. All three are Unix timestamps in seconds, not milliseconds — a factor-of-1000 mistake here produces expiry dates in the far future.

What is the "alg: none" vulnerability?

An old but instructive flaw. Some libraries honored a header claiming the algorithm was "none" and skipped signature verification entirely, letting anyone forge a token by editing the payload. Current libraries reject it, and the general lesson holds: the algorithm must be decided by the verifying server, never read from the token.

Can I edit a token here and have it still work?

No. Changing any part of the payload invalidates the signature, and any correctly implemented server will reject it. Producing a valid modified token requires the signing key.

References & Further Reading

RFC 7519 — JSON Web Token (JWT) — The core specification and registered claim definitions
RFC 8725 — JSON Web Token Best Current Practices — Documents the alg:none and algorithm-confusion attacks and their mitigations
RFC 4648 — Base64url encoding — Section 5 defines the URL-safe alphabet JWT segments use
RFC 7518 — JSON Web Algorithms (JWA) — Section 3.1 is the full alg table with its Required/Recommended/Optional column; 3.2 sets the HMAC minimum key size; 3.6 says implementations "MUST NOT accept Unsecured JWSs by default"
PyJWT — API reference — Source for the warning not to compute the algorithms parameter from the token's own alg header

By OnlineToolHubs Team • September 2026