# Using Postman for Signatures

Postman is commonly used for debugging backend HTTP services. We can use the POST's Pre-request Script feature to construct the HTTP request header to complete the signature.

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As shown in the figure, fill in the necessary information for signature in Headers. The variable parameters are defined in the {{}} modifier, and the parameters in {{}} in POSTMAN are global variables. We need two global variables in the example: the parameter values of X-Gw-Timestamp and X-Gw-Signatured.

Switch to the Pre-request Script menu, Pre-request Script has built-in javascript interpreter, combined with POSTMAN API, you can modify HTTP request parameters. Due to the different versions of POSTMAN, there are differences in POSTMAN API. For specific API, [please refer to this](https://learning.getpostman.com/docs/postman/scripts/intro_to_scripts).

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The code is as follows:

``` 
var stage = "user_prd_stage";
var appKey = "76976226127534999";
var appSecret = "d1bb0aead0fd4f36b7e58eb55de46543";
var timeStamp = Math.ceil(Date.now() / 1000)
var signedString = "X-Gw-Timestamp:"+timeStamp+",X-Gw-App-Key:"+appKey;
var signature = CryptoJS.HmacSHA256(signedString , appSecret).toString(CryptoJS.enc.Base64);
postman.setGlobalVariable("timeStamp" , timeStamp);
postman.setGlobalVariable("X-Gw-SignedString",signedString);
postman.setGlobalVariable("X-Gw-Signature",signature);
```