How to encrypt one entry in web.config

You can't encrypt a single entry - the infrastructure only allows for encryption of whole config sections.

One option is to place the entry in its own config section and encrypt that.

In c# and .Net 4.5 I had to use this to read the encrypted setting:

string password = ((System.Collections.Specialized.NameValueCollection)ConfigurationManager.GetSection("secureAppSettings"))["Password"];

but otherwise works a treat.

You could put the password into a separate section and encrypt this section only. For example:

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
    <configSections>
        <section name="secureAppSettings" type="System.Configuration.NameValueSectionHandler, System, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" />
    </configSections>

    <appSettings>
        <add key="Host" value="www.foo.com" />
        <add key="Token" value="qwerqwre" />
        <add key="AccountId" value="123" />
        <add key="DepartmentId" value="456" />
        <add key="SessionEmail" value="[email protected]" />
        <add key="DefaultFolder" value="789" />  
    </appSettings>

    <secureAppSettings>
        <add key="Password" value="asdfasdf" />
    </secureAppSettings>  
</configuration>

and then (note that I am using DPAPI in my example so adapt the provider for RSA):

aspnet_regiis -pef secureAppSettings . -prov DataProtectionConfigurationProvider

Once encrypted the file will look like this:

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
    <configSections>
        <section name="secureAppSettings" type="System.Configuration.NameValueSectionHandler, System, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" />
    </configSections>

    <appSettings>
        <add key="Host" value="www.foo.com" />
        <add key="Token" value="qwerqwre" />
        <add key="AccountId" value="123" />
        <add key="DepartmentId" value="456" />
        <add key="SessionEmail" value="[email protected]" />
        <add key="DefaultFolder" value="789" />  
    </appSettings>

    <secureAppSettings configProtectionProvider="DataProtectionConfigurationProvider">
        <EncryptedData>
            <CipherData>
                <CipherValue>AQAAANCMnd.......</CipherValue>
            </CipherData>
        </EncryptedData>
    </secureAppSettings>  
</configuration>

The way you would access those settings in your application once the file is encrypted is still the same and completely transparent:

var host = ConfigurationManager.AppSettings["Host"];
var password = ConfigurationManager.AppSettings["Password"];