Caesar's cipher encryption

text$="In cryptography, a Caesar cipher, also known as a Caesar's cipher, the shift cipher, Caesar's code or Caesar shift, is one of the simplest and most widely known encryption techniques(Wikipedia)"
shift=255
ciphr$=""
for n = 1 to length(text$)
letter$= mid(text$, n, 1)
m = asc(letter$)
ciphr$=ciphr$+chr(m+shift)
next n
print ciphr$