Monitor File for Read and Change Data?

Is there any way, using C#, to monitor a specific file then change its contents before it is read by specific applications?

Here is the situation:

I have a Windows 2003 Server running 开发者_高级运维ASP.NET with a configuration file (xml) which contains LDAP information. I want to have the LDAP password encrypted. I'm trying to devise a way to monitor that file, and whenever it is read, decrypt the LDAP password and pass that to whatever is reading it. Is there any way to tell which program is doing the read? I aldready have the encrypt/decrypt working but it is built into the ASP.NET installation; I would like to make it external. The encrypt/decrypt is RSA using key's from the key store.

If you want the encrypt/decrypt external to your main application, what about creating a separate .dll or webservice that does that. Then your call in your ASP.NET application is to your webserice or .dll.

Something like (Warining: Not Compiled- you'll need to clean this up)

WebServiceInstance instance = new WebServiceInstance();
string password = instance.PerformGetPassword();

Then, your ASP.NET service is unaware of the encrypted password at all. Additionally, if you have other applications which need to access the same file, they can call the same webservice.

I think this would be much better accomplished by using NTFS permissions on the file. Grant access only to certain users/groups, and ensure that any process requiring access to the secured data is running under the security context of a user that has the correct ACL permissions.