Encrypting configuration information of web.config

ASP.NET IIS Registration Tool (Aspnet_regiis.exe) is use for encrypt or decrypt sections of a Web configuration file. ASP.NET will automatically decrypt encrypted configuration elements when the Web.config file is processed.
The following command encrypts the connectionStrings element in the Web.config file for the application SampleApplication. Because no -site option is included, the application is assumed to be from Web site 1 (most commonly Default Web Site in IIS). The encryption is performed using the RsaProtectedConfigurationProvider specified in the machine configuration.
aspnet_regiis -pe "connectionStrings" -app "/SampleApplication" -prov "RsaProtectedConfigurationProvider"
The following command decrypts the connectionStrings element in the Web.config file for the ASP.NET application SampleApplication:
aspnet_regiis -pd "connectionStrings" -app "/SampleApplication"
Link: http://www.beansoftware.com/ASP.NET-Tutorials/Encrypting-Connection-String.aspx