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JWT Decoder

Encoding & Encryption · Free Online Tool

Decode JWT tokens and inspect header and payload.

By EZ4Code Team

This tool runs interactively in your browser. Please visit ez4code.com/tools/jwt-decoder to use it.

How to Use JWT Decoder

  1. 1Enter or paste your data into the input field.
  2. 2Select the desired encoding, hashing, or encryption options.
  3. 3Click the action button to process your data.
  4. 4Copy the result from the output field.

Features

  • ✓100% free — no signup, no subscription, no ads
  • ✓Runs entirely in your browser — data never leaves your device
  • ✓Fast and lightweight — instant results
  • ✓Cross-platform — works on desktop, tablet, and mobile

What is JWT Decoder?

JSON Web Token (JWT), defined by RFC 7519, is a compact, URL-safe token format for securely transmitting claims between parties. A JWT consists of three Base64URL-encoded parts separated by dots: Header.Payload.Signature. It is widely used for API authentication, OAuth 2.0, and SSO.

How does JWT Decoder work?

The Header specifies the signing algorithm (e.g., HS256, RS256). The Payload contains claims (registered, public, private) — statements about an entity. The Signature is computed over the encoded Header and Payload using the algorithm and a secret/private key. To verify, recompute the signature and compare.

Common Use Cases

  • ✓API authentication — bearer tokens in Authorization headers
  • ✓OAuth 2.0 — access tokens and ID tokens
  • ✓SSO — share identity across multiple services
  • ✓Stateless sessions — store user info without server-side sessions

JWT vs Session Cookies

Sessions store state server-side; JWTs are stateless and self-contained. JWTs scale better (no session store) but are harder to revoke before expiry. Sessions are more secure for sensitive apps (can revoke instantly); JWTs suit distributed/microservice architectures. Always use HTTPS for both.

Security & Privacy

This tool decodes JWTs locally in your browser — your token never leaves your device. IMPORTANT: Decoding a JWT only reads its contents; it does NOT verify the signature. Never trust JWT claims without server-side signature verification. JWTs are visible to anyone who intercepts them — use HTTPS.

Technical Details

Standard: RFC 7519. Structure: Base64URL(Header).Base64URL(Payload).Base64URL(Signature). Header: alg (HS256/RS256/ES256/none), typ. Registered claims: iss (issuer), sub (subject), aud (audience), exp (expiry), nbf (not before), iat (issued at), jti (JWT ID). Signature: HMAC(SHA-256) for HS256, RSA for RS256. 'alg: none' tokens must always be rejected.

Frequently Asked Questions

Is decoding a JWT the same as verifying it?

No. Decoding reads the payload; verification checks the signature cryptographically. Anyone can decode a JWT; only the server with the secret can verify it. Never trust unverified JWT claims.

What is the 'alg: none' vulnerability?

If a server accepts 'alg: none' tokens, attackers can forge tokens without a signature. Always reject 'none' algorithm and use a hardcoded allowed-algorithms list.

How long should a JWT live?

Access tokens: 15-60 minutes. Refresh tokens: days to weeks. Short-lived access tokens limit damage if leaked. Use refresh tokens to maintain sessions.

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