<configuration>
       <appSettings/>
       <connectionStrings>
  <add name="NorthwindConnectionString" connectionString="Data Source=ARAS02-XP;Initial Catalog=Northwind;User ID=sa"
   providerName="System.Data.SqlClient" />
</connectionStrings>
       <system.web>
             <compilation debug="true"/>
             <authentication mode="Windows"/>
    <pages theme="Theme1" />
             </system.web>
</configuration>

using System.Web.Configuration;
using System.Web.Security;
using System.Configuration;
 
public void EncryptConnString()
    {
 
Configuration config = WebConfigurationManager.OpenWebConfiguration(Request.ApplicationPath);
       ConfigurationSection section = config.GetSection("connectionStrings");
        if (!section.SectionInformation.IsProtected)
        {
            section.SectionInformation.ProtectSection("RsaProtectedConfigurationProvider");
            config.Save();
        }
    }
 
when this section runs it will produce a ne encrypted web.config
<?xml version="1.0"?>
<!-- 
    Note: As an alternative to hand editing this file you can use the 
    web admin tool to configure settings for your application. Use
    the Website->Asp.Net Configuration option in Visual Studio.
    A full list of settings and comments can be found in 
    machine.config.comments usually located in 
    \Windows\Microsoft.Net\Framework\v2.x\Config 
-->
<configuration>
    <appSettings/>
    <connectionStrings configProtectionProvider="RsaProtectedConfigurationProvider">
        <EncryptedData Type="http://www.w3.org/2001/04/xmlenc#Element"
            xmlns="http://www.w3.org/2001/04/xmlenc#">
            <EncryptionMethod Algorithm="http://www.w3.org/2001/04/xmlenc#tripledes-cbc" />
            <KeyInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
                <EncryptedKey xmlns="http://www.w3.org/2001/04/xmlenc#">
                    <EncryptionMethod Algorithm="http://www.w3.org/2001/04/xmlenc#rsa-1_5" />
                    <KeyInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
                        <KeyName>Rsa Key</KeyName>
                    </KeyInfo>
                    <CipherData>
                        <CipherValue>jWmekeNh1TC5Bf5RG2RWo8TU0qLoSF9IdSSpWMgiAjeCUqvPfo/XQr/zzLz4kdHUvCbbrSPX92YOpfv0YKSKO1mlEwE9LA57W9oo/0H7E8feO0flheoNdow9Tw8RVaM7meM8CqODladWD8Vr8G9mk17gWBFByWboIBMzWQ6Rp7U=</CipherValue>
                    </CipherData>
                </EncryptedKey>
            </KeyInfo>
            <CipherData>
                <CipherValue>7goUfwnWEqyrTFZXMBcD2eW+15j+eYyzq/YS/GpMX2NTMOrfJ6BHFy4Xr+kGEhLsckrGARfbQFsNLctL7wPBAMucnS0g2nbeMLKH1PPGjvBXjsdrvDUJ50w9CyvQ0dOqBb2Kdx0aEvmxCfCy/xLbkYPE6t/LGjVHUJFySVs4SjWhR4sLxzkxuTRSA3kq+2woobOfzIUSqOsO035SYiOYynQf2QcfodYZgT4U2KVsflUHR6Zk/iiTIh0+t1y0cMioFHkkHM8NDdjnYHToNhAP67GrulM/nAsTiMuAW4ElX/MomWAFngKmJvDqo8oKVWXY</CipherValue>
            </CipherData>
        </EncryptedData>
    </connectionStrings>
    <system.web>
        <!-- 
            Set compilation debug="true" to insert debugging 
            symbols into the compiled page. Because this 
            affects performance, set this value to true only 
            during development.
        -->
        <compilation debug="false" />
        <!--
            The <authentication> section enables configuration 
            of the security authentication mode used by 
            ASP.NET to identify an incoming user. 
        -->
        <authentication mode="Windows" />
        <!--
            The <customErrors> section enables configuration 
            of what to do if/when an unhandled error occurs 
            during the execution of a request. Specifically, 
            it enables developers to configure html error pages 
            to be displayed in place of a error stack trace.
 
        <customErrors mode="RemoteOnly" defaultRedirect="GenericErrorPage.htm">
            <error statusCode="403" redirect="NoAccess.htm" />
            <error statusCode="404" redirect="FileNotFound.htm" />
        </customErrors>
        -->
    </system.web>
</configuration>

public void DecryptConnString()
    {
        Configuration config = WebConfigurationManager.OpenWebConfiguration(Request.ApplicationPath);
        ConfigurationSection section = config.GetSection("connectionStrings");
        if (section.SectionInformation.IsProtected)
        {
            section.SectionInformation.UnprotectSection();
            config.Save();
        }
    }