RFC 7519 規格と HMAC 検証6 分で読める最終更新日:
JWTトークンの構造と署名検証：RFC 7519 規格技術解説
JWT のヘッダーとペイロードをローカルでデコードし、HMAC-SHA256 署名を検証。トークンの有効期限を安全に確認。
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WEBioDD エンジニアリングチーム•100% ブラウザ内完結＆ゼロトラスト
重要ポイント
- JWT はドットで区切られた3つの Base64URL 要素（Header、Payload、Signature）で構成されます。
- 署名は改ざんを防止しますが暗号化はしません。ペイロードは誰でも閲覧可能です。
- リプレイ攻撃を防ぐため、exp（有効期限）および nbf（利用開始日時）クレームを必ず検証してください。
- 本番環境のシークレットキーやトークンを信頼できないオンラインツールに貼り付けないでください。
100% In-Browser & Local Execution
JWT トークンを安全にデコード・デバッグ
秘密鍵を外部に送信することなく、ブラウザ内でヘッダー、ペイロード、署名を解析・検証。
- RFC 7519 compliant parser
- HMAC-SHA256 signature verify
- Live expiration countdown
- 100% client-side execution
01.JWT Token Anatomy: Header, Payload, and Signature
A JSON Web Token (JWT) is a compact, URL-safe string divided into three Base64URL-encoded segments: Header.Payload.Signature. The Header declares the signing algorithm (e.g., HS256 for HMAC-SHA256). The Payload contains claims (user ID, roles, expiration). The Signature is computed by hashing the encoded header and payload with a secret key, ensuring the token has not been tampered with.
typescriptClient-side JWT parsing without server transmission
// JWT Decoding Algorithm
function decodeJWT(token: string) {
const [headerB64, payloadB64, signatureB64] = token.split('.');
const header = JSON.parse(atob(headerB64.replace(/-/g, '+').replace(/_/g, '/')));
const payload = JSON.parse(atob(payloadB64.replace(/-/g, '+').replace(/_/g, '/')));
return { header, payload, signature: signatureB64 };
}02.HMAC-SHA256 Signature Verification Process
To verify a JWT signature locally, concatenate the Base64URL-encoded header and payload with a dot, then compute HMAC-SHA256 using your secret key via Web Crypto API. Compare the result against the provided signature. A mismatch indicates tampering or an incorrect secret.
Never paste production secret keys into online JWT debuggers. Our tool runs 100% locally in your browser memory.
03.Token Expiration & Timestamp Claims
The exp claim (expiration time) is a Unix timestamp defining when the token becomes invalid. The nbf (not-before) claim prevents premature token usage. Always validate these claims on the server and display countdown timers to users in dashboards.
Local JWT Debugger vs. Remote Online Validators
Technical architectural comparison between local browser execution and cloud server processing.
| Security Aspect | Local Browser Tool | Third-Party JWT.io Sites |
|---|---|---|
| Secret Key Exposure | Never leaves browser memory | Transmitted over HTTPS POST |
| Token Payload Privacy | Decoded in local RAM only | Sent to remote servers & logged |
| Validation Latency | Instant (sub-millisecond) | 150ms - 500ms network delay |
| Offline Capability | Works without internet | Fails when disconnected |
How to Decode and Verify JWT Tokens
Step-by-step instructions for JWT inspection and validation.
1
Paste JWT Token
Copy your JWT string from API responses or browser cookies and paste into the debugger input field.
2
Inspect Header & Payload
View decoded JSON claims including user ID, roles, issued-at (iat), and expiration (exp) timestamps.
3
Verify Signature (Optional)
Enter your secret key to validate HMAC signature integrity (never use production keys on public tools).
4
Check Expiration Status
Review the live countdown timer showing time remaining until token expiry.
JWT Security Best Practices
- ✓Always use HTTPS to prevent JWT interception during transmission.
- ✓Set short expiration times (15-60 minutes) and refresh tokens via secure endpoints.
- ✓Never store sensitive data in JWT payloads; they are NOT encrypted.
- ✓Validate signatures and expiration timestamps on every API request server-side.
Frequently Asked Questions
No. JWT payloads are Base64URL-encoded, which is easily decoded. Never store passwords or PII in JWT claims.