JWT Encoder — Create, Sign & Verify Tokens Securely

Subhead Encode and sign JSON Web Tokens (JWT) in your browser with Base64URL-safe encoding and HS/RS/ES support. Decode tokens, verify signatures, validate claims, and download — all processed client-side unless you choose otherwise.

Primary CTA Encode & Sign JWT

PRIMARY ARTICLE

What is a JWT and why use a JWT Encoder?

A JWT (JSON Web Token) is a compact, URL-safe token standard (RFC 7519) that represents claims between parties. It consists of three Base64URL parts: header, payload (claims), and signature.

A JWT Encoder helps developers safely build tokens: it constructs the header and payload, applies Base64URL encoding, signs with a selected algorithm (e.g., HS256, RS256, ES256), and optionally verifies signatures and claims. Use an encoder to prototype tokens, debug auth flows, or learn JWT internals.

Core features (clear bullets + benefits)

• Encode & Decode JWTs — Build tokens from header + payload or decode existing tokens to inspect claims.
• Sign Tokens (HS/RS/ES/PSS) — Support HMAC (HS256/384/512), RSA (RS256/384/512, PS256), and ECDSA (ES256/384/512).
• Verify Signatures — Verify with shared secrets or public keys (PEM/JWK).
• Base64URL Encoding — Use URL-safe Base64 without padding (required by RFC 7515).
• Claim Validation — Validate exp, nbf, iat, aud, and iss with time-safe checks.
• Keypair Generation — Generate test RSA/ECDSA keypairs or import JWKs for verification.
• Copy / Download — One-click copy and download .jwt or .txt.
• Client-side by default — Tokens are generated and verified in the browser; no server uploads unless you opt in.
• Examples & How-tos — Code snippets for Node (jsonwebtoken, jose), Python (PyJWT), and browser usage.
• Security Warnings — In-tool prompts prevent developers from pasting production secrets.

How JWT encoding & signing works (simple pipeline)

1. Header & Payload — Developer supplies a JSON header (algorithm + typ) and payload (claims).
2. Base64URL encode — Both header and payload are serialized to UTF-8, then Base64URL-encoded (no padding).
3. Signing input — The signing input is base64url(header) + '.' + base64url(payload).
4. Signature — Sign the input using the chosen algorithm and secret/private key. Signature is Base64URL-encoded and appended.
5. Result — Final token: header.payload.signature.

Important: JWT uses Base64URL, not plain Base64. This avoids + and / characters and removes padding =.

Common pitfalls & security gotchas

• Using alg: none — Never accept tokens with "alg": "none" in production. Some buggy libraries once did.
• Wrong algorithm handling — Don’t trust the token’s alg header blindly; the server should enforce allowed algorithms.
• Weak secrets — Use sufficiently random shared secrets for HS*, and keep private keys private for RS/ES.
• Clock skew & exp validation — Validate exp/nbf with a small clock-skew allowance (e.g., ±5s).
• Reusing production secrets in client-side tools — Never paste real production private keys or secrets in public tools. Use test keys.
• Signature verification pitfalls — For RSA/ECDSA, validate using the correct key format (PEM vs JWK) and library.

Before & After Example

Header

Payload

HS256 Signing (example using a secret "secret") — result

Verify: Use HMAC SHA-256 with the same secret to recompute the signature and compare.

Technical depth: algorithms, libs & best practices

Algorithms explained:

• HS256/384/512 (HMAC): symmetric; fast; use strong secrets.
• RS256 / PS256 (RSA): asymmetric; sign with private key, verify with public key. PS = RSASSA-PSS (recommended over PKCS#1 v1.5 in some contexts).
• ES256/384/512 (ECDSA): asymmetric & smaller signatures, suitable for mobile/IoT.

Libraries & tools:

• Node: jsonwebtoken (simple), jose (modern, RFC-complete).
• Python: PyJWT, jose variants.
• Go/Java/.NET: official JWT libraries per language.
• Browser: jose works in browsers; careful not to expose secrets.

Best practices:

• Prefer RS/ES for third-party tokens; keep private keys secure.
• Rotate keys regularly and use JWK sets for key discovery (jwks).
• Validate aud, iss, exp, nbf, and iat.
• Avoid storing sensitive data in JWT payloads if token is stored client-side.

Client-side vs Server-side: when to use the tool

Client-side (this tool): prototyping, debugging, inspecting, generating test tokens, learning. Tokens are created & verified in the browser—fast & private.

Server-side (production): never use browser-generated secrets for production auth. Production signing should happen on trusted servers or HSMs/KMS.

Comparison: Langstop JWT Encoder vs Common tools (text table)

Sources reviewed: jwt.io, Auth0/jwt debugger, jwt.ms, token.dev, FusionAuth tools. ([JSON Web Tokens - jwt.io][1])

Primary keywords (short-tail)

• jwt encoder
• jwt generator
• encode jwt online

Secondary keywords

• jwt encoder online
• jwt debugger
• jwt sign verify

18 long-tail keywords (natural phrasing)

1. How to encode JWT online for testing
2. Generate HS256 JWT with secret in browser
3. Verify RS256 JWT signature with public key
4. Create JWT with ECDSA ES256 for mobile apps
5. Encode JWT without padding Base64URL rules
6. Debug JWT claims and expiration errors
7. Convert header payload to JWT and sign
8. How to validate aud and iss claims in JWT
9. JWT generator with JWK import and key rotation
10. Client-side JWT encoder privacy best practices
11. How to avoid alg none vulnerability in JWT
12. JWT token generation for OAuth2 test flows
13. Create JWT with custom claims online
14. How to verify JWT signature in browser with jose
15. JWT encoder that supports PS256 and ES512
16. Create and download JWT token for API testing
17. Generate JWT for local development without secrets
18. Best JWT encoder for developers and QA

FAQ — People Also Ask (20 concise answers; two deeper ones)

1. What is a JWT? A compact, URL-safe token that carries claims as JSON, typically used for authentication and authorization.

2. How does JWT signing work? Header and payload are Base64URL-encoded, concatenated, and signed with a secret or private key; the signature is appended.

3. What is Base64URL? A URL-safe variant of Base64 that replaces + and / with - and _ and omits padding =.

4. Can I generate JWTs in the browser? Yes for testing and debugging, but never use real production secrets client-side.

5. What algorithms are supported? Commonly HS256/384/512 (HMAC), RS256/384/512 (RSA), PS*, and ES* (ECDSA).

6. How do I verify a JWT signature? Recompute the signature using the header/payload and the appropriate key, then compare in constant time.

7. What does exp mean? exp is the expiration time; tokens after this UNIX timestamp should be rejected.

8. Can JWTs be encrypted? Yes — JWE (JSON Web Encryption) encrypts payloads; JWS is for signing.

9. Is Base64URL padding optional? JWT spec uses no padding; libraries typically accept both padded and unpadded forms.

10. What’s the difference between JWS and JWE? JWS signs payloads (verifiable integrity), JWE encrypts payloads (confidentiality).

11. Should I store secrets in the browser? No — never store production secrets in client-side code or tools.

12. How to handle clock drift when validating iat/exp? Allow small clock skew (e.g., 5–60 seconds) when validating time-based claims.

13. Can JWTs be revoked? Not natively; implement token revocation via server-side blacklists or short expiration + refresh tokens.

14. What is alg: none and is it safe? alg: none means unsigned tokens; treat them as unauthenticated and block in production.

15. How to import JWK or PEM keys for verification? Convert PEM to JWK or import directly if your tool accepts PEM; ensure correct key type for algorithm.

16. How to create RS256 keys? Use openssl genpkey -algorithm RSA -pkeyopt rsa_keygen_bits:2048 for test keys, and keep private keys secure.

17. Can JWT carry sensitive data? (longer answer ~120 words) JWTs are not encrypted by default; the payload is base64url-encoded but easily decoded. Do not store secrets, passwords, or PII you can't afford to leak. If you must send sensitive data, use JWE (encrypted JWT) or keep sensitive data server-side and store only non-sensitive claims in tokens. Also, use short-lived tokens and rotate keys frequently. Ensure tokens are transmitted over TLS and validated thoroughly. For highly sensitive workflows, prefer server-side session tokens or token binding mechanisms.

18. How to debug a JWT that fails verification? Check algorithm mismatch, key format (PEM vs JWK), clock skew (exp/nbf), and ensure the signing key matches verification key.

19. Can I generate JWTs for OAuth testing? Yes—encoders help craft test tokens for OAuth flows, but use test credentials and avoid production secrets.

20. What is a JWK Set (JWKS)? (longer answer ~120 words) A JWKS is a JSON Web Key Set — a JSON document that contains public keys (JWKs) used by servers to publish their verification keys. Clients fetch the JWKS endpoint (commonly /.well-known/jwks.json) to verify tokens dynamically and handle key rotation without manual key updates. Implement caching and key-id (kid) lookups. When building a JWT tool, allow importing JWKS URLs to automatically fetch public keys for signature verification.

Suggested internal links

• JWT Debugger
• JSON Web Token Guide
• JSON Formatter
• JSON Encoder
• API Testing Tool
• Security Best Practices (blog)
• Tools Hub
• CLI Integration Guide

Suggested external authoritative references

• RFC 7519 — JSON Web Token (JWT)
• RFC 7515 — JSON Web Signature (JWS)
• RFC 7516 — JSON Web Encryption (JWE)
• jwt.io — Introduction & Libraries
• OWASP — Authentication Cheat Sheet

CTAs: Encode & Sign JWT • Decode JWT • Verify Signature • Generate Keypair

Privacy reassurance: Tokens are encoded and verified in your browser by default. Never paste production secrets — the tool warns if a likely production secret is detected.

• Encode & Sign → Primary
• Decoded — View header & claims → Success state
• Verification failed — invalid signature or wrong key → Error state

Error messages & fix tips:

• Invalid JWT format → Ensure token has three dot-separated parts.
• Signature verification failed → Check public key / secret and algorithm.
• Malformed key → Ensure PEM/JWK is valid; convert if necessary.

Competitive Intelligence

• Security-first UX: warn on pasting private keys/secrets and provide test-key mode.
• Deeper dev guidance: include code snippets for jose, jsonwebtoken, PyJWT.
• Algorithm coverage: include PS* (PSS) and ES* examples.
• Claim validation UI: let users assert expected aud/iss and show pass/fail.
• Key management: allow JWK import + JWKS URL fetch for verification.
• Downloadable asset: “JWT Best Practices — Checklist” (PDF) for link building.

Short content brief (6 bullets for editor/dev)

1. Publish path: /jwt-encoder with canonical meta variants above; hero CTA opens in-page encode/sign UI.
2. Default client-side: implement encoding and verification in Web Worker; optional server-side bulk signing behind auth. Warn on secrets.
3. Structured data: add WebPage + SoftwareApplication + FAQPage JSON-LD; include potentialAction for Encode action.
4. Developer UX: include code snippets (Node jose/jsonwebtoken, Python PyJWT) and key import (PEM/JWK) UI.
5. Security features: secret detection, test-key mode, JWKS import for verification, and claim validation toggles.
6. Extras: downloadable “JWT Best Practices” PDF, two extra examples (JWE and JWKS usage), and an interactive demo (generate token & verify with public key).