25. | Learn Encryption Techniques with BASIC and C++

Learn Encryption Techniques with BASIC and C++
(Publisher: Wordware Publishing, Inc.)
Author(s): Gil Held
ISBN: 1556225989
Publication Date: 10/01/98

Table of Contents

Modifying the GROUPBY5 Subroutine

As previously constructed, this subroutine performs the actual printing of characters in groups of five. Now we will modify this subroutine so that it fills the last group with X’s if there are less than five characters in the group. Listing 2.13 shows the statement in this modified subroutine.

Listing 2.13 The modified GROUPBY5 subroutine.

 GROUPBY5:    L = 1    FOR I = 1 TO MSGLEN        PRINT MID$(TEXT$, I, 1);        IF I / 5 = L THEN GOTO AGROUP        GOTO NGROUP AGROUP:   PRINT " ";        L = L + 1 NGROUP: NEXT I    NUMX = 5 - MSGLEN MOD 5    IF NUMX = 5 OR NUMX = 0 THEN GOTO PRTBLK    FOR I = 1 TO NUMX    PRINT "X";    NEXT I PRTBLK: PRINT RETURN

When the subroutine is invoked, the variable L is set to 1 and the FOR-NEXT loop cycles through the contents of the string variable TEXT$, printing one character at a time. If the index I of the FOR-NEXT loop divided by 5 equals L, five characters have been printed and a branch to the label AGROUP occurs. At that location, the PRINT statement prints a space, providing a space between groups of five characters. Next, the variable L is incremented and the loop ends. If a group of five characters was not printed, a branch to the label NGROUP occurs, bypassing the printing of the space.

Once the FOR-NEXT loop is completed, you must determine if a group did not have five characters, a condition that can only occur when the last line in the enciphered message is processed. Here, the value of the variable NUMX determines the number of X’s that must be added to fill the last group. Only if NUMX is greater than 0 or less than 5 do we invoke the next FOR-NEXT loop to print the terminating X’s. Otherwise, a branch to the label PRTBLK occurs, which terminates a previous PRINT statement ending with a semicolon so that the next cycle through the subroutine GROUPBY5 will have succeeding groups placed on a new line.

The CIPHER4.BAS Program

The entire program previously developed as modules will be listed to facilitate recognizing how the previously mentioned subroutines and calling program relate to each other. This program is contained in the directory BASIC in the file CIPHER4.BAS on the CD-ROM and is given in Listing 2.14.

Listing 2.14 The CIPHER4.BAS program listing.

 REM PROGRAM CIPHER4.BAS DIM PLAINTEXT$(26), CIPHERTEXT$(26) CLS     GOSUB INITIALIZE     PRINT "CIPHER4.BAS PROGRAM accepts spaces between words in a                 plaintext "     PRINT "message and selectively enciphers the message using a            simple"     PRINT "monoalphabetic substitution process." 1    INPUT "Enter UPPERCASE Alphabetic Shift Key: ", K$     FOR I = 0 TO 25     IF K$ = PLAINTEXT$(I) GOTO 2     NEXT I     PRINT "You must enter a letter from A to Z"     GOTO 1 2    REM Position I represents shift key letter GOSUB FORMCIPHER          'create cipher alphabet GOSUB MSGFILE          'assign I/O files, place message on a file GOSUB CONVERTSTORE         'convert and store ciphertext on a file GOSUB PRTOUT           'print results STOP INITIALIZE:     REM Initialize plaintext values     FOR I = 0 TO 25     READ PLAINTEXT$(I)     NEXT I     DATA "A","B","C","D","E","F","G","H","I","J","K","L","M","N"     DATA "O","P","Q","R","S","T","U","V","W","X","Y","Z" RETURN FORMCIPHER:     REM Routine to form CIPHERTEXT alphabet based upon defined shift key     J = I + 1     FOR K = 0 TO 25     CIPHERTEXT$(K) = PLAINTEXT$((K + J) MOD 26)     NEXT K RETURN MSGFILE:     REM Routine to assign I/O files and accept keyboard or file input     REM and remove spaces between words         INPUT "Enter filename to store plaintext message,                           default=MESSAGE.DAT", INFILE$         IF INFILE$ = "" THEN INFILE$ = "MESSAGE.DAT"         INPUT "Enter filename to store enciphered message,                           default=CIPHERTX.DAT", OUTFILE$         IF OUTFILE$ = "" THEN OUTFILE$ = "CIPHERTX.DAT"         INPUT "Select keyboard (k) or file (f) message input: ", IN$         IF IN$ = "F" OR IN$ = "f" THEN RETURN                OPEN INFILE$ FOR OUTPUT AS #1     REM Routine to place message on a file removing spaces between words        PRINT "Enter your message - place a / at the beginning of                          each line"        PRINT "that should remain in plaintext and a \ on a                                  separate line"        PRINT "to indicate the end of the enciphered message"        PRINT AGN:     LINE INPUT TEXT$        IF MID$(TEXT$, 1, 1) = "/" THEN GOTO XT        NTEXT$ = ""        FOR I = 1 TO LEN(TEXT$)        NTEXT$ = NTEXT$ + LTRIM$(MID$(TEXT$, I, 1))        NEXT I        WRITE #1, NTEXT$        IF MID$(TEXT$, 1, 1) = "\" GOTO DONE        GOTO AGN XT:     WRITE #1, TEXT$        GOTO AGN DONE:    CLOSE #1 RETURN CONVERTSTORE:       REM Routine to convert and store ciphertext on a file       OPEN INFILE$ FOR INPUT AS #1       OPEN OUTFILE$ FOR OUTPUT AS #2       DO UNTIL EOF(1)           INPUT #1, TEXT$           MSGLEN = LEN(TEXT$)           IF MID$(TEXT$, 1, 1) = "/" THEN GOTO CLEARTXT           IF MID$(TEXT$, 1, 1) = "\" THEN GOTO DONE1           REM Convert plaintext to ciphertext              FOR I = 1 TO MSGLEN              FOR J = 0 TO 25              IF MID$(TEXT$, I, 1) = PLAINTEXT$(J) THEN GOTO GOTIT              NEXT J GOTIT:         MID$(TEXT$, I, 1) = CIPHERTEXT$(J)              NEXT I CLEARTXT         WRITE #2, TEXT$    LOOP DONE1:         CLOSE #2 RETURN PRTOUT: REM Subroutine to print results    INPUT "Press Return key to display resulting enciphered message", p$    CLS: TEMP$ = ""    PRINT "Resulting enciphered message is:"    OPEN OUTFILE$ FOR INPUT AS #2    DO UNTIL EOF(2)       INPUT #2, TEXT$       IF MID$(TEXT$, 1, 1) = "/" THEN PRINT RIGHT$(TEXT$,                          LEN(TEXT$) - 1)       IF MID$(TEXT$, 1, 1) = "/" THEN GOTO NOGROUP       IF TEMP$ = "" GOTO BLANK        'blank string       TEMP$ = TEMP$ + TEXT$       'concatenate       TEXT$ = TEMP$       TEMP$ = "" BLANK:  MSGLEN = LEN(TEXT$)       IF MSGLEN >= 25 THEN GOTO BIG       TEMP$ = TEXT$       GOTO NOGROUP                     'less than 25 characters BIG:            FOR I = 26 TO MSGLEN       'place remainder of string                                                     'into temp        TEMP$ = TEMP$ + MID$(TEXT$, I, 1)        NEXT I        MSGLEN = 25        GOSUB GROUPBY5:                  'print 25 characters NOGROUP: LOOP        TEXT$ = TEMP$                    'print remainder        MSGLEN = LEN(TEXT$)        IF MSGLEN <= 25 THEN GOSUB GROUPBY5        IF MSGLEN <= 25 THEN RETURN      'done printing      WHILE MSGLEN > 25        TEMP$ = LEFT$(TEXT$, 25)         'get first 25 characters                                                          'in string        TEMPR$ = RIGHT$(TEXT$, MSGLEN - 25)        TEXT$ = TEMP$        MSGLEN = LEN(TEXT$)        GOSUB GROUPBY5        TEXT$ = TEMPR$        MSGLEN = LEN(TEXT$)      WEND        TEXT$ = TEMPR$        MSGLEN = LEN(TEXT$)        GOSUB GROUPBY5 RETURN GROUPBY5:    L = 1    FOR I = 1 TO MSGLEN        PRINT MID$(TEXT$, I, 1);        IF I / 5 = L THEN GOTO AGROUP        GOTO NGROUP AGROUP:   PRINT " ";        L = L + 1 NGROUP:NEXT I        NUMX = 5 - MSGLEN MOD 5        IF NUMX = 5 OR NUMX = 0 THEN GOTO PRTBLK        FOR I = 1 TO NUMX        PRINT "X";        NEXT I PRTBLK:  PRINT RETURN END

Note that the main portion of CIPHER4.BAS very closely resembles CIPHER3.BAS. However, calls using the GOSUB statement were added to access the subroutines MSGFILE, CONVERTSTORE, and PRTOUT—the latter subroutine accesses the subroutine GROUPBY5 as previously discussed. You should also note the inclusion of several PRINT statements at the beginning of the program to better describe its operation.

Table of Contents