String Manipulation

Sir,
i am writing a program to convert text in my richtext box to another format.

for a=1 to len(richtextbox1.text)
b=mid(richtextbox1.text,a,1)
if b="A" then
c=c & "Z"
elseif b="B" then
c=c & "G"
elseif b="C" then
c=c & "T"
elseif b="D" then
c=c & "H"
elseif b="E" then
c=c & "K"
......
......
.....
else
c= b
endif
next

above is an abstract of my code. what my problem is when converting large text it takes very much time. how can my speed up my program? i wan to check one character by character. please help....

raman

if you use a select case for the letters, with the most common first, no further cases are evaluated after the case is found, so this should save some time