How can I use the whole potential of DES? I have recently programmed the DES algorithm. Right now it works just with integers (or better to say chars) in the range &lt;0;255>. How can I use its whole potential - using just ASCII would be a waste (there are many characters I would never use).
Programming your own DES is usually a bad idea (unless as an exercise to understand the algorithm better), but OK. By definition the algorithm works with blocks of data that consist of 8 bytes (i.e. integers in the range 0..255), and the key is 7 of those bytes (or 8 with unused parity bits). 

What the data means (text or binary files like Office documents or zip files) etc. is irrelevant. I don't see what you mean by "full potential". You can just use it for ASCII text. It doesn't really matter what range the bytes are in. The encoding of the data is up to you (you can pack lower ASCII data into 7 bit characters, that overflow into the next byte, as SMS does, e.g.), and not a part of DES or any crypto-algorithm. Bytes are bytes. DES handles bytes.