Crypto File in a webservice asp.net

we have a webservice used to store and recall file in the file system. With some call to the webservice is possibile to writefile to the filesystem with another call , giving the name of file, the webservice giveback the file. All file start with charachters J in the filename. What we need is a change in this webservice, still in place, that introduce the crypting of the file using the asp.net library for crypting. THe webservice will be able to answer to 2 new request. Request A: Crypt file xxxxx with password yyyyy the webserice will find the stated file in the filesystem ( first charachters is J) and crypt it. After crypting it change the first charachters from J to X. Scope of this first character is to recognize uncrypted file ( start with J ), from crupted file ( start with X). if the file is not found, it give back an error. Request B: Decrypt file xxxxx with password yyyy and give it me back xxxxx is the file name yyyyy is the password it check, if file not exist give back en error. xf file exist AND password is correct it : -copy the file in a temprary position -uncrypt the file with the password -give back the uncrypted file -destroy the file in the temporary position if file exist but password is wrong it send back an error