Code:

1 DIM T%(1000), L1(26), L2(26), TA%(1000), TB%(1000), TC%(1000)4 DIM TP%(50), R%(1000), C%(1000), B(11), LB(11)5 DIM KS%(9, 9), RL%(26), CL%(26)10 REM DETERMINE PERIOD20 REM OPEN FILES, READ AND PURGE25 GOSUB 90000: REM Set UP And OPEN FILES30 GOSUB 10000: REM READ DATA FROM FILE40 GOSUB 10200: REM REMOVE UNWANTED CHARACTERS & SPACES41 PRINT "DO YOU KNOW THE PERIOD? TYPE Y OR N."42 A$ = "": A$ = INKEY$: IF A$ = "" GOTO 4243 IF UCASE$(A$) = "Y" GOTO 30045 PRINT "PERIOD IC"50 FOR P = 3 TO 20    55 FOR Q = 1 TO 26: L1(Q) = 0: NEXT Q    60 HP = INT(P / 2)    70 OD = P - 2 * HP    75 S = 0    80 TA% = 0: K = 0    90 FOR B = 0 TO J - P STEP P        100 FOR X = 1 TO HP + OD            110 F = T%(B + X)            120 G = T%(B + X + HP)            125 S = S + 1            130 GOSUB 1000        140 NEXT X        150 FOR X = 1 TO HP            160 L1(T%(X + B)) = L1(T%(X + B)) + 1            170 L2(T%(B + P + 1 - X)) = L2(T%(B + P + 1 - X)) - 1        180 NEXT X    190 NEXT B    200 IC = 0    210 GOSUB 2000    220 GOSUB 3000    230 PRINT P; INT((IC * 625) + .5); INT(CS + .5): REM MODIFIED TO SHOW INTEGER VALUES ONLY240 NEXT P250 REM PLACE THE TIP (SUSPECTED WORD OR PHRASE ENCIPHERED WITHIN THE MESSAGE)300 INPUT "WHAT IS THE PERIOD? "; P320 HP = INT(P / 2)330 OD = P - 2 * HP340 INPUT "ENTER THE TIP -- NO BLANKS "; A$355 TL = LEN(A$)360 FOR I = 1 TO TL    370 TP%(I) = ASC(MID$(A$, I, 1)) - 64: REM INDEX OMITTED IN ORIGINAL LISTING380 NEXT I390 REM MAKE VECTORS OF ROW AND COLUMN ELEMENTS: AR = 1, AC = 101400 K = 1401 REM SEE ALSO: ALTERNATE CODE ON P. 243410 FOR B = 0 TO J - P STEP P    415 C%(K) = T%(B + 1)    420 FOR X = 1 TO HP + OD        430 R%(K) = T%(B + X)        440 R%(K + 1) = T%(B + X) - 100        445 C%(K + 1 - OD) = T%(B + X + HP) + 100        450 C%(K + 2 - OD) = T%(B + X + HP + 1)        460 K = K + 2    470 NEXT X    480 K = K - OD490 NEXT B495 B(11) = 10000: FOR XX = 1 TO 10: B(XX) = 0: NEXT XX500 IF OD = 0 GOTO 610510 REM FIND ODD PERIOD NATURALS520 FOR I = 0 TO B - TL    530 C = 0    540 FOR K = 1 TO TL        550 IF R%(I + K) = TP%(K) THEN C = C + 1        560 IF C%(I + K) = TP%(K) + 100 THEN C = C + 1    570 NEXT K    580 GOSUB 4000590 NEXT I600 GOTO 700610 REM DO IDENTICALS FOR EVEN PERIOD620 FOR I = 0 TO B - TL    630 C = 0    640 FOR K = 1 TO TL        650 IF (TP%(K) = R%(I + K)) AND (TP%(K + 1) = C%(I + K)) THEN C = C + 2    670 NEXT K    680 GOSUB 4000690 NEXT I700 PRINT: PRINT "PT=" ': REM CHECK THIS ORIGINALLY 700 PRINT: PRINT "PT='710 FOR I = 1 TO TL    720 PRINT CHR$(TP%(I) + 64);730 NEXT I735 PRINT740 FOR I = 10 TO 1 STEP -1    745 IF B(I) = 0 GOTO 830    750 PRINT LB(I),    760 FOR X = 1 TO TL        765 T = R%(LB(I) + X): IF T > 100 THEN T = T - 100: PRINT "reverse";        770 PRINT CHR$(T + 64); "reverse off";    780 NEXT X    790 PRINT: PRINT ,    800 FOR X = 1 TO TL        805 T = C%(LB(I) + X): IF T > 100 THEN T = T - 100: PRINT "reverse";        810 PRINT CHR$(T + 64); "reverse off";    820 NEXT X    825 PRINT830 NEXT I833 CLOSE #2835 A$ = "": A$ = INKEY$: IF A$ = "" THEN GOTO 835840 STOP1000 FOR Y = 1 TO K    1010 IF (F = TA%(Y)) AND (G = TB%(Y)) THEN TC%(Y) = TC%(Y) + 1: RETURN1020 NEXT Y1030 K = K + 11040 TA%(K) = F1050 TB%(K) = G1060 TC%(K) = I1070 RETURN2000 FOR Y = 1 TO K    2010 IC = IC + TC%(Y) * (TC%(Y) - 1)2020 NEXT Y2030 IC = IC / (S * (S - 1))2040 RETURN3000 CS = 03010 FOR Z = 1 TO 26    3015 IF L1(Z) + L2(Z) = 0 GOTO 3030    3020 CS = CS + (L1(Z) - L2(Z)) ^ 2 / (L1(Z) + L2(Z))3030 NEXT Z3040 RETURN4000 IF C < B(1) THEN RETURN4010 FOR XX = 2 TO 11    4020 IF C < B(XX) THEN B(XX - 1) = C: LB(XX - 1) = I: RETURN    4030 B(XX - 1) = B(XX)    4040 LB(XX - 1) = LB(XX)4050 NEXT XX4060 RETURN10000 REM Read in the text10010 J = 010020 F = 010030 CLS: PRINT10040 PRINT "ENTERING TEXT FROM FILE"10045 PRINT: PRINT10050 LINE INPUT #1, A$10055 PRINT A$10060 FOR I = 1 TO LEN(A$)    10070 X = ASC(MID$(A$, I, 1)) - 64    10080 IF X = -17 THEN F = 1    10090 J = J + 1    10100 T%(J) = X10110 NEXT I10120 IF F = 0 GOTO 1005010121 SLEEP 5: PRINT: PRINT10130 J = J - 110135 CLOSE #110140 RETURN10200 REM SQUEESE OUT THE BLANKS10210 I = 010220 FOR K = 1 TO J    10230 IF T%(K) < 1 OR T%(K) > 26 GOTO 10260    10240 I = I + 1    10250 T%(I) = T%(K)10260 NEXT K10270 J = I10280 RETURN90000 REM OPEN INPUT AND OUTPUT FILE90005 SCREEN _NEWIMAGE(800, 600, 256): REM SET SCREEN PROPERTIES90010 CLS90020 INPUT "WHAT IS THE NAME OF THE INPUT FILE  > "; A$90030 OPEN A$ FOR INPUT AS #190040 PRINT: PRINT90050 INPUT "WHAT IS THE NAME OF THE OUTPUT FILE > "; A$90060 OPEN A$ FOR OUTPUT AS #290070 RETURN