10 'ROT-13 ENCODER/DECODER 
20 '***********************************************************************
30 'FILENAME: OOO31.BAS
40 'DATE: 16/05/1998
50 '
60 'AIM: 1.Receive input from keyboard or file
61 '     2.Apply ROT-13 encryption algorithm 
62 '     3.Save output onto disk.
70 ' (Only alphabets will undergo transformation, everything else
90 ' remain unchanged and are also saved onto the disk).
100 '
110 'Total TIME DEDICATED: 21hours.
120 '
130 'Output will be save to the file FILENAME$
140 'Encryption formula is as below.  x is input, y is output.
150 '(ie. x is the decimal value of the input character,
160 '     y is the decimal value of the output character)
170 '
180 'In addition, z is the value of y processed so that y becomes restricted
190 'within the 52-alphabet-mod-field.
200 'z is the final value which will be saved onto disk.
210 '
220 'Using QBASIC, the index of x can get to as high as 4 or 5
230 'but with GWBASIC the power of 2 is the highest. Powers higher than
240 'these will result in overflow. This problem can be
250 'overcome by replacing MOD function with a loop of division and
260 'subtraction. Eg. keep dividing y by 256 until quotient <=1 then
270 'start subtracting things to obtain a remainder.
300 '
310 'Tested on Pentium-100. MSDOS6.22 and MSDOS7
480 '***********************************************************************
490 '
500 CLS : CLEAR : KEY OFF
510 PRINT "***** ROT-13 ENCRYPTION/DECRIPTION *****"
520 INPUT "Please Enter Output File Name: ", FILENAME$
530 OPEN FILENAME$ FOR OUTPUT AS #1
540 PRINT "Ready to recieve text now. Press ~ anytime to end"
550 WHILE SLT$ <> "~"
560      SLT$ = INPUT$(1)
570      PRINT SLT$;
580 X=ASC(SLT$)
590 '
600 '
610 '                   vvvvvvv ENCRYPTION FORMULA vvvvvvv
620                                Y = X - 1
630 '                   ^^^^^^^ ENCRYPTION FORMULA ^^^^^^^
640 '
650 '
660 '
670 ENED=Y
680  '^^^ The FORMULA needs to be one-to-one function
690  '
700  'vvvvvvv Picks out alphabets and leaving out anything else vvvvvvvv
710  IF ASC(SLT$) >= 65 AND ASC(SLT$) <= 90 THEN TRANS = 65: CIS = TRANS ELSE IF ASC(SLT$) >= 97 AND ASC(SLT$) <= 122 THEN TRANS = 97: CIS = TRANS ELSE TRANS = ENED: CIS = ASC(SLT$)
720  '^^^^^^^ Picks out alphabets and leaving out anything else ^^^^^^^^
730  '
740  '
750 ' vvvvv Making sure the difference is within 26 vvvvv
760     IF ENED >= TRANS AND (ENED MOD TRANS) < 26 THEN Z = (ENED MOD TRANS) + CIS
770     IF ENED >= TRANS AND (ENED MOD TRANS) >= 26 THEN Z = ((ENED MOD TRANS) MOD 26) + CIS
780     IF ENED < TRANS AND (ENED MOD TRANS) < 26 THEN Z = ENED - TRANS + CIS + 26
790     IF ENED < TRANS AND (ENED MOD TRANS) >= 26 THEN Z = -((TRANS - ENED) MOD 26) + CIS + 26
800 ' ^^^^^ Making sure the difference is within 26 ^^^^^
810 '
820 '
830 IF SLT$ = "~" THEN SLT$ = "~" ELSE PRINT #1, CHR$(Z); 'LIMIT SET TO MOD-52-ALPHABET FIELD
840 '^^^ The use of ; to have continuous screen and file output
850 WEND
860 CLOSE
870 PRINT : PRINT "Encryption saved to this file => "; FILENAME$
880 SYSTEM