JWT Decoder

A JWT (JSON Web Token, pronounced "jot") is a compact, URL-safe way to transmit claims between parties. It is the most common stateless authentication mechanism on the web. Every OAuth 2.0 flow, every OpenID Connect login, and most single-page applications use JWTs to identify users without server-side sessions.

JWT structure: header.payload.signature

A JWT looks like three Base64url-encoded strings separated by dots:

eyJhbGciOiJIUzI1NiJ9.eyJzdWIiOiIxMjM0NTY3ODkwIn0.dozjgNryP4J3jVmNHl0w5N_XgL0n3I9PlFUP0THsR8U

• Header: Metadata — signing algorithm (HS256, RS256, ES256) and token type (JWT).
• Payload: The claims — user ID (sub), expiration (exp), issuer (iss), and any custom data the server chose to include.
• Signature: A cryptographic hash of the header and payload, used to verify the token has not been tampered with.

Standard JWT claims

Claim	Name	Description
sub	Subject	The principal this token is about (usually a user ID)
iss	Issuer	Who created and signed this token
aud	Audience	Who this token is intended for
exp	Expiration	Unix timestamp after which the token is invalid
nbf	Not Before	Unix timestamp before which the token is invalid
iat	Issued At	Unix timestamp when the token was created
jti	JWT ID	Unique identifier to prevent token replay

Decoding vs. verifying

Anyone can decode a JWT. The header and payload are Base64url-encoded, not encrypted. Paste a token above to see its contents immediately. Never put secrets (passwords, API keys, session tokens) in a JWT payload — assume anyone who captures the token can read it.

Verification proves authenticity. To verify that a JWT was genuinely issued by the claimed authority and has not been modified, provide the secret key (for HMAC algorithms) or the public key (for RSA/ECDSA). The tool uses WebCrypto in your browser to verify the signature — no token data leaves your machine.

Common JWT algorithms

Algorithm	Type	Key	Use case
HS256	HMAC-SHA256	Shared secret	Single-service tokens, internal APIs
RS256	RSA-SHA256	Public/private key pair	Multi-service, third-party verification
ES256	ECDSA P-256	Public/private key pair	Smaller signatures, mobile/performance

HS256 uses a shared secret — anyone who knows the secret can both create and verify tokens. RS256 and ES256 use asymmetric keys: the issuer signs with a private key, and anyone with the public key can verify. This is essential for OAuth 2.0, where the authorization server signs tokens and resource servers verify them without sharing secrets.