To save having to visit external links, in C:\Windows\Microsoft.NET\Framework\v4.0.30319 (for .NET 4 / 4.5)
aspnet_regiis.exe -pe "connectionStrings" -app "/YourWebSiteName" -prov "DataProtectionConfigurationProvider"
To decrypt connectionStrings section using this tool, you can specify following command in aspnet_iisreg.exe tool.
aspnet_regiis.exe -pd "connectionStrings" -app "/YouWebSiteName"