What is JSON Web Token?

JSON Web Token (JWT) is an open standard (RFC 7519) that defines a compact and self-contained way for securely transmitting information between parties as a JSON object. This information can be verified and trusted because it is digitally signed. JWTs can be signed using a secret (with the HMAC algorithm) or a public/private key pair using RSA or ECDSA.

JSON Web Token (JWT) Overview

JWTs represent a set of claims as a JSON object that is encoded in a JWS and/or JWE structure. This JSON object is the JWT Claims Set. As per Section 4 of RFC 7159 [RFC7159], the JSON object consists of zero or more name/value pairs (or members), where the names are strings and the values are arbitrary JSON values. These members are the claims represented by the JWT. This JSON object MAY contain whitespace and/or line breaks before or after any JSON values or structural characters, in accordance with Section 2 of RFC 7159 [RFC7159].

The member names within the JWT Claims Set are referred to as Claim Names. The corresponding values are referred to as Claim Values.

The contents of the JOSE Header describe the cryptographic operations applied to the JWT Claims Set. If the JOSE Header is for a JWS, the JWT is represented as a JWS and the claims are digitally signed or MACed, with the JWT Claims Set being the JWS Payload. If the JOSE Header is for a JWE, the JWT is represented as a JWE and the claims are encrypted, with the JWT Claims Set being the plaintext encrypted by the JWE. A JWT may be enclosed in another JWE or JWS structure to create a Nested JWT, enabling nested signing and encryption to be performed.

A JWT is represented as a sequence of URL-safe parts separated by period ('.') characters. Each part contains a base64url-encoded value. The number of parts in the JWT is dependent upon the representation of the resulting JWS using the JWS Compact Serialization or JWE using the JWE Compact Serialization.

Jones, et al. Standards Track [Page 6]

RFC 7519 JSON Web Token (JWT) May 2015

3.1. Example JWT

The following example JOSE Header declares that the encoded object is a JWT, and the JWT is a JWS that is MACed using the HMAC SHA-256 algorithm:

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

To remove potential ambiguities in the representation of the JSON object above, the octet sequence for the actual UTF-8 representation used in this example for the JOSE Header above is also included below. (Note that ambiguities can arise due to differing platform representations of line breaks (CRLF versus LF), differing spacing at the beginning and ends of lines, whether the last line has a terminating line break or not, and other causes. In the representation used in this example, the first line has no leading or trailing spaces, a CRLF line break (13, 10) occurs between the first and second lines, the second line has one leading space (32) and no trailing spaces, and the last line does not have a terminating line break.) The octets representing the UTF-8 representation of the JOSE Header in this example (using JSON array notation) are:

[123, 34, 116, 121, 112, 34, 58, 34, 74, 87, 84, 34, 44, 13, 10, 32, 34, 97, 108, 103, 34, 58, 34, 72, 83, 50, 53, 54, 34, 125]

Base64url encoding the octets of the UTF-8 representation of the JOSE Header yields this encoded JOSE Header value:

 eyJ0eXAiOiJKV1QiLA0KICJhbGciOiJIUzI1NiJ9

The following is an example of a JWT Claims Set:

 {"iss":"joe",
  "exp":1300819380,
  "http://example.com/is_root":true}

The following octet sequence, which is the UTF-8 representation used in this example for the JWT Claims Set above, is the JWS Payload:

[123, 34, 105, 115, 115, 34, 58, 34, 106, 111, 101, 34, 44, 13, 10, 32, 34, 101, 120, 112, 34, 58, 49, 51, 48, 48, 56, 49, 57, 51, 56, 48, 44, 13, 10, 32, 34, 104, 116, 116, 112, 58, 47, 47, 101, 120, 97, 109, 112, 108, 101, 46, 99, 111, 109, 47, 105, 115, 95, 114, 111, 111, 116, 34, 58, 116, 114, 117, 101, 125]

Although JWTs can be encrypted to also provide secrecy between parties, we will focus on signed tokens. Signed tokens can verify the integrity of the claims contained within it, while encrypted tokens hide those claims from other parties. When tokens are signed using public/private key pairs, the signature also certifies that only the party holding the private key is the one that signed it.

When should you use JSON Web Tokens?

Here are some scenarios where JSON Web Tokens are useful:

Authorization: This is the most common scenario for using JWT. Once the user is logged in, each subsequent request will include the JWT, allowing the user to access routes, services, and resources that are permitted with that token. Single Sign On is a feature that widely uses JWT nowadays, because of its small overhead and its ability to be easily used across different domains.
Information Exchange: JSON Web Tokens are a good way of securely transmitting information between parties. Because JWTs can be signed—for example, using public/private key pairs—you can be sure the senders are who they say they are. Additionally, as the signature is calculated using the header and the payload, you can also verify that the content hasn't been tampered with.

What is the JSON Web Token structure?

In its compact form, JSON Web Tokens consist of three parts separated by dots (.), which are:

Header
Payload
Signature

Therefore, a JWT typically looks like the following.

xxxxx.yyyyy.zzzzz

Let's break down the different parts.

Header

The header typically consists of two parts: the type of the token, which is JWT, and the signing algorithm being used, such as HMAC SHA256 or RSA.

For example:

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

Then, this JSON is Base64Url encoded to form the first part of the JWT.

Payload

The second part of the token is the payload, which contains the claims. Claims are statements about an entity (typically, the user) and additional data. There are three types of claims: registered, public, and private claims.

Registered claims: These are a set of predefined claims which are not mandatory but recommended, to provide a set of useful, interoperable claims. Some of them are: iss (issuer), exp (expiration time), sub (subject), aud (audience), and others.
Notice that the claim names are only three characters long as JWT is meant to be compact.
Public claims: These can be defined at will by those using JWTs. But to avoid collisions they should be defined in the IANA JSON Web Token Registry or be defined as a URI that contains a collision resistant namespace.
Private claims: These are the custom claims created to share information between parties that agree on using them and are neither registered or public claims.

An example payload could be:

{
  "sub": "1234567890",
  "name": "John Doe",
  "admin": true
}

The payload is then Base64Url encoded to form the second part of the JSON Web Token.

Do note that for signed tokens this information, though protected against tampering, is readable by anyone. Do not put secret information in the payload or header elements of a JWT unless it is encrypted.

Signature

To create the signature part you have to take the encoded header, the encoded payload, a secret, the algorithm specified in the header, and sign that.

For example if you want to use the HMAC SHA256 algorithm, the signature will be created in the following way:

HMACSHA256(
  base64UrlEncode(header) + "." +
  base64UrlEncode(payload),
  secret)

The signature is used to verify the message wasn't changed along the way, and, in the case of tokens signed with a private key, it can also verify that the sender of the JWT is who it says it is.

Putting all together

The output is three Base64-URL strings separated by dots that can be easily passed in HTML and HTTP environments, while being more compact when compared to XML-based standards such as SAML.

The following shows a JWT that has the previous header and payload encoded, and it is signed with a secret.

If you want to play with JWT and put these concepts into practice, you can use jwt.io Debugger to decode, verify, and generate JWTs.

How do JSON Web Tokens work?

In authentication, when the user successfully logs in using their credentials, a JSON Web Token will be returned. Since tokens are credentials, great care must be taken to prevent security issues. In general, you should not keep tokens longer than required.

You also should not store sensitive session data in browser storage due to lack of security.

Whenever the user wants to access a protected route or resource, the user agent should send the JWT, typically in the Authorization header using the Bearer schema. The content of the header should look like the following:

Authorization: Bearer <token>

This can be, in certain cases, a stateless authorization mechanism. The server's protected routes will check for a valid JWT in the Authorization header, and if it's present, the user will be allowed to access protected resources. If the JWT contains the necessary data, the need to query the database for certain operations may be reduced, though this may not always be the case.

If the token is sent in the Authorization header, Cross-Origin Resource Sharing (CORS) won't be an issue as it doesn't use cookies.

The following diagram shows how a JWT is obtained and used to access APIs or resources:

The application or client requests authorization to the authorization server. This is performed through one of the different authorization flows. For example, a typical OpenID Connect compliant web application will go through the /oauth/authorize endpoint using the authorization code flow.
When the authorization is granted, the authorization server returns an access token to the application.
The application uses the access token to access a protected resource (like an API).

Do note that with signed tokens, all the information contained within the token is exposed to users or other parties, even though they are unable to change it. This means you should not put secret information within the token.

Why should we use JSON Web Tokens?

Let's talk about the benefits of JSON Web Tokens (JWT) when compared to Simple Web Tokens (SWT) and Security Assertion Markup Language Tokens (SAML).

As JSON is less verbose than XML, when it is encoded its size is also smaller, making JWT more compact than SAML. This makes JWT a good choice to be passed in HTML and HTTP environments.

Security-wise, SWT can only be symmetrically signed by a shared secret using the HMAC algorithm. However, JWT and SAML tokens can use a public/private key pair in the form of a X.509 certificate for signing. Signing XML with XML Digital Signature without introducing obscure security holes is very difficult when compared to the simplicity of signing JSON.

JSON parsers are common in most programming languages because they map directly to objects. Conversely, XML doesn't have a natural document-to-object mapping. This makes it easier to work with JWT than SAML assertions.

Regarding usage, JWT is used at Internet scale. This highlights the ease of client-side processing of the JSON Web token on multiple platforms, especially mobile.

Comparison of the length of an encoded JWT and an encoded SAML

If you want to read more about JSON Web Tokens and even start using them to perform authentication in your own applications, browse to the JSON Web Token landing page at Auth0.

WHAT IS JSON WEB TOKEN?

JSON Web Token (JWT) is an open standard (RFC 7519) that defines a compact and self-contained way for securely transmitting information between parties as a JSON object. This information can be verified and trusted because it is digitally signed. JWTs can be signed using a secret (with HMAC algorithm) or a public/private key pair using RSA.

Let’s explain some concepts of this definition further.

Compact: Because of its size, it can be sent through an URL, POST parameter, or inside an HTTP header. Additionally, due to its size its transmission is fast.
Self-contained: The payload contains all the required information about the user, to avoid querying the database more than once.

Interested in getting up-to-speed with JWTs as soon as possible? DOWNLOAD THE FREE EBOOK

WHEN SHOULD YOU USE JSON WEB TOKENS?

These are some scenarios where JSON Web Tokens are useful:

Authentication: This is the typical scenario for using JWT, once the user is logged in, each subsequent request will include the JWT, allowing the user to access routes, services, and resources that are permitted with that token. Single Sign On is a feature that widely uses JWT nowadays, because of its small overhead and its ability to be easily used among systems of different domains.
Information Exchange: JWTs are a good way of securely transmitting information between parties, because as they can be signed, for example using a public/private key pair, you can be sure that the sender is who they say they are. Additionally, as the signature is calculated using the header and the payload, you can also verify that the content hasn’t changed.

WHICH IS THE JSON WEB TOKEN STRUCTURE?

JWTs consist of three parts separated by dots (.), which are:

Header
Payload
Signature

Therefore, a JWT typically looks like the following.

xxxxx.yyyyy.zzzzz

Let’s break down the different parts.

Header

The header typically consists of two parts: the type of the token, which is JWT, and the hashing algorithm such as HMAC SHA256 or RSA.

For example:

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

Then, this JSON is Base64Url encoded to form the first part of the JWT.

Payload

The second part of the token is the payload, which contains the claims. Claims are statements about an entity (typically, the user) and additional metadata. There are three types of claims: reserved, public, and private claims.

- Reserved claims: These are a set of predefined claims, which are not mandatory but recommended, thought to provide a set of useful, interoperable claims. Some of them are: iss (issuer), exp (expiration time), sub (subject), aud (audience), among others.

Notice that the claim names are only three characters long as JWT is meant to be compact.

Public claims: These can be defined at will by those using JWTs. But to avoid collisions they should be defined in the IANA JSON Web Token Registry or be defined as a URI that contains a collision resistant namespace.
Private claims: These are the custom claims created to share information between parties that agree on using them.

An example of payload could be:

{
  "sub": "1234567890",
  "name": "John Doe",
  "admin": true
}

The payload is then Base64Url encoded to form the second part of the JWT.

Signature

To create the signature part you have to take the encoded header, the encoded payload, a secret, the algorithm specified in the header, and sign that.

For example if you want to use the HMAC SHA256 algorithm, the signature will be created in the following way.

HMACSHA256(
  base64UrlEncode(header) + "." +
  base64UrlEncode(payload),
  secret)

The signature is used to verify that the sender of the JWT is who it says it is and to ensure that the message was’t changed in the way.

Putting all together

The output is three Base64 strings separated by dots that can be easily passed in HTML and HTTP environments, while being more compact compared to XML-based standards such as SAML.

The following shows a JWT that has the previous header and payload encoded and it is signed with a secret.

You can browse to jwt.io where you can play with a JWT and put these concepts in practice. jwt.io allows you to decode, verify and generate JWT.

HOW JSON WEB TOKENS WORK?

You also should not store sensitive session data in browser storage due to lack of security.

Whenever the user wants to access a protected route, it should send the JWT, typically in the Authorization header using the Bearer schema. Therefore the content of the header should look like the following.

Authorization: Bearer <token>

This is a stateless authentication mechanism as the user state is never saved in the server memory. The server’s protected routes will check for a valid JWT in the Authorization header, and if there is, the user will be allowed. As JWTs are self-contained, all the necessary information is there, reducing the need of going back and forward to the database.

This allows to fully rely on data APIs that are stateless and even make requests to downstream services. It doesn’t matter which domains are serving your APIs, as Cross-Origin Resource Sharing (CORS) won’t be an issue as it doesn’t use cookies.\

WHY SHOULD YOU USE JSON WEB TOKENS?

Let’s talk about the benefits of JSON Web Tokens (JWT) comparing it to Simple Web Tokens (SWT) and Security Assertion Markup Language Tokens (SAML).

As JSON is less verbose than XML, when it is encoded its size is also smaller; making JWT more compact than SAML. This makes JWT a good choice to be passed in HTML and HTTP environments.

Security-wise, SWT can only be symmetric signed by a shared secret using the HMAC algorithm. While JWT and SAML tokens can also use a public/private key pair in the form of a X.509 certificate to sign them. However, signing XML with XML Digital Signature without introducing obscure security holes is very difficult compared to the simplicity of signing JSON.

JSON parsers are common in most programming languages, because they map directly to objects, conversely XML doesn’t have a natural document-to-object mapping. This makes it easier to work with JWT than SAML assertions.

Regarding usage, JWT is used at an Internet scale. This highlights the ease of client side processing of JWTs on multiple platforms, especially, mobile.\

HOW WE USE JSON WEB TOKENS IN AUTH0?

In Auth0, we issue JWTs as a result of the authentication process. When the user logs in using Auth0, a JWT is created, signed, and sent to the user. Auth0 supports signing JWT with both HMAC and RSA algorithms. This token will be then used to authenticate and authorize with APIs which will grant access to their protected routes and resources.

We also use JWTs to perform authentication and authorization in Auth0’s API v2, replacing the traditional usage of regular opaque API keys. Regarding authorization, JSON Web Tokens allow granular security, that is the ability to specify a particular set of permissions in the token, which improves debuggability.

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