JWT 基本概念详解

发表于 2025/12/01

作者 deathwhispers

10 分钟阅读

JWT 基本概念详解

JWT 基本概念详解
- 什么是 JWT?
- JWT 由哪些部分组成？
  - Header
  - Payload
  - Signature
- 如何基于 JWT 进行身份验证？
- 如何防止 JWT 被篡改？
- 如何加强 JWT 的安全性？

什么是 JWT?

JWT （JSON Web Token）是目前最流行的跨域认证解决方案，是一种基于 Token 的认证授权机制。从 JWT 的全称可以看出，JWT 本身也是 Token，一种规范化之后的 JSON 结构的 Token。

JWT 自身包含了身份验证所需要的所有信息，因此，我们的服务器不需要存储 Session 信息。这显然增加了系统的可用性和伸缩性，大大减轻了服务端的压力。

可以看出，JWT 更符合设计 RESTful API 时的「Stateless（无状态）」原则 。

并且，使用 JWT 认证可以有效避免 CSRF 攻击，因为 JWT 一般是存在在 localStorage 中，使用 JWT 进行身份验证的过程中是不会涉及到 Cookie 的。

我在 JWT 优缺点分析这篇文章中有详细介绍到使用 JWT 做身份认证的优势和劣势。

下面是 RFC 7519open in new window 对 JWT 做的较为正式的定义。

JSON Web Token (JWT) is a compact, URL-safe means of representing claims to be transferred between two parties. The claims in a JWT are encoded as a JSON object that is used as the payload of a JSON Web Signature (JWS) structure or as the plaintext of a JSON Web Encryption (JWE) structure, enabling the claims to be digitally signed or integrity protected with a Message Authentication Code (MAC) and/or encrypted. ——JSON Web Token (JWT)open in new window

JWT 由哪些部分组成？

JWT 本质上就是一组字串，通过（.）切分成三个为 Base64 编码的部分：

Header
描述 JWT 的元数据，定义了生成签名的算法以及 Token 的类型。
Payload
用来存放实际需要传递的数据
Signature（签名） ：服务器通过 Payload、Header 和一个密钥(Secret)使用 Header 里面指定的签名算法（默认是 HMAC SHA256）生成。

JWT 通常是这样的：xxxxx.yyyyy.zzzzz。

示例：

```plain text eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9. eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiaWF0IjoxNTE2MjM5MDIyfQ. SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c

你可以在 [jwt.ioopen in new window](https://jwt.io/) 这个网站上对其 JWT 进行解码，解码之后得到的就是 Header、Payload、Signature 这三部分。

Header 和 Payload 都是 JSON 格式的数据，Signature 由 Payload、Header 和 Secret(密钥)通过特定的计算公式和加密算法得到。

![](/assets/images/learning/sysdesign/jwt-basic-concepts-explanation/c63e4c498565f8b1918aaa6512181be9.png)

### Header

Header 通常由两部分组成：

- typ
（Type）：令牌类型，也就是 JWT。
- alg
（Algorithm） ：签名算法，比如 HS256。

示例：

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

JSON 形式的 Header 被转换成 Base64 编码，成为 JWT 的第一部分。

Payload

Payload 也是 JSON 格式数据，其中包含了 Claims(声明，包含 JWT 的相关信息)。

Claims 分为三种类型：

Registered Claims（注册声明） ：预定义的一些声明，建议使用，但不是强制性的。
Public Claims（公有声明）IANA JSON Web Token Registryopen in new window ：JWT 签发方可以自定义的声明，但是为了避免冲突，应该在中定义它们。
Private Claims（私有声明） ：JWT 签发方因为项目需要而自定义的声明，更符合实际项目场景使用。

下面是一些常见的注册声明：

iss （issuer）：JWT 签发方。
iat （issued at time）：JWT 签发时间。
sub （subject）：JWT 主题。
aud （audience）：JWT 接收方。
exp （expiration time）：JWT 的过期时间。
nbf （not before time）：JWT 生效时间，早于该定义的时间的 JWT 不能被接受处理。
jti （JWT ID）：JWT 唯一标识。

示例：

```plain text { “uid”: “ff1212f5-d8d1-4496-bf41-d2dda73de19a”, “sub”: “1234567890”, “name”: “John Doe”, “exp”: 15323232, “iat”: 1516239022, “scope”: [“admin”, “user”] }

Payload 部分默认是不加密的，**一定不要将隐私信息存放在 Payload 当中！！！**

JSON 形式的 Payload 被转换成 Base64 编码，成为 JWT 的第二部分。

### Signature

Signature 部分是对前两部分的签名，作用是防止 JWT（主要是 payload） 被篡改。

这个签名的生成需要用到：

- Header + Payload。
- 存放在服务端的密钥(一定不要泄露出去)。
- 签名算法。

签名的计算公式如下：

```plain text
HMACSHA256(
  base64UrlEncode(header) + "." +
  base64UrlEncode(payload),
  secret)

算出签名以后，把 Header、Payload、Signature 三个部分拼成一个字符串，每个部分之间用”点”（.）分隔，这个字符串就是 JWT 。

如何基于 JWT 进行身份验证？

在基于 JWT 进行身份验证的的应用程序中，服务器通过 Payload、Header 和 Secret(密钥)创建 JWT 并将 JWT 发送给客户端。客户端接收到 JWT 之后，会将其保存在 Cookie 或者 localStorage 里面，以后客户端发出的所有请求都会携带这个令牌。

简化后的步骤如下：

用户向服务器发送用户名、密码以及验证码用于登陆系统。
如果用户用户名、密码以及验证码校验正确的话，服务端会返回已经签名的 Token，也就是 JWT。
用户以后每次向后端发请求都在 Header 中带上这个 JWT 。
服务端检查 JWT 并从中获取用户相关信息。

两点建议：

建议将 JWT 存放在 localStorage 中，放在 Cookie 中会有 CSRF 风险。
请求服务端并携带 JWT 的常见做法是将其放在 HTTP Header 的 Authorization 字段中（ Authorization: Bearer Token ）。

spring-security-jwt-guideopen in new window 就是一个基于 JWT 来做身份认证的简单案例，感兴趣的可以看看。

如何防止 JWT 被篡改？

有了签名之后，即使 JWT 被泄露或者解惑，黑客也没办法同时篡改 Signature 、Header 、Payload。

这是为什么呢？因为服务端拿到 JWT 之后，会解析出其中包含的 Header、Payload 以及 Signature 。服务端会根据 Header、Payload、密钥再次生成一个 Signature。拿新生成的 Signature 和 JWT 中的 Signature 作对比，如果一样就说明 Header 和 Payload 没有被修改。

不过，如果服务端的秘钥也被泄露的话，黑客就可以同时篡改 Signature 、Header 、Payload 了。黑客直接修改了 Header 和 Payload 之后，再重新生成一个 Signature 就可以了。

密钥一定保管好，一定不要泄露出去。JWT 安全的核心在于签名，签名安全的核心在密钥。

如何加强 JWT 的安全性？

使用安全系数高的加密算法。
使用成熟的开源库，没必要造轮子。
JWT 存放在 localStorage 中而不是 Cookie 中，避免 CSRF 风险。
一定不要将隐私信息存放在 Payload 当中。
密钥一定保管好，一定不要泄露出去。JWT 安全的核心在于签名，签名安全的核心在密钥。
Payload 要加入 exp （JWT 的过期时间），永久有效的 JWT 不合理。并且，JWT 的过期时间不易过长。

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JWT

本文由作者按照 CC BY 4.0 进行授权

JWT 基本概念详解

什么是 JWT?

JWT 由哪些部分组成？

Payload

如何基于 JWT 进行身份验证？

如何防止 JWT 被篡改？

如何加强 JWT 的安全性？

热门标签