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SubjectRe: [PATCH 3/4] Twofish cipher - i586 assembler
Nice push/pop design!

A couple of questions:

1) Would it be worth moving encrypt_round's pop %edi earlier, like
encrypt_first_round does? Scheduling loads as early as possible is
just good general principles.

2) Is it really worth having special first & last round definitions?

encrypt_first round just has one more instruction that encrypt_round
(ror $16,%eax) that could be moved to the pre-round setup, thereby
eliminating the entire encrypt_first_round macro.

And the only difference in encrypt_last_round is the absence of a
"push b ### D" that could be delayed until the end of the macro and
moved into the start of the next encrypt_round.

Oh... and a change from "rol $15, c ## D" to "ror $1, c ## D".
It might be worth living with the single extra instruction for
the code simplicity.

Then you'd have a single encrypt_round of:

/*
a input register containing a (prerotated 16 bits)
b input register containing b
c input register containing c
d input register containing d (prerotated 1 bit left)
operations on a and b are interleaved to increase performance
*/
#define encrypt_round(a,b,c,d,round)\
push d ## D;\
movzx b ## B, %edi;\
mov s1(%ebp,%edi,4),d ## D;\
movzx a ## B, %edi;\
mov s2(%ebp,%edi,4),%esi;\
movzx b ## H, %edi;\
ror $16, b ## D;\
xor s2(%ebp,%edi,4),d ## D;\
movzx a ## H, %edi;\
ror $16, a ## D;\
xor s3(%ebp,%edi,4),%esi;\
movzx b ## B, %edi;\
xor s3(%ebp,%edi,4),d ## D;\
movzx a ## B, %edi;\
xor (%ebp,%edi,4), %esi;\
movzx b ## H, %edi;\
ror $15, b ## D;\
xor (%ebp,%edi,4), d ## D;\
movzx a ## H, %edi;\
xor s1(%ebp,%edi,4),%esi;\
pop %edi;\
add d ## D, %esi;\
add %esi, d ## D;\
add k+round(%ebp), %esi;\
xor %esi, c ## D;\
rol $15, c ## D;\
add k+4+round(%ebp),d ## D;\
xor %edi, d ## D;

which would be called by:
twofish_enc_blk:
push %ebp /* save registers according to calling convention*/
push %edi
push %ebx
push %esi

mov ctx + 16(%esp), %ebp /* abuse the base pointer: set new base bointer to the crypto ctx */
mov in_blk+16(%esp),%edi /* input adress in edi */

mov (%edi), %eax
mov b_offset(%edi), %ebx
mov c_offset(%edi), %ecx
mov d_offset(%edi), %edx
input_whitening(%eax,%ebp,a_offset)
input_whitening(%ebx,%ebp,b_offset)
input_whitening(%ecx,%ebp,c_offset)
input_whitening(%edx,%ebp,d_offset)
rol $16, %eax

encrypt_round(R0,R1,R2,R3,0)
encrypt_round(R2,R3,R0,R1,8)
encrypt_round(R0,R1,R2,R3,2*8)
encrypt_round(R2,R3,R0,R1,3*8)
encrypt_round(R0,R1,R2,R3,4*8)
encrypt_round(R2,R3,R0,R1,5*8)
encrypt_round(R0,R1,R2,R3,6*8)
encrypt_round(R2,R3,R0,R1,7*8)
encrypt_round(R0,R1,R2,R3,8*8)
encrypt_round(R2,R3,R0,R1,9*8)
encrypt_round(R0,R1,R2,R3,10*8)
encrypt_round(R2,R3,R0,R1,11*8)
encrypt_round(R0,R1,R2,R3,12*8)
encrypt_round(R2,R3,R0,R1,13*8)
encrypt_round(R0,R1,R2,R3,14*8)
encrypt_round(R2,R3,R0,R1,15*8)

rol $16, %ecx
output_whitening(%eax,%ebp,c_offset)
output_whitening(%ebx,%ebp,d_offset)
output_whitening(%ecx,%ebp,a_offset)
output_whitening(%edx,%ebp,b_offset)

mov out_blk+16(%esp),%edi;
mov %ecx, (%edi)
mov %edx, b_offset(%edi)
mov %eax, c_offset(%edi)
mov %ebx, d_offset(%edi)

pop %edi
pop %esi
pop %ebx
pop %ebp
mov $1, %eax
ret

I'm also trying to figure out why the encrypt_round and decrypt_round
macros are different. Normally, a Feistel cipher just requires that
the round subkeys be reversed to reverse the cipher; the F function is
unmodified.
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