10 'This is a sample program which uses the DES module to decrypt files
20 'in either CBC or ECB mode. Mode controlled by 'CIPHR.MOD$' string.
30 '
100 DEFINT A-Z
110 CIPHR.SEG=&H1F00          'This is the absolute load addr. (offset=0)
112	          'The DES module MUST be loaded at offset 0.
114                           '(The segment may be in any unused area of mem.)
115 '
120 DEF SEG=CIPHR.SEG
130 BLOAD "DESMODUL.BSV",0         'Load the module.
140 DEF SEG
145 '
146 CIPHR.MOD$="CBC"          'Set ciphering mode to CBC
148 DEC.FNC=&HFF	          'ENCRYPT/DECRYPT code (ENC=0; DEC=0FFh)
149                           'This prog. will NOT ENCRYPT in CBC mode.
150 CPR.TXT$="########"+""    'Make room for ciphertext (see Basic manual,
152                           'page C-9).
153 PLN.TXT$="########"+""    'Save dummy string into PLN.TXT too (same reason)
155                           ' This is also the Initializing Vector for CBC
160 SET.KEY=15                'Entry point for SET.KEY subroutine (offset)
170 DO.XOR=12                 'Entry point for XOR subroutine
180 CIPHER=9                  'Entry point for CIPHER subroutine
182 '
184 INPUT "Ciphertext file"; CTXT$
186 INPUT "Plaintext file"; PTXT$
188 '
190 OPEN CTXT$ AS #1 LEN=8
200 OPEN PTXT$ AS #2 LEN=8
210 FIELD #1, 8 AS A$
220 FIELD #2, 8 AS B$         'Open files for random I/O
230 '
260 DEF SEG=CIPHR.SEG
270 POKE 0,DEF.FNC            'Set 'ENCRYPT' flag
280 DEF SEG
290 '
300 HEX.KEY$="AAAAAAAA"       'Key (=41h,41h,41h,...in this example)
310 DEF SEG=CIPHR.SEG
320 CALL SET.KEY(HEX.KEY$)    'Make subkeys for ENCRYPT
330 DEF SEG
340 '
350 FOR J=1 TO (LOF(1)+7)\8
360 GET #1,J
365 CPR.TXT.LAST$=CPR.TXT$     'Save old ciphertext block for XOR.
370 CPR.TXT$=A$                'Get new ciphertext block
380 '
390 DEF SEG=CIPHR.SEG
410 CALL CIPHER(CPR.TXT$,PLN.TXT$)    'Perform decryption
415 IF (CIPHR.MOD$="CBC") THEN CALL DO.XOR(PLN.TXT$,CPR.TXT$)
416                           'ARG1=ARG1 XOR ARG2
420 DEF SEG
430 '
440 LSET B$=PLN.TXT$
455 IF J MOD 128 = 0 THEN PRINT J\128; "k ";
450 PUT #2,J
460 NEXT J
470 CLOSE
480 '
490 END