# DSA
## Digital Signature Algorithm
## # 先瞭解的概念
1\.
```
數位加密：用公鑰加密，只有用私鑰解開，因為私鑰只有你自己有，所以他保證了數據不能被別人看到
數位簽名：用私鑰加密，只能用公鑰解密，任何人都可以用公鑰驗證。因為私鑰只有你自己有，所以它可以保證數據只能是你發出的，不可能有別人發出，除非你得私鑰丟失或被第三方破解出來
```
2\.
```
RSA 與 DSA 都是非對稱加密算法。其中RSA的安全性是基於極其困難的大整數的分解（兩個質數的乘積）；DSA 的安全性是基於整數有限域離散對數難題。基本上可以認為相同密鑰長度的 RSA 算法與 DSA 算法安全性相當。
```
3\.
```
DSA 只能用於數字簽名，而無法用於加密（某些擴展可以支持加密）；RSA 即可作為數字簽名，也可以作為加密算法
```
## #簽章過程:(使用Openssl)
> 查看相關指令
```
openssl gendsa
```
1. 產生一個1024bits的參數檔案
```
openssl dsaparam -out dsa_param.pem 1024
```
2.從剛才的參數檔案產生一個private key 並且用AES-128之password protect 也可用aes-192或aes-256
```
openssl gendsa -out dsa_privatekey.pem -aes128 dsa_param.pem
```
> PS:unable to write ''random state" 可參考此
>
>
>
> 但其不影響
3.接著再從私鑰產生一把公鑰
```
openssl dsa -in dsa_privatekey.pem -pubout -out dsa_publickey.pem
```
4.之後我們新增一個我們要用來簽章的文件 ，並且在裡面寫一點字
```
vim document.txt
```
然後用私鑰對文件簽章產生一個sig
```
openssl dgst -dss1 -sign dsa_privatekey.pem -out document.sig document.txt
```
5.最後用公鑰驗證
```
openssl dgst -dss1 -verify dsa_publickey.pem -signature document.sig document.txt
```
### #使用Node.js
先參考此文，產生pem
```javascript
var fs = require('fs');
var crypto = require('crypto');
var pem = require('pem')
var privateKey = fs.readFileSync('./dsa_priv0.pem');
var publicKey = fs.readFileSync('./dsa_pub.pem');
var buffLen = 128;
var sign = crypto.createSign('dsaWithSHA1');
sign.update('apple');
// 注意 這裡是用私鑰 簽名 如果公鑰會出現錯誤
var res = sign.sign(privateKey, 'hex');
var verify = crypto.createVerify('dsaWithSHA1');
verify.update('apple');
var rst = verify.verify(publicKey, res, "hex");
console.log(rst); // Prints success. means the key pair works.
```
---
# Agent Instructions: Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.
Perform an HTTP GET request on the current page URL with the `ask` query parameter:
```
GET https://easonwang.gitbook.io/crypto/dsa.md?ask=
```
The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.
Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.