Java Security

I am writing a program to implements the caesar cipher. For the program I am using ASCII code so my encryption algorithm is:

c = (p + k) mod 128

The only problem is, is that depending on the characters being encrypted and depending on the key I use some of the characters are encrypted to non-printing ascii characters such as 'del'.

So my code reads in the plain text from a file converts each character to ascii, encrypts each character using the key. It then writes the encrypted ascii values to a file as text (i.e. the character equivalent of the ascii).

If I do then encounter a non-printing ascii value it is written to the file as a square shape. Is there any way I can get around this?

Thanks for your help on the matter.

Wallace