public static string base64encode(string filename)
{
FileStream fs = new FileStream(filename,
FileMode.Open,
FileAccess.Read);
byte[] filebytes = new byte[fs.Length];
fs.Read(filebytes, 0, Convert.ToInt32(fs.Length));
string encodedData =
Convert.ToBase64String(filebytes,
Base64FormattingOptions.InsertLineBreaks);
return encodedData;
}
public static void base64decode(string text, string filename)
{
byte[] filebytes = Convert.FromBase64String(text);
FileStream fs = new FileStream(filename,
FileMode.CreateNew,
FileAccess.Write,
FileShare.None);
fs.Write(filebytes, 0, filebytes.Length);
fs.Close();
}

Использовать так:

string inputFile = "./aa.exe.mdb";
string encoded = base64encode(inputFile);
Console.Write(encoded);
base64decode(encoded, inputFile + ".new");

Скачать пример