HPAVC/HPAVC CD-ROM.iso/pc/SECDR13A.ZIP/CRYPT2.ASM

Assembly Source File | 1993-11-19 | 19.2 KB | 763 lines

        %PAGESIZE        59        ; Turbo assembler formatting codes
        %BIN        13
        %LINUM        3

; Copyright (c) 1993 Colin Plumb.  This code may be freely
; distributed under the terms of the GNU General Public Licence.
; Internet <[e-mail]>

        .model large
        .code

; 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


; Args are in, out, key
        public        _Idea2
_Idea2 proc far
        cld
        push        bp           ; Args start at [bp+6]
        mov        bp,sp
        push        si
        push        di
        push        ds        ; 6 more words here, so args are at [sp+12]
        lds        si,[bp+6]        ; in
        lodsw
        xchg        ah,al
        mov        dx,ax
        lodsw
        xchg        ah,al
        mov        x1,ax
        lodsw
        xchg        ah,al
        mov        x2,ax
        lodsw
        xchg        ah,al
        mov        x3,ax
        lds        si,[bp+14]        ; key

        mov        x0,dx

        call        Core

        mov        ax,x0
        mov        bp,sp
        les        di,[bp+16]
        xchg        ah,al
        stosw
        mov        ax,x1
        xchg        ah,al
        stosw
        mov        ax,x2
        xchg        ah,al
        stosw
        mov        ax,dx
        xchg        ah,al
        stosw

        pop        ds
        pop        di
        pop        si
        pop        bp

        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]
        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]
        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