PCMania 10/Pcmania_Ep2_10_CD-01.iso/ARTICULOS/tecnologia/SECD13C.ZIP/SECTSR.ASM

Assembly Source File | 1994-03-28 | 24.7 KB | 1,192 lines

; TSR disk encryptor V1.3b

DISKDATA STRUC
DDDRV      DB ?       ;PHYSICAL DRIVE (HD)
DRVLET      DB ?       ;DRIVE LETTER
FIRSTCYL  DW ?       ;FIRST CYLINDER TO ENCRYPT
FIRSTHD   DB ?       ;FIRST HEAD TO ENCRYPT
FIRSTSEC  DB ?       ;FIRST SECTOR (BOOT SECTOR)
LASTCYL   DW ?       ;LAST CYLINDER TO ENCRYPT
MAXSEC      DB ?       ;NUMBER OF SECTORS ON DEVICE
MAXHD      DB ?       ;NUMBER OF HEADS ON DEVICE
SECSIZE   DW ?       ;BLOCK SIZE IN BYTES
SERIAL      DW ?       ;DISK SERIAL NUMBER, FOR IV
      DW ?       ;REST OF DISK SERIAL NUMBER
ACTIVE      DB ?       ;CRYPT THIS DISK? FLAG
      DB ?       ;PAD TO EVEN BOUND
DISKDATA ENDS

MAXDRV=4       ;MAX HD PARTITIONS IN SAFE MODE EWS

;; .MODEL TINY
;; .CODE
_TEXT SEGMENT PARA PUBLIC 'CODE'
 ASSUME CS:_TEXT,DS:_TEXT
 ORG 0100h
START:
 MOV AX,0800h
 MOV DL,0F0h
 INT 13h
 MOV DX,OFFSET ALRINS
 CMP AX,0EDCCh ;1.3 Already Installed?
 JE  ERREX1
 CMP AX,0EDCBh ;IS Previous Version ALREADY INSTALLED?
 JNE LOADTSR
ERREX1:
 MOV AH,9
 INT 21h ;PRINT ERROR AND EXIT
 INT 20h
LOADTSR:
 MOV AH,9
 MOV DX,OFFSET FKEY
 INT 21h ;PRINT SIGN-ON

 MOV AX,3513h
 INT 21h ;GET INT VECTOR
 MOV WORD PTR[INT_JOUT+1],BX
 MOV WORD PTR[INT_JOUT+3],ES
 MOV WORD PTR REALBIOS,BX
 MOV WORD PTR REALBIOS+2,ES

 MOV DX,OFFSET(I13_ENTRY)
 MOV AX,2513h
 INT 21h ;SET NEW INT VECTOR

 MOV AX,3100h
 MOV DX,(END_OF_FILE-START)/16+17
 INT 21h ;GO TSR
 ALIGN 16
TSRSTACK: ;OUR STACK GOES HERE
 NOP

I13_ENTRY:
 CLI
 MOV WORD PTR CS:STKSEG,SS
 MOV WORD PTR CS:STKOFS,SP
 MOV WORD PTR CS:IV,CS
 MOV SS,WORD PTR CS:IV
 MOV SP,OFFSET TSRSTACK ;SETUP OUR STACK
 STI
 PUSH AX
 PUSH BX
 PUSH CX
 PUSH DX
 PUSH SI
 PUSH DI
 PUSH BP
 PUSH DS
 PUSH ES
 PUSH CS
 POP DS

;DECIDE IF THIS REQUEST NEEDS PROCESSING
 CMP AH,2 ;READ SECTOR
 JE RDORWR
 CMP AH,3 ;WRITE SECTOR
 JE RDORWR
 CMP AH,8 ;REQUEST FOR ADDRESS?
 JNE TONOPROC
 CMP DL,0F0h ;CORRECT DRIVE NUMBER?
 JNE TONOPROC
 MOV BP,SP
 MOV [BP+10],OFFSET LOADDATA ;DX=DATA ADDRESS
 MOV [BP+12],CS ;CX=CODE SEGMENT
 MOV AX,0EDCCh; ;THIS MEANS V 1.3+ INSTALLED (V1.2- = EDCB)
 JMP RETPROG

TONOPROC:
 JMP NOPROC
RDORWR:
 MOV BYTE PTR RQTYPE,AH ;STORE REQUEST TYPE
 MOV BYTE PTR NUMSEC,AL ;STORE NUMBER OF SECTORS
 MOV BYTE PTR STSEC,CL    ;STORE SECTOR; MUST BE MASKED
 AND BYTE PTR STSEC,00111111b ;MASK OFF HIGH 2 BITS
 MOV WORD PTR CALLCX,CX ;KEEP CX FOR DISK ACCESS
 MOV AH,CL        ;WE NEED TO...
 MOV AL,CH        ;SWAP THESE
 MOV CL,6        ;ROTATE 6 BITS...
 SHR AH,CL        ;TO THE RIGHT, NOW AX CONTAINS CYLINDER
 MOV BYTE PTR DRIVE,DL    ;STORE DRIVE
 MOV WORD PTR STCYL,AX    ;STORE CYLINDER
 MOV BYTE PTR STHD,DH    ;STORE HEAD
 MOV WORD PTR BUFOS,BX    ;STORE BUFFER ADDRESS
 MOV WORD PTR BUFSG,ES    ;STORE BUFFER SEGMENT

 CMP DL,1           ;FLOPPY DRIVE? EWS
 JBE FLOPPY
;;; CMP BYTE PTR HDNUM,DL ;IS IT ACTIVE HARD DRIVE?
;;; JNE NOTHARD
 MOV BP,OFFSET HD1   ;SEARCH DISKDATA BLOCKS
 MOV CX,MAXDRV
SHDLP:
 CMP DL,[BP].DDDRV
 JNE SHDCNT
 CMP AX,[BP].FIRSTCYL
 JB  SHDCNT
 CMP AX,[BP].LASTCYL
 JA  SHDCNT
 CMP [BP].ACTIVE,1    ;ACTIVE HARD DISK PARTITION?
 JE  SHDFND          ;YES
 MOV BYTE PTR BADKEY,1
 JMP BACKFROMBIOS     ;NO. TO RETURN NOT READY
SHDCNT:
 ADD BP,HD2-HD1
 LOOP SHDLP
 JMP NOPROC          ;NOT ACTIVE & NOT SAFE. PASS THRU
SHDFND:
 MOV AX,OFFSET HKEY
 MOV WORD PTR KEY,AX ;SET KEY
 JMP PROCRQ
NOTHARD:
FLOPPY:
 MOV AX,OFFSET FKEY
 MOV WORD PTR KEY,AX ;SET FLOPPY KEY
 CMP DL,0         ;IS IT DRIVE A?
 JNE NOTDRIVEA
 MOV BP,OFFSET FDA   ;SET DISKDATA BLOCK
 JMP PROCRQ
NOTDRIVEA:
 CMP DL,1         ;IS IT DRIVE B?
 JNE NOTDRIVEB
 MOV BP,OFFSET FDB   ;SET DISKDATA BLOCK
 JMP PROCRQ
NOTDRIVEB:
NOPROC2:
 JMP NOPROC

PROCRQ: ;RIGHT DRIVE

 MOV BYTE PTR BADKEY,0 ;CLEAR BAD KEY FLAG

;HERE WE ENCRYPT THE DATA IF IT WAS A WRITE
 CMP BYTE PTR RQTYPE,3 ;IS IT WRITE?
 JNE NOENCRYPT ;IF NOT, DON'T ENCRYPT
 MOV BYTE PTR ENCRYPT,1 ;SET ENCRYPT MODE
 CALL CRYPTIT ;ENCRYPT THE DATA
 CMP BYTE PTR BADKEY,1
 JE BACKFROMBIOS ;SKIP WRITE IF BAD KEY/NOT LOGGED IN

;HERE WE DO THE ACTUAL DISK OPERATION
NOENCRYPT:
 PUSHF
 PUSH CS
 MOV AX,OFFSET BACKFROMBIOS
 PUSH AX ;HERE WE ARE FAKING AN INT
 MOV AH,BYTE PTR RQTYPE
 MOV AL,BYTE PTR NUMSEC
 MOV CX,WORD PTR CALLCX
 MOV DH,BYTE PTR STHD
 MOV DL,BYTE PTR DRIVE
 MOV ES,WORD PTR BUFSG
 MOV BX,WORD PTR BUFOS
 JMP INT_JOUT ;GO TO BIOS
BACKFROMBIOS:
 PUSHF
 PUSH AX

;HERE WE DECRYPT THE DATA
 MOV BYTE PTR ENCRYPT,0 ;SET DECRYPT MODE
 CALL CRYPTIT ;DECRYPT THE DATA

;HERE WE RETURN TO THE CALLING PROGRAM
 POP AX ;INT RETURN CODE
 POP BX ;FLAGS

 CMP BYTE PTR BADKEY,1 ;RETURN FAKE ERROR?
 JNE NOERRRET
 OR BL,1      ;SET CARRY IF KEY BAD
 MOV AH,80h   ;SIMULATE NO DISK PRESENT ERROR

NOERRRET:
 MOV ES,WORD PTR STKSEG
 MOV SI,WORD PTR STKOFS
 MOV ES:[SI+4],BX ;PUT NEW FLAGS WHERE IRET WILL RECALL THEM

RETPROG:
 POP ES
 POP DS
 POP BP
 POP DI
 POP SI
 POP DX
 POP CX
 POP BX
 CLI
 MOV SS,WORD PTR CS:STKSEG
 MOV SP,WORD PTR CS:STKOFS ;RESTORE USER STACK
 STI
 IRET ;RETURN TO PROGRAM, LOADING MODIFIED FLAGS

NOPROC: ;REQUESTS NOT PROCESSED JUMP HERE
 POP ES
 POP DS
 POP BP
 POP DI
 POP SI
 POP DX
 POP CX
 POP BX
 POP AX
 CLI
 MOV SS,WORD PTR CS:STKSEG
 MOV SP,WORD PTR CS:STKOFS ;RESTORE USER STACK
 STI

INT_JOUT:
 DB 0EAh,00,00,00,00 ;JMP FAR

CRYPTIT: ;THIS ENCRYPTS OR DECRYPTS THE BUFFER
 MOV AL,BYTE PTR NUMSEC
 MOV BYTE PTR SECLFT,AL ;LOAD SECTOR COUNTER
 MOV AX,WORD PTR STCYL
 MOV WORD PTR CURCYL,AX ;LOAD CURRENT CYLINDER
 MOV AL,BYTE PTR STHD
 MOV BYTE PTR CURHD,AL    ;LOAD CURRENT HEAD
 MOV AL,BYTE PTR STSEC
 MOV BYTE PTR CURSEC,AL ;LOAD CURRENT SECTOR
 MOV AX,WORD PTR BUFOS
 MOV WORD PTR BUFPS,AX    ;LOAD BUFFER POINTER

CRYPTLOOP:
 CMP BYTE PTR SECLFT,0
 JE DONECRYPT ;CHECK FOR LAST SECTOR

 MOV AX,WORD PTR CURCYL
 CMP AX,[BP].FIRSTCYL
 JB DONTCRYPTSEC
 JNE NOTBOOTSEC

 MOV AL,BYTE PTR CURHD
 CMP AL,[BP].FIRSTHD
 JB DONTCRYPTSEC
 JNE NOTBOOTSEC

 MOV AL,BYTE PTR CURSEC
 CMP AL,[BP].FIRSTSEC
;DO WE NEED A JB HERE?
 JNE NOTBOOTSEC

 CALL LOADBS ;LOAD DATA FROM BOOT SECTOR
 JMP DONTCRYPTSEC

NOTBOOTSEC:
 MOV AX,WORD PTR CURCYL
 CMP AX,[BP].LASTCYL
 JA DONTCRYPTSEC

 CMP [BP].ACTIVE,1
 JE DOCRYPTSEC      ;CHECK ACTIVE FLAG
 CMP BYTE PTR DRIVE,1
 JBE DONTCRYPTSEC
 MOV BYTE PTR BADKEY,1
 JMP DONTCRYPTSEC
DOCRYPTSEC:
 CALL CRYPTSEC ;ENCRYPT CURRENT SECTOR
DONTCRYPTSEC:
 CALL NEXTSEC  ;GO ON TO NEXT SECTOR
 JMP CRYPTLOOP       ;LOOP
DONECRYPT: RET

CRYPTSEC: ;HERE WE CRYPT ONE SECTOR
;SET UP IV
 MOV AX,WORD PTR CURCYL
 MOV WORD PTR IV,AX
 MOV AL,BYTE PTR CURHD
 MOV BYTE PTR IV+2,AL
 MOV AL,BYTE PTR CURSEC
 MOV BYTE PTR IV+3,AL
 MOV AX,[BP].SERIAL
 MOV WORD PTR IV+4,AX
 MOV AX,[BP].SERIAL+2
 MOV WORD PTR IV+6,AX ;SET UP IV

;PRE-ENCRYPT IV
 MOV AX,1  ;ACTUALLY ZERO BLOCKS, WILL JUST PRE-ENCRYPT IV
 PUSH AX   ;STORE NUMBER OF BLOCKS
 SUB SP,8 ;PRETEND TO PUSH PLAINTEXT/CIPHERTEXT ADDRESSES
 MOV AX,WORD PTR KEY
 PUSH CS
 PUSH AX
 MOV AX,OFFSET IV
 PUSH CS
 PUSH AX
 PUSH AX ;MAKE IT LOOK LIKE A FAR CALL FOR IDEACFB
 CALL _IDEACFB
 ADD SP,20 ;REMOVE EXTRA WORD

;ENCRYPT/DECRYPT THE BLOCK
 MOV AX,[BP].SECSIZE
 MOV CL,3
 SHR AX,CL ;SECSIZE/8
 INC AX    ;ONE BLOCK (IV) NOT USED
 PUSH AX   ;STORE NUMBER OF BLOCKS
 MOV AX,WORD PTR BUFSG
 MOV BX,WORD PTR BUFPS
 PUSH AX
 PUSH BX
 PUSH AX
 PUSH BX
 MOV AX,WORD PTR KEY
 PUSH CS
 PUSH AX
 MOV AX,OFFSET IV
 PUSH CS
 PUSH AX
 PUSH AX ;MAKE IT LOOK LIKE A FAR CALL FOR IDEACFB
 CMP BYTE PTR ENCRYPT,1
 JNE DECRYPT
 CALL _IDEACFB
 JMP DONE_CRYPTSEC
DECRYPT:
 CALL _IDEACFBX
DONE_CRYPTSEC:
; ADD SP,18
 ADD SP,20 ;REMOVE EXTRA WORD
 RET

NEXTSEC: ;INCREMENT HEAD, SECTOR, CYLINDER AS NEEDED TO GO TO NEXT SECTOR
 MOV AX,[BP].SECSIZE
 ADD WORD PTR BUFPS,AX ;GO TO NEXT BLOCK IN MEMORY
 DEC BYTE PTR SECLFT   ;COUNT DOWN SECTORS LEFT
 INC BYTE PTR CURSEC   ;INCREMENT SECTOR
 MOV AL,BYTE PTR CURSEC
 CMP AL,[BP].MAXSEC
 JBE DONE_COUNT
 MOV BYTE PTR CURSEC,1 ;RESET SECTOR
 INC BYTE PTR CURHD    ;INCREMENT HEAD
 MOV AL,BYTE PTR CURHD
 CMP AL,[BP].MAXHD
 JB DONE_COUNT
 MOV BYTE PTR CURHD,0  ;RESET HEAD
 INC WORD PTR CURCYL   ;THEN INCREMENT CYLINDER
DONE_COUNT: RET

LOADBS:
 CMP BYTE PTR DRIVE,1
 JA IGNOREBS             ;DON'T ALTER PARAMS FOR HARD DRIVE
 MOV [BP].ACTIVE,0         ;DEFAULT OFF
 MOV ES,WORD PTR BUFSG
 MOV SI,WORD PTR BUFPS         ;GET ACCESS TO BUFFER
 CMP WORD PTR ES:[SI+510],0AA55h ;CHECK FOR BOOT-RECORD SIGNATURE
 JNE IGNOREBS             ;BAILOUT IF BOOT SECTOR LOOKS BAD
 PUSH SI             ;Check both offsets for CRYP 1.3C
 ADD  SI,WORD PTR INCFOF
 CALL CKCRPF
 POP  SI
 JE   DKCRYPT
 PUSH SI
 ADD  SI,WORD PTR OUTCFOF
 CALL CKCRPF
 POP  SI
 JE   DKCRYPT
 JMP  NOTCRYPT
DKCRYPT:
 PUSH SI            ;CHECK BOTH CKWD OFFSETS 1.3B
 ADD  SI,WORD PTR INCFOF
 CALL CKKEY
 POP  SI
 JE   GOODKEY
 PUSH SI
 ADD  SI,WORD PTR OUTCFOF
 CALL CKKEY
 POP  SI
 JE   GOODKEY
 JMP  NOTGOODKEY


NOTGOODKEY:
 MOV BYTE PTR BADKEY,1
 JMP NOTCRYPT
GOODKEY:
 MOV [BP].ACTIVE,1 ;TURN ON ENCRYPTION
NOTCRYPT:
 MOV AL,ES:[SI+18h]
 MOV [BP].MAXSEC,AL ;STORE MAX SECTOR
 MOV AL,ES:[SI+1Ah]
 MOV [BP].MAXHD,AL  ;STORE MAX HEAD
 MOV AX,ES:[SI+0Bh]
 MOV [BP].SECSIZE,AX ;STORE BYTES PER SECTOR
 MOV AX,ES:[SI+27h]
 MOV [BP].SERIAL,AX  ;STORE FIRST WORD OF SERIAL NUMBER
 MOV AX,ES:[SI+29h]
 MOV [BP].SERIAL+2,AX ;STORE SECOND WORD OF SERIAL NUMBER
IGNOREBS:
 RET

CKCRPF PROC NEAR
 CMP WORD PTR ES:[SI],'RC'
 JNE CKCRPFR
 CMP WORD PTR ES:[SI+2],'PY'
CKCRPFR:
 RET
CKCRPF ENDP

CKKEY PROC NEAR
 MOV AX,WORD PTR FKEYCHK
 CMP WORD PTR ES:[SI+4],AX
 JNE CKKEYRET
 MOV AX,WORD PTR FKEYCHK+2
 CMP WORD PTR ES:[SI+6],AX
CKKEYRET:
 RET
CKKEY ENDP

;VARIABLES HERE WILL BE LOADED BY LOGIN
    ALIGN 4
LOADDATA:
TSRVER    DB '130C'            ;VERSION FOR CHECK
REALBIOS LABEL DWORD
    DW 0,0
FKEYCHK:DW 2 DUP (0ffffh)
FKEY:    DB 13,10,'Secure Drive Version 1.3c',13,10
    DB 'TSR Installed',13,10,'$'
ALRINS: DB 13,10,'Secure Drive TSR already resident',13,10,'$'
    DB (104-($-FKEY)) DUP (0aah)      ;FLOPPY ENCRYPTION KEY
FKEYV10 DB 0
    DB "PPP"              ;PAD FOR ALIGNMENT
HKEYCHK:DW 2 DUP (0ffffh)
HKEY    LABEL BYTE
    DB 104 DUP (0bbh)          ;HARD DRIVE ENCRYPTION KEY
HKEYV10 DB 0
    DB "PPP"              ;PAD FOR ALIGNMENT
INCFOF    DW 3                  ;INPUT CHECKWORD OFFSET IN BOOT REC  1.3B
OUTCFOF DW 502                  ;OUTPUT CHECKWORD OFFSET IN BOOT REC 1.3B
FDA:    DISKDATA <0,'A',0,0,1,0FFFFh,18,2,512,0,0,0,'P'>
FDB:    DISKDATA <0,'B',0,0,1,0FFFFh,18,2,512,0,0,0,'P'>
HD1:    DISKDATA <0,'Z',0FFFFh,0,0,0,0,0,512,0,0,0,'P'>
HD2    LABEL BYTE
    REPT MAXDRV-1
    DISKDATA <0,'Z',0FFFFh,0,0,0,0,0,512,0,0,0,'P'>
    ENDM

    ALIGN 4
;VARIABLES HERE STAY PUT
STKSEG: DW ? ;SEGMENT OF USER STACK
STKOFS: DW ? ;OFFSET OF USER STACK

RQTYPE: DB ? ;REQUEST TYPE, AH FROM CALL
DRIVE:    DB ? ;DRIVE FROM CALL
NUMSEC: DB ? ;NUMBER OF SECTORS TO READ (AL FROM CALL)
SECLFT: DB ? ;NUMBER OF SECTORS LEFT TO PROCESS
CALLCX: DW ? ;ORIGINAL CX
KEY:    DW ? ;CURRENTLY SELECTED KEY
BADKEY: DB ? ;KEY BAD FLAG

STCYL:    DW ? ;START CYLINDER FROM CALL
CURCYL: DW ? ;CURRENT CYLINDER BEING PROCESSED

STSEC:    DB ? ;START SECTOR FROM CALL
CURSEC: DB ? ;CURRENT SECTOR BEING PROCESSED

STHD:    DB ? ;START HEAD FROM CALL
CURHD:    DB ? ;CURRENT HEAD BEING PROCESSED

BUFOS:    DW ? ;OFFSET OF TARGET BUFFER
BUFPS:    DW ? ;CURRENT POSITION WITHIN BUFFER
BUFSG:    DW ? ;SEGMENT OF TARGET BUFFER

ENCRYPT: DB ? ;1 FOR ENCRYPT, 0 FOR DECRYPT
IV:  DB 8 DUP (?)   ;IV FOR CFB

; Copyright (c) 1993 Colin Plumb.  This code may be freely
; distributed under the terms of the GNU General Public Licence.

; A core operation in IDEA is multiplication modulo 65537.
; The valid inputs, 1 through 66636 inclusive are represented in
; 16-bit registers modulo 65536.  I.e. a value of 0 means 65536,
; or -1.  Thus, we need to test for that specially.  -x, modulo
; 65537, is 65537-x = 1-x.
; For any other number, represent the product as a*65536+b.  Since
; 65536 = -1 (mod 65537), this is the same number as b-a.  Should
; this result be negautive (generate a borrow), -n mod 65537 = 1-n
; mod 65536.  Or in other words, if you add the borrow bit back on,
; you get the right answer.

; This is what the assembly code does.    It forms a zero, and adds
; that on with carry.

; Another useful optimisation takes advantage of the fact that
; a and b are equal only if the answer is congruent to 0 mod 65537.
; Since 65537 is prime, this happens only if one of the inputs is
; congruent to 0 mod 65537.  Since the inputs are all less than 65537,
; this means it must have been zero.

; The code below tests for a zero result of the subtraction, and if
; one arises, it branches out of line to figure out what happened.


; This code implemets the IDEA encryption algorithm.
; It follows in pseudo-C, where the * operator operates
; modulo 65537, as Idea needs.    (If you don't understand,
; learn IDEA better.)

; IDEA is works on 16-bit units.  If you're processing bytes,
; it's defined to be big-endian, so an Intel machine needs to
; swap the bytes around.

; void Idea(u_int16 *in, u_int16 *out, u_int16 *key)
; {
;     register u+int16 x0, x1, x2, x3, s1, s2, round;
;
;     x0 = *in++;  x1 = *in++;  x2 = *in++;    x3 = *in;
;
;     for (round = 0; round < 8; round++) {
;         x0 *= *key++;
;         x1 += *key++;
;         x2 += *key++;
;         x3 *= *key++;
;
;         s1  = x1;  s2    = x2;
;         x2 ^= x0;  x1 ^= x3;
;
;         x2 *= *key++;
;         x1 += x2;
;         x1 *= *key++;
;         x2 += x1;
;
;         x0 ^= x1;  x3 ^= x2;
;         x1 ^= s2;  x2 ^= s1;
;     }
;     *out++ = x0 * *key++;
;     *out++ = x2 + *key++;          /* Yes, this is x2, not x1 */
;     *out++ = x1 + *key++;
;     *out    = x3 * *key;
; }

; ds:si points to key, ax, dx are temps, args in bx, cx, di, bp
; Trashes *all* registers.  direction flag must be clear.
; Leaves es zero.

; Since there is no spare register to hold the loop count, I make
; clever use of the stack, pushing the start of the loop several
; times and using a ret instruction to do the return.

; Annoyingly, lods is fastest on 8086's, but other techniques are
; best on 386's.  Well, that's what the manual says, but real
; life is different.  USELODS wins on a 386SX, at least.
; Leave it set for all platforms.

USELODS        equ      1

; bp must be x0 for some of the code below to work
x0      equ         bp
x1      equ         bx
x2      equ         cx
x3      equ         di
; di must be x3 for some of the code below to work

;; Now, this is rather interesting.  We test for zero arguments
;; after the multiply.    Assuming random inputs, one or both are
;; zero (2^17-1)/2^32, or approximately 1/32786 of the time.
;; Encryption in any feedback mode produces essentially random
;; inputs, so average-case analysis is okay.  While we don't
;; want the out-of-line code to waste time, it is not worth
;; slowing down the in-line case to speed it up.
;;
;; Basically, we start inverting the source x, and if that was 0,
;; we use the inverse of the key instead.

Core1Z:
    neg       x0
    jnz       Core1Za
if USELODS
    sub       x0,[si-2]
else
    sub       x0,[si]
endif
Core1Za:
    inc       x0
    jmp       Core1done
Core2Z:
    neg       x3
    jnz       Core2Za
if USELODS
    sub       x3,[si-2]
else
    sub       x3,[si+6]
endif
Core2Za:
    inc       x3
    jmp       Core2done
Core3Z:
    neg       x2
    jnz       Core3Za
if USELODS
    sub       x2,[si-2]
else
    sub       x2,[si+8]
endif
Core3Za:
    inc       x2
    jmp       Core3done
Core4Z:
    neg       x1
    jnz       Core4Za
if USELODS
    sub       x1,[si-2]
else
    sub       x1,[si+10]
endif
Core4Za:
    inc       x1
    jmp       Core4done

; We need a constant 0 that we can move into a register without affecting
; the carry flag (as the classic xor ax,ax is wont to do), so we use the
; es register for a constant 0 source.    This is okay even in protected
; mode.  (I *told* you this was tricky code!)

; BTW, since you wanted to know, this is 8 + 78*4 + 16 = 336 instructions.

Core        proc    near
    xor       ax,ax
    mov       es,ax
    mov       ax,OFFSET Finish
    push        ax
    mov       ax,OFFSET Coreloop
    push        ax          ; Loop 3 times, then return
    push        ax
    push        ax

Coreloop:
if USELODS
    lodsw
else
    mov       ax,[si]          ; x0 *= *key++
endif
    mul       x0
    sub       ax,dx
    jz      Core1Z
    mov       x0,es
    adc       x0,ax
Core1done:

if USELODS
    lodsw
    add       x1,ax
    lodsw
    add       x2,ax
else
    add       x1,[si+2]        ; x1 += *key++
    add       x2,[si+4]        ; x2 += *key++
endif

if USELODS
    lodsw
else
    mov       ax,[si+6]        ; x3 += *key++
endif
    mul       x3
    sub       ax,dx
    jz      Core2Z
    mov       x3,es
    adc       x3,ax
Core2done:

    push        x1            ; s1 = x1
    push        x2              ; s2 = x2

    xor       x1,x3        ; x1 ^= x3
    xor       x2,x0        ; x2 ^= x0

if USELODS
    lodsw
else
    mov       ax,[si+8]        ; x2 *= *key++
endif
    mul       x2
    sub       ax,dx
    jz      Core3Z
    mov       x2,es
    adc       x2,ax
Core3done:

    add       x1,x2        ; x1 += x2

if USELODS
    lodsw
else
    mov       ax,[si+10]         ; x1 *= *key++
endif
    mul       x1
    sub       ax,dx
    jz      Core4Z
    mov       x1,es
    adc       x1,ax
Core4done:

    add       x2,x1        ; x2 += x1

    xor       x0,x1        ; x0 ^= x1
    xor       x3,x2        ; x3 ^= x2

    pop       dx
    xor       x1,dx        ; x1 ^= s2
    pop       dx
    xor       x2,dx        ; x2 ^= s1

; Second unrolling of loop
if USELODS
    lodsw
else
    mov    ax,[si+12]        ; x0 *= *key++
endif
    mul       x0
    sub       ax,dx
    jz      Core5Z
    mov       x0,es
    adc       x0,ax
Core5done:

if USELODS
    lodsw
    add       x1,ax
    lodsw
    add       x2,ax
else
    add       x1,[si+14]         ; x1 += *key++
    add       x2,[si+16]         ; x2 += *key++
endif

if USELODS
    lodsw
else
    mov       ax,[si+18]         ; x3 *= *key++
endif
    mul       x3
    sub       ax,dx
    jz      Core6Z
    mov       x3,es
    adc       x3,ax
Core6done:

    push        x1             ; s1 = x1
    push        x2              ; s2 = x2

    xor       x1,x3        ; x1 ^= x3
    xor       x2,x0        ; x2 ^= x0

if USELODS
    lodsw
else
    mov       ax,[si+20]         ; x2 *= *key++
endif
    mul       x2
    sub       ax,dx
    jz      Core7Z
    mov       x2,es
    adc       x2,ax
Core7done:

    add       x1,x2        ; x1 += x2

if USELODS
    lodsw
else
    mov       ax,[si+22]         ; x1 *= *key++
endif
    mul       x1
    sub       ax,dx
    jz      Core8Z
    mov       x1,es
    adc       x1,ax
Core8done:

    add       x2,x1        ; x2 += x1

    xor       x0,x1        ; x0 ^= x1
    xor       x3,x2        ; x3 ^= x2

    pop       dx
    xor       x1,dx        ; x1 ^= s2
    pop       dx
    xor       x2,dx        ; x2 ^= s1

ife USELODS
    lea       si,[si+24]
endif

    ret       ; Used as a loop instruction!

Core5Z:
    neg       x0
    jnz       Core5Za
if USELODS
    sub       x0,[si-2]
else
    sub       x0,[si+12]
endif
Core5Za:
    inc       x0
    jmp       Core5done
Core6Z:
    neg       x3
    jnz       Core6Za
if USELODS
    sub       x3,[si-2]
else
    sub       x3,[si+18]
endif
Core6Za:
    inc       x3
    jmp       Core6done
Core7Z:
    neg       x2
    jnz       Core7Za
if USELODS
    sub       x2,[si-2]
else
    sub       x2,[si+20]
endif
Core7Za:
    inc       x2
    jmp       Core7done
Core8Z:
    neg       x1
    jnz       Core8Za
if USELODS
    sub       x1,[si-2]
else
    sub       x1,[si+22]
endif
Core8Za:
    inc       x1
    jmp       Core8done
Core9Z:
    neg       x0
    jnz       Core9Za
if USELODS
    sub       x0,[si-2]
else
    sub       x0,[si]
endif
Core9Za:
    inc       x0
    jmp       Core9done
; Special: compute into dx (zero on entry)
Core10Z:
    sub       dx,x3
    jnz       Core10Za
if USELODS
    sub       dx,[si-2]
else
    sub       dx,[si+6]
endif
Core10Za:
    inc       dx
;     jmp        Core10done
    ret


Finish:
if USELODS
    lodsw
else
    mov       ax,[si]          ; x0 *= *key++
endif
    mul       x0
    sub       ax,dx
    jz      Core9Z
    mov       x0,es
    adc       x0,ax
Core9done:

    xchg        x1,x2
if USELODS
    lodsw
    add       x1,ax
    lodsw
    add       x2,ax
else
    add       x1,[si+2]        ; x1 += *key++
    add       x2,[si+4]       ; x2 += *key++
endif

; This is special: compute into dx, not x3
if USELODS
    lodsw
else
    mov       ax,[si+6]        ; x3 *= *key++
endif
    mul       x3
    sub       ax,dx
    mov       dx,es
    jz      Core10Z
    adc       dx,ax
Core10done:

    ret

    endp

; Okay, the basic plan for the CFB kernel is
; get x0,x1,x2,x3
; get key pointer
; call core
; get buffer pointers
;Loop:
; lodsw
; xor         ax,x0
; mov    x0,ax
; stosw
; lodsw
; xor         ax,x1
; mov         x0,ax
; stosw
; lodsw
; xor         ax,x2
; mov         x0,ax
; stosw
; lodsw
; xor         ax,x3
; mov         x3,ax
; stosw
; push buffer pointers
; get key pointer
; call          core
; pop buffer pointers
; loop
; lodsw/xor/etc.
;
;
; This function is designed to go in the middle of a byte-granularity
; CFB engine.  It performs "len" encryptions of the IV, encrypting
; 8*(len-1) bytes from the source to the destination.  The idea is
; that you first xor any odd leading bytes, then call this function,
; then xor up to 8 trailing bytes.

; The main loop in this is 38 instructions, plus the 336 for the core
; makes 374 total.  That's 46.75 instructions per byte.
; (It's the same for IdeaCFBx)

; IV, key, plain, cipher, len
;     public        _IdeaCFB
;_IdeaCFB proc far     ; Args are at [sp+4]
_IDEACFB:
    cld
    push        bp
    push        si
    push        di
    push        ds          ; 8 more words here, so args are at [sp+12]
; To be precise, IV is at 12, key at 16, plain at 20,
; cipher at 24 and len at 28
    mov       bp,sp
    lds       si,[bp+12]         ; IV
; Load and byte-swap IV
    mov       ax,[si]
    xchg        ah,al
    mov       x1,[si+2]
    mov       x2,[si+4]
    xchg        bh,bl
    xchg        ch,cl
    mov       dx,[si+6]
    xchg        dh,dl

    lds       si,[bp+16]         ; Key
    mov       x0,ax
    mov       x3,dx

    call        Core
IdeaCFBLoop:
;     mov        ax,x0
;     mov        bp,sp
;     dec        WORD PTR [bp+28]        ; Decrement count
;     jz       IdeaCFBEnd
;     lds        si,[bp+20]
;     les        di,[bp+24]
;     mov        x0,ax
; Alternate code: (which is faster?  Two moves or three segment overrides?)
    mov       si,sp
    dec       WORD PTR ss:[si+28]
    jz      IdeaCFBEnd
    les       di,ss:[si+24]
    lds       si,ss:[si+20]

    lodsw
    xchg        ah,al
    xor       ax,x0
    mov       x0,ax
    xchg        ah,al
    stosw
    lodsw
    xchg        ah,al
    xor       ax,x1
    mov       x1,ax
    xchg        ah,al
    stosw
    lodsw
    xchg        ah,al
    xor       ax,x2
    mov       x2,ax
    xchg        ah,al
    stosw
    lodsw
    xchg        ah,al
    xor       ax,dx
    mov       dx,ax
    xchg        ah,al
    stosw

;     mov        ax,x0
;     mov        bp,sp
;     mov        [bp+20],si          ; Save source offset
;     mov        [bp+24],di          ; Save destination offset
;     lds        si,[bp+16]          ; Key
;     mov        x0,ax         ; Get x0 in place for another iteration
; Alternate code for the above: (which is faster?  One move or three ss:?)
    mov       ax,si
    mov       si,sp
    mov       ss:[si+20],ax
    mov       ss:[si+24],di
    lds       si,ss:[si+16]

    mov       x3,dx        ; Get x3 in place
    mov       ax,OFFSET IdeaCFBLoop
    push        ax
    jmp       Core

IdeaCFBEnd:
;     lds        si,[bp+12]
    lds       di,ss:[si+12]    ; Get IV for writing back

    mov       ax,x0
    xchg        ah,al
    mov       [di],ax          ; Use stosw?
    xchg        bh,bl
    xchg        ch,cl
    mov       [di+2],x1
    mov       [di+4],x2
    xchg        dh,dl
    mov       [di+6],dx

    pop       ds
    pop       di
    pop       si
    pop       bp

    ret

    endp

; This decoding step is similar, except that instead of
;     lods
;     xor        x0,ax
;     mov        ax,x0
;      stos
; the feedback step is
;     lods
;     xchg         x0,ax
;     xor        ax,x0
;     stos

; IV, key, cipher, plain, len
;     public        _IdeaCFBx
;_IdeaCFBx proc far      ; Args are at [sp+4]
_IDEACFBX:
    cld
    push        bp
    push        si
    push        di
    push        ds          ; 8 more words here, so args are at [sp+12]
    mov       bp,sp
    lds       si,[bp+12]         ; IV
; Load and byte-swap IV
    mov       ax,[si]
    xchg        ah,al
    mov       x1,[si+2]
    mov       x2,[si+4]
    xchg        bh,bl
    xchg        ch,cl
    mov       dx,[si+6]
    xchg        dh,dl

    lds       si,[bp+16]         ; Key
    mov       x0,ax
    mov       x3,dx

    call        Core
IdeaCFBxLoop:
;     mov        ax,x0
;     mov        bp,sp
;     dec        WORD PTR [bp+28]        ; Decrement count
;     jz       IdeaCFBxEnd
;     lds        si,[bp+20]
;     les        di,[bp+24]
;     mov        x0,ax
; Alternate code: (which is faster?  Two moves or three segment overrides)
    mov       si,sp
    dec       WORD PTR ss:[si+28]
    jz      IdeaCFBxEnd
    les       di,ss:[si+24]
    lds       si,ss:[si+20]

    lodsw
    xchg        ah,al
    xchg        x0,ax
    xor       ax,x0
    xchg        ah,al
    stosw
    lodsw
    xchg        ah,al
    xchg        x1,ax
    xor       ax,x1
    xchg        ah,al
    stosw
    lodsw
    xchg        ah,al
    xchg        x2,ax
    xor       ax,x2
    xchg        ah,al
    stosw
    lodsw
    xchg        ah,al
    xchg        dx,ax
    xor       ax,dx
    xchg        ah,al
    stosw

;     mov        ax,x0
;     mov        bp,sp
;     mov        [bp+20],si          ; Save source offset
;     mov        [bp+24],di          ; Save destination offset
;     lds        si,[bp+16]          ; Key
;     mov        x0,ax         ; Get x0 in place for another iteration
; Alternate code for the above: (which is faster?  One move or three ss:?)
    mov       ax,si
    mov       si,sp
    mov       ss:[si+20],ax
    mov       ss:[si+24],di
    lds       si,ss:[si+16]

    mov       x3,dx        ; Get x3 in place
    mov       ax,OFFSET IdeaCFBxLoop
    push        ax
    jmp       Core

IdeaCFBxEnd:
;     lds        si:[bp+12]
    lds       di,ss:[si+12]    ; Get IV for writing back

    mov       ax,x0
    xchg        ah,al
    mov       [di],ax          ; Use stosw?
    xchg        bh,bl
    xchg        ch,cl
    mov       [di+2],x1
    mov       [di+4],x2
    xchg        dh,dl
    mov       [di+6],dx


    pop       ds
    pop       di
    pop       si
    pop       bp

    ret

    endp
END_OF_FILE:
_TEXT ENDS
END START