c# code: string InputFile = Application .StartupPath + "\\ encrptsofttechsolving.xml " ; // this is excel file, i.e I wanna to deencrypt string OutputFile = = Application .StartupPath + "\\softtechsolving.xls " ; // softtechsolving is file name after decryption of encrptsofttechsolving.xml XmlDocument XDoc1 = new XmlDocument (); try { string password = @"xxxxxxx" ; // Your Key Here UnicodeEncoding UE = new UnicodeEncoding (); byte [] key = UE.GetBytes(password); FileStream fsCrypt = new FileStream (inputFile, FileMode .Open); RijndaelManaged RMCrypto = new RijndaelManaged (); CryptoStream cs = new CryptoStream (fsCrypt, RMCrypto.CreateDecryptor(key, key), CryptoStreamMode .Read);