> For the complete documentation index, see [llms.txt](https://huy312100.gitbook.io/software-development/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://huy312100.gitbook.io/software-development/authentication-and-authorization-standards/more/jwt.md).
# JWT
JWT is a means of representing transfer requests between the two Client - Server parties, the information in the JWT string is formatted in JSON. In which the Token string must have 3 parts: header , payload part and signature part separated by "."
So according to the above theory, I will have a Token chain as follows:
```
header.payload.signature
```
## Structure of JSON Web Token
As mentioned above JSON Web Token consists of 3 parts, separated by a dot (.):
### 1. Header
Will contains what type of token and encryption algorithm
```json
{
"typ": "JWT",
"alg": "HS256"
}
```
* **“typ” (type)** indicates that the object is a **JWT**
* **“alg” (algorithm)** defines the encryption algorithm for the string as HS**256**
### 2. Payload
The **payload** will contain the information you want to put in the **Token** string such as **username , userId , author , ...**
```json
{
"user_name": "admin",
"user_id": "1513717410",
"authorities": "ADMIN_USER",
"jti": "474cb37f-2c9c-44e4-8f5c-1ea5e4cc4d18"
}
```
> Note do not put too much information in the **Payload** chain because it will affect the delay when the **Server** has to confirm a **Token** that is too long.
### 3. Signature
This signature will be generated by encrypting the header , payload with a **secret key**, for example:
```
data = base64urlEncode( header ) + "." + base64urlEncode( payload );
signature = Hash( data, secret );
```
* **base64UrlEncoder** : header and payload encoding algorithm
The above code after encoding the **header** and **payload** using the **base64UrlEncode** algorithm, we will have the following string
```java
// header
eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9
// payload
eyJhdWQiOlsidGVzdGp3dHJlc291cmNlaWQiXSwidXNlcl9uYW1lIjoiYWRtaW4iLCJzY29wZSI6WyJyZWFkIiwid3JpdGUiXSwiZXhwIjoxNTEzNzE
```
Then encrypt the above 2 strings with secret **(secret key)** with the **HS256** algorithm, we will have the following signature string:
```
9nRhBWiRoryc8fV5xRpTmw9iyJ6EM7WTGTjvCM1e36Q
```
#### Final
Combining the above 3 strings we will get a complete JWT chain
