dsa数字签名
系统功能需求：
1.对文件进行签名，生成一个独立于原文的签名文件；
2.可以对文件进行签名，在文件后追加签名信息；
3.具有密钥管理的功能，比如密钥的生成、导入、导出；
4.可以对word文档实施操作，对word文档的内容进行签名；
5.能够比对已经实施的签名，防止进行重复的签名，以防止重放攻击；
6.可以对已经签名的文件进行单独的源文件和签名信息的分离操作。(DSA digital signature
System functional requirements:
1. Sign the document and generate a signature file independent of the original text.
2. The document can be signed and the signature information can be added after the document.
3. It has the function of key management, such as key generation, import and export.
4. Word documents can be operated and the contents of Word documents can be signed.
5. Ability to compare existing signatures and prevent duplicate signatures to prevent replay attacks;
6. It is possible to separate the source file from the signature information of the signed file.)