;            SCRAMBLE.ASM ver 2.1
;
;SCRAMBLE is a program to scramble CP/M files using an 8 byte
;password.
;
;03/14/79 Originally written by Ward Crhistensen
;
;07/13/81 Moved stack init to beginning so default stack not
;         used.  Added fix to write routine for proper
;         operation under CP/M 2.x.  Expanded Macros so program
;         may be assembled with ASM.  By Keith Petersen, W8SDZ
;
;12/30/82 Removed loop called MIXUP and MVI H,0 just before
;         it.  Comment was "Scramble awhile to mix up the
;         seed".  Loop occurred before the password was moved
;         into location, so loop had no effect on "seed".
;         Added CALL ERXIT in FINISH.  If an error had
;         occurred program would have crashed on the error
;         message itself.  Added more comments around pseudo-
;         random number generator to better understand the
;         coding.  By Bob Hageman
;
MONTH   EQU     12      ;LAST..
DAY     EQU     30      ;..MODIFICATION..
YEAR    EQU     82      ;..DATE
;
;Scrambling is done in place, i.e. the file is modified on top
;of itself.  The same password used to scramble the file is
;used to unscramble it, using the exact same command.  This is
;because the scrambling code is exclusive-ORed with the data
;file, and two same exclusive ORs result in the original value
;being returned.
;
;Command format:
;
;       SCRAMBLE filename.type PASSWORD
;
;Where PASSWORD is any 8 character string which
;is allowable as a file name (i.e. no '.', etc).
;
       ORG     100H
;
;Init local stack
       LXI     H,0
       DAD     SP
       SHLD    STACK
       LXI     SP,STACK
;
;Print sign-on message
       CALL    START
       DB      'SCRAMBLE.COM as of '
       DB      '0'+MONTH/10
       DB      '0'+MONTH MOD 10,'/'
       DB      '0'+DAY/10
       DB      '0'+DAY MOD 10,'/'
       DB      '0'+YEAR/10
       DB      '0'+YEAR MOD 10
       DB      0DH,0AH,'$'
;
START   POP     D       ;GET ID
       MVI     C,PRINT
       CALL    BDOS    ;PRINT ID
;
;Start of program execution
;
;See that the password is 8 characters
       LDA     FCB2+8
       CPI     ' '
       JNZ     PWIS8
       CALL    ERXIT
       DB      '++ PASSWORD NOT 8 BYTES ++$'
;
;Save the password
;
PWIS8   LXI     H,FCB2+1 ;POINT TO PASSWORD
       LXI     D,PASSWD ;OUR PASSWORD AREA
       LXI     B,8      ;8 CHARS
       CALL    MOVER    ;MOVE IT
;
;Password is 8 bytes, now make sure no character
;is repeated more than 2 times
       LXI     H,PASSWD
       MVI     B,8     ;8 CHARS TO TEST
;
DUPTEST CALL    CKDUP   ;ABORTS IF 3 = CHARS
       INX     H       ;TO NEXT CHAR
       DCR     B
       JNZ     DUPTEST
;
;See that the input file exists
       PUSH    B
       PUSH    D
       PUSH    H
       MVI     C,OPEN
       LXI     D,FCB
       CALL    BDOS
       POP     H
       POP     D
       POP     B
       INR     A       ;OK?
       JNZ     SCRAMLP ;YES, SCRAMBLE IT
       CALL    ERXIT
       DB      '++NO SUCH FILE++$'
;
;Read the file, scramble a sector, re-write it.
;
SCRAMLP CALL    RDSECT  ;READ A SECTOR
       JC      FINISH  ;EXIT LOOP IF EOF
       CALL    SCRAMBL ;SCRAMBLE IT
       CALL    BACKUP  ;RE-POSITION FOR WRITE
       CALL    WRSECT  ;RE-WRITE THE SECTOR
       JMP     SCRAMLP ;LOOP UNTIL EOF
;
;All done - on a "normal" CP/M system we wouldn't have to do
;anything because we re-wrote in place.  However, for systems
;which use sector deblocking we must explicitly close the file
;in order to flush the memory-resident disk buffers.
;
FINISH  PUSH    B
       PUSH    D
       PUSH    H
       MVI     C,CLOSE
       LXI     D,FCB
       CALL    BDOS
       POP     H
       POP     D
       POP     B
       INR     A       ;THIS BETTER WORK..
       JNZ     EXIT
       CALL    ERXIT
       DB      '++ CLOSE ERROR - FILE LEFT IN '
       DB      'UNKNOWN CONDITION ++$'
;
;Sector read routine
;
RDSECT  PUSH    B
       PUSH    D
       PUSH    H
       MVI     C,READ
       LXI     D,FCB
       CALL    BDOS
       POP     H
       POP     D
       POP     B
       ORA     A
       RZ              ;ALL OK
;
;Read error or EOF
;
       CPI     1       ;EOF?
       STC             ;CARRY SHOWS EOF
       RZ              ;RET, CARRY SET
       CALL    ERXIT
       DB      '++ READ ERROR - FILE MAY BE '
       DB      'DESTROYED ++$'
;
;Scramble the sector
;
SCRAMBL LXI     H,80H   ;POINT TO SECTOR
;
SCRLP   CALL    PSEURAN ;GET PSEUDO RANDOM #
       XRA     M       ;SCRAMBLE
       MOV     M,A
       INR     L       ;MORE IN SECTOR?
       JNZ     SCRLP
       RET
;
;Backup the file pointer for the re-write
;
BACKUP  LDA     FCBRNO  ;GET SECTOR #
       DCR     A       ;BACK UP
       STA     FCBRNO
       RP              ;RETURN IF OK
;
;We backed up into previous extent, will have to re-open it
;(this only works for 16k extent size systems).
;
       LDA     FCBEXT  ;GET EXTENT
       DCR     A       ;BACK UP 1
       STA     FCBEXT
       PUSH    B
       PUSH    D
       PUSH    H
       MVI     C,OPEN  ;RE-OPEN
       LXI     D,FCB
       CALL    BDOS
       POP     H
       POP     D
       POP     B
       INR     A
       JNZ     OPEN2OK
       CALL    ERXIT
       DB      '++ RE-OPENING EXTENT FAILED',0DH,0AH
       DB      '++ FILE IS CLOBBERED $'
;
OPEN2OK MVI     A,7FH   ;GET HI SECTOR
       STA     FCBRNO
       RET
;
;Write back the sector
;
WRSECT  LDA     FCB+14
       ANI     1FH     ;RESET S2 FLAG FOR CP/M 2.x
       STA     FCB+14
       PUSH    B
       PUSH    D
       PUSH    H
       MVI     C,WRITE
       LXI     D,FCB
       CALL    BDOS
       POP     H
       POP     D
       POP     B
       ORA     A
       RZ
       CALL    ERXIT
       DB      '++ WRITE ERROR - FILE CLOBBERED ++$'
;
;Get a Pseudo-Random 8 bit number using the password as a seed
;
;       For speed, this routine does no register
;       PUSHes and POPs, however HL aren't used.
;
PSEURAN MVI     C,4     ;GRAB EVERY 4TH PSEU. #
;
; The following is done four times for each character in the
;file being scrambled.  After four password shifts a value is
;returned in A to the calling routine.
;
PSEULP0 MVI     B,8     ;SHIFT THRU 8 BYTES
       LXI     D,PASSWD
       ORA     A       ;CLEAR INITIAL CARRY
;
; PSEULP1 shifts the 8 byte sequence of the password one bit to
;the right filling the left most bit with 0 (for the first of
;the four passes, after that bit may receive 0 or 1 from carry)
;and ending with the right most bit moved to the carry.
;
PSEULP1 LDAX    D       ;GET A CHAR
       RAR             ;SHIFT
       STAX    D       ;Put shifted char back in place
       INX     D       ;Point to next char
       DCR     B       ;Count down
       JNZ     PSEULP1
;Exclusive-OR the last few bits into the first one
       DCX     D       ;BACK UP TO LAST
       RAR             ;Shift the 8th byte twice more thru
       RAR             ;  itself and the carry
       XCHG
       XRA     M       ;Mix treble shifted 8th byte in A with
                       ;  the single shifted 8th byte in M
       RRC             ;SHIFT LO BIT INTO HI discarding the
                       ;  carry bit (4th shift of 8th byte)
       ANI     80H     ;ISOLATE SINGLE BIT, A will contain
                       ;  either 80H or 00H
       LXI     H,PASSWD ;GET FIRST BYTE
       ORA     M       ;'OR' IN THE BIT
       MOV     M,A     ;MOVE IT BACK, whatever is in A when
                       ;  C=0 will be the value to be XORed
                       ;  with the next byte in the current
                       ;  sector.  This value changes for each
                       ;  and every byte of the file.
       XCHG            ;RESTORE HL
       DCR     C
       JNZ     PSEULP0 ;LOOP IF MORE PASSES
       RET
;
;Routine to check for duplicate chars in password
;
CKDUP   MVI     C,3     ;DUP CHAR COUNTER
       LXI     D,PASSWD
       MVI     A,8     ;CHAR COUNT
;
CKDLP   PUSH    PSW     ;SAVE COUNT
       LDAX    D       ;GET CHAR
       CMP     M       ;DUP?
       JNZ     CKNDUP
       DCR     C       ;COUNT DUPS
       JNZ     CKNDUP
       STA     DUPCHAR ;SAVE FOR PRINT
       CALL    ERXIT
       DB      '++ NO CHARACTER MAY APPEAR MORE '
       DB      'THAN TWICE IN THE PASSWORD.  ',0DH,0AH
       DB      ''''
DUPCHAR DB      $-$,''' DOES IN YOURS ++$'
;
CKNDUP  INX     D
       POP     PSW     ;GET COUNT
       DCR     A
       JNZ     CKDLP
       RET             ;OK, NOT 3 DUP
;
;Move subroutines
;
MOVER   MOV     A,M
       STAX    D
       INX     H
       INX     D
       DCX     B
       MOV     A,B
       ORA     C
       JNZ     MOVER
       RET
;
;Exit with error message
;
MSGEXIT EQU     $       ;EXIT W/"INFORMATIONAL" MSG
ERXIT   POP     D       ;GET MSG
       MVI     C,PRINT
       CALL    BDOS
;
;Exit, restoring stack and return
;
EXIT    LHLD    STACK
       SPHL
       RET             ;TO CCP
;
PASSWD  DS      8       ;PASSWORD KEPT HERE
       DS      40H     ;STACK AREA
STACK   DS      2
;
;BDOS equates
;
RDCON   EQU     1
WRCON   EQU     2
PRINT   EQU     9
CONST   EQU     11
OPEN    EQU     15
CLOSE   EQU     16
SRCHF   EQU     17
SRCHN   EQU     18
ERASE   EQU     19
READ    EQU     20
WRITE   EQU     21
MAKE    EQU     22
REN     EQU     23
STDMA   EQU     26
BDOS    EQU     5
FCB     EQU     5CH
FCB2    EQU     6CH
FCBEXT  EQU     FCB+12
FCBRNO  EQU     FCB+32
;
       END


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