I wrote a program which encodes text.. its basic (like a = z , b = y etc) but i added an "encryption key" to make it a bit better..
The program chooses between two lists (eg : in the first list a = z, but in the second 'a' might equal 'j')
The encryption key is a series of 1's and 2's which tells the program which list to use..
Then the 1's and 2's are changed to character using the CHR$() function...it starts at a certain value and increments by 1 or two based on the key (eg : if the start value was 60 and the key was 11221, the text version would be CHR$(61),CHR$(62),CHR$(64),CHR$(66),CHR$(67) )
it usually works fine, but sometimes it skips a letter
Theoretically the number of letters that the program outputs is equal to the number of the letters that the user inputs... however, i conducted a test run by typing 'hello' and checking the output.. every third trial or so , it skipped a letter and when i decoded it (the decoder program is separate: it works fine), it would result in a jumbled mess of characters..

This is the Source Code(its very long because i couldn't figure out how to compress it ) :



SCREEN _NEWIMAGE(1600, 900, 32)
f& = _LOADFONT("C:\WINDOWS\Fonts\Lucon.ttf", 21)
_FONT f&
_FULLSCREEN
codenum = 1

DIM SHARED k
DIM SHARED letter$
DIM SHARED Key$
DIM SHARED message$
DIM SHARED EncryptionKey$
DIM SHARED Start
DIM SHARED Translation$
RANDOMIZE TIMER
Start = INT(RND * 73) + 34
PRINT "Your Start value is : "; Start
PRINT
setKey
encodeKey
PRINT "'"; k; "'"
PRINT "'"; Key$; "'"
PRINT "Your Encryption Key is : "; EncryptionKey$
PRINT
LINE INPUT "Enter Message : "; message$
encode
PRINT
PRINT "Translated : "; Translation$


SUB setKey
RANDOMIZE TIMER
FOR x = 0 TO 4
k = k + ((INT(RND * 2) + 1) * (10 ^ x))
NEXT x
Key$ = RIGHT$(STR$(k), 5)
END SUB

SUB encodeKey
FOR x = 1 TO 5
SELECT CASE (RIGHT$(LEFT$(Key$, x), 1))
CASE "1"
Start = Start + 1
EncryptionKey$ = EncryptionKey$ + CHR$(Start)
CASE "2"
Start = Start + 2
EncryptionKey$ = EncryptionKey$ + CHR$(Start)
END SELECT
NEXT x
END SUB


SUB encode
currentPlace = 0
place = LEN(message$) + 1
DO
FOR x = 1 TO 5
currentPlace = currentPlace + 1
IF currentPlace >= place THEN GOTO subend

codenum = VAL(RIGHT$(LEFT$(Key$, x), 1))
IF codenum = 2 THEN
SELECT CASE LCASE$(RIGHT$(LEFT$(message$, currentPlace), 1))
CASE "a"
letter$ = "z"
CASE "b"
letter$ = "y"
CASE "c"
letter$ = "x"
CASE "d"
letter$ = "w"
CASE "e"
letter$ = "v"
CASE "f"
letter$ = "u"
CASE "g"
letter$ = "t"
CASE "h"
letter$ = "s"
CASE "i"
letter$ = "r"
CASE "j"
letter$ = "q"
CASE "k"
letter$ = "p"
CASE "l"
letter$ = "o"
CASE "m"
letter$ = "n"
CASE "n"
letter$ = "m"
CASE "z"
letter$ = "a"
CASE "y"
letter$ = "b"
CASE "x"
letter$ = "c"
CASE "w"
letter$ = "d"
CASE "v"
letter$ = "e"
CASE "u"
letter$ = "f"
CASE "t"
letter$ = "g"
CASE "s"
letter$ = "h"
CASE "r"
letter$ = "i"
CASE "q"
letter$ = "j"
CASE "p"
letter$ = "k"
CASE "o"
letter$ = "l"
CASE " "
letter$ = " "
END SELECT
ELSEIF codenum = 1 THEN
SELECT CASE LCASE$(RIGHT$(LEFT$(message$, currentPlace), 1))
CASE " "
letter$ = " "
CASE "a"
letter$ = "/"
CASE "b"
letter$ = "."
CASE "c"
letter$ = ","
CASE "d"
letter$ = "'"
CASE "e"
letter$ = ""
CASE "f"
letter$ = "\"
CASE "g"
letter$ = "["
CASE "h"
letter$ = "1"
CASE "i"
letter$ = "2"
CASE "j"
letter$ = "3"
CASE "k"
letter$ = "4"
CASE "l"
letter$ = "5"
CASE "m"
letter$ = "6"
CASE "n"
letter$ = "7"
CASE "z"
letter$ = "8"
CASE "y"
letter$ = "9"
CASE "x"
letter$ = "+"
CASE "w"
letter$ = "-"
CASE "v"
letter$ = "`"
CASE "u"
letter$ = "~"
CASE "t"
letter$ = "|"
CASE "s"
letter$ = "*"
CASE "r"
letter$ = "="
CASE "q"
letter$ = "_"
CASE "p"
letter$ = "}"
CASE "o"
letter$ = "{"
CASE "/"
letter$ = "a"
CASE "."
letter$ = "b"
CASE ","
letter$ = "c"
CASE "'"
letter$ = "d"
CASE ""
letter$ = "e"
CASE "\"
letter$ = "f"
CASE "["
letter$ = "g"
CASE "1"
letter$ = "h"
CASE "2"
letter$ = "i"
CASE "3"
letter$ = "j"
CASE "4"
letter$ = "k"
CASE "5"
letter$ = "l"
CASE "6"
letter$ = "m"
CASE "7"
letter$ = "n"
CASE "8"
letter$ = "z"
CASE "9"
letter$ = "y"
CASE "+"
letter$ = "x"
CASE "-"
letter$ = "w"
CASE "`"
letter$ = "v"
CASE "~"
letter$ = "u"
CASE "|"
letter$ = "t"
CASE "*"
letter$ = "s"
CASE "="
letter$ = "r"
CASE "_"
letter$ = "q"
CASE "}"
letter$ = "p"
CASE "{"
letter$ = "o"
END SELECT
ELSE
PRINT "Not Valid"; codenum; " "
END IF
Translation$ = Translation$ + letter$
NEXT x
LOOP WHILE currentPlace <= place
subend:
END SUB