首先添加引用: using System.Web.Configuration;

加密操作如下:
private void ProtectSection(string sectionName, string provider)
    {
        Configuration config =
            WebConfigurationManager.
                OpenWebConfiguration(Request.ApplicationPath);

        ConfigurationSection section = config.GetSection(sectionName);

        if (section != null && !section.SectionInformation.IsProtected)
        {
            section.SectionInformation.ProtectSection(provider);
            config.Save();
        }
    }

解密操作如下:
 private void UnProtectSection(string sectionName)
    {
        Configuration config =
            WebConfigurationManager.
                OpenWebConfiguration(Request.ApplicationPath);

        ConfigurationSection section = config.GetSection(sectionName);

        if (section != null && section.SectionInformation.IsProtected)
        {
            section.SectionInformation.UnprotectSection();
            config.Save();
        }
    }
实践:
加密前的配置文件:
<?xml version="1.0"?>
<configuration>
    <appSettings>
  <add key="name" value="shy520" />
  <add key="address" value="cnblogs" />
 </appSettings>
    <system.web>
        <compilation debug="true"/>
  </system.web>
</configuration>
加密后的配置文件:
<?xml version="1.0"?>
<configuration>
    <appSettings configProtectionProvider="DataProtectionConfigurationProvider">
  <EncryptedData>
   <CipherData>
    <CipherValue>
      AQAAANCMnd8BFdERjHoAwE/Cl+s
      BAAAABi1ATlNkEUGEf0XyWGL2Xg
      QAAAACAAAAAAADZgAAqAAAABAAA
      ABIhxMWlazAntwIIpST1CDXAAAA
      AASAAACgAAAAEAAAAPz/YKYx07c
      b+h4fqdr4fkLgAAAAX1Ieyc+WSx
      AfsDW1vn2C/fXsG2TAnYeUgaCov
      8e3nGFTHHsH91gLiqKregFPYzGR
      vW1xrez/3VwOmJI9eS7EFKrcXej
      NnHL66kg2iNRk3ntLNZlZtTs3cZ
      9w63u47VKAjs6miWsGBz2GntL/9
      UGHLELigrJcr3YJ+lsjOscExQnv
      HGvA48EfxpD+tEiFBtgXeHsFkQX
      cqGySshx16vCt2GUNUC3ZmEAhBh
      UsAFkPIYqelYHd4+m9a/xPe2tqw
      GIbla1wbW2NDEfrzJPwnkfmpNqR
      hXijKImipwXbDVYy6o0UAAAAs8D
      suYNOhJ7qAjJa2c/4eUC7sks=
      </CipherValue>
   </CipherData>
  </EncryptedData>
 </appSettings>
    <system.web>
        <compilation debug="true"/>
  </system.web>
</configuration>