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OpenSSL: make final reduction in Montgomery multiplication constant-time.

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(The issue was reported by Shay Gueron.)

The final reduction in Montgomery multiplication computes if (X >= m) then X =
X - m else X = X

In OpenSSL, this was done by computing T = X - m,  doing a constant-time
selection of the *addresses* of X and T, and loading from the resulting
address. But this is not cache-neutral.

This patch changes the behaviour by loading both X and T into registers, and
doing a constant-time selection of the *values*.

TODO(fork): only some of the fixes from the original patch still apply to
the 1.0.2 code.
This commit is contained in:
Adam Langley 2014-06-20 12:00:00 -07:00
parent b36a3156b6
commit 75b833cc81
3 changed files with 40 additions and 43 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

15
crypto/bn/asm/x86-mont.pl
View File

@ -570,16 +570,15 @@ $sbit=$num;
&jge (&label("sub"));
&sbb ("eax",0); # handle upmost overflow bit
&and ($tp,"eax");
&not ("eax");
&mov ($np,$rp);
&and ($np,"eax");
&or ($tp,$np); # tp=carry?tp:rp
&set_label("copy",16); # copy or in-place refresh
&mov ("eax",&DWP(0,$tp,$num,4));
&mov (&DWP(0,$rp,$num,4),"eax"); # rp[i]=tp[i]
&mov (&DWP($frame,"esp",$num,4),$j); # zap temporary vector
&mov ("edx",&DWP(0,$tp,$num,4));
&mov ($np,&DWP(0,$rp,$num,4));
&xor ("edx",$np); # conditional select
&and ("edx","eax");
&xor ("edx",$np);
&mov (&DWP(0,$tp,$num,4),$j) # zap temporary vector
&mov (&DWP(0,$rp,$num,4),"edx"); # rp[i]=tp[i]
&dec ($num);
&jge (&label("copy"));

55
crypto/bn/asm/x86_64-mont.pl
View File

@ -273,22 +273,21 @@ $code.=<<___;
mov %rax,($rp,$i,8) # rp[i]=tp[i]-np[i]
mov 8($ap,$i,8),%rax # tp[i+1]
lea 1($i),$i # i++
dec $j # doesnn't affect CF!
dec $j # doesn't affect CF!
jnz .Lsub
sbb \$0,%rax # handle upmost overflow bit
xor $i,$i
and %rax,$ap
not %rax
mov $rp,$np
and %rax,$np
mov $num,$j # j=num
or $np,$ap # ap=borrow?tp:rp
.align 16
.Lcopy: # copy or in-place refresh
mov ($ap,$i,8),%rax
mov (%rsp,$i,8),$ap
mov ($rp,$i,8),$np
xor $np,$ap # conditional select:
and %rax,$ap # ((ap ^ np) & %rax) ^ np
xor $np,$ap # ap = borrow?tp:rp
mov $i,(%rsp,$i,8) # zap temporary vector
mov %rax,($rp,$i,8) # rp[i]=tp[i]
mov $ap,($rp,$i,8) # rp[i]=tp[i]
lea 1($i),$i
sub \$1,$j
jnz .Lcopy
@ -643,7 +642,6 @@ my @ri=("%rax","%rdx",$m0,$m1);
$code.=<<___;
mov 16(%rsp,$num,8),$rp # restore $rp
mov 0(%rsp),@ri[0] # tp[0]
pxor %xmm0,%xmm0
mov 8(%rsp),@ri[1] # tp[1]
shr \$2,$num # num/=4
lea (%rsp),$ap # borrow ap for tp
@ -681,35 +679,36 @@ $code.=<<___;
mov @ri[2],16($rp,$i,8) # rp[i]=tp[i]-np[i]
sbb \$0,@ri[0] # handle upmost overflow bit
mov @ri[0],%xmm0
punpcklqdq %xmm0,%xmm0 # extend mask to 128 bits
mov @ri[3],24($rp,$i,8) # rp[i]=tp[i]-np[i]
xor $i,$i # i=0
and @ri[0],$ap
not @ri[0]
mov $rp,$np
and @ri[0],$np
lea -1($num),$j
or $np,$ap # ap=borrow?tp:rp
movdqu ($ap),%xmm1
movdqa %xmm0,(%rsp)
movdqu %xmm1,($rp)
mov $num,$j
pxor %xmm5,%xmm5
jmp .Lcopy4x
.align 16
.Lcopy4x: # copy or in-place refresh
movdqu 16($ap,$i),%xmm2
movdqu 32($ap,$i),%xmm1
movdqa %xmm0,16(%rsp,$i)
movdqu %xmm2,16($rp,$i)
movdqa %xmm0,32(%rsp,$i)
movdqu %xmm1,32($rp,$i)
.Lcopy4x: # copy or in-place refresh
movdqu (%rsp,$i),%xmm2
movdqu 16(%rsp,$i),%xmm4
movdqu ($rp,$i),%xmm1
movdqu 16($rp,$i),%xmm3
pxor %xmm1,%xmm2 # conditional select
pxor %xmm3,%xmm4
pand %xmm0,%xmm2
pand %xmm0,%xmm4
pxor %xmm1,%xmm2
pxor %xmm3,%xmm4
movdqu %xmm2,($rp,$i)
movdqu %xmm4,16($rp,$i)
movdqa %xmm5,(%rsp,$i) # zap temporary vectors
movdqa %xmm5,16(%rsp,$i)
lea 32($i),$i
dec $j
jnz .Lcopy4x
shl \$2,$num
movdqu 16($ap,$i),%xmm2
movdqa %xmm0,16(%rsp,$i)
movdqu %xmm2,16($rp,$i)
___
}
$code.=<<___;

13
crypto/bn/asm/x86_64-mont5.pl
View File

@ -330,17 +330,16 @@ $code.=<<___;
sbb \$0,%rax # handle upmost overflow bit
xor $i,$i
and %rax,$ap
not %rax
mov $rp,$np
and %rax,$np
mov $num,$j # j=num
or $np,$ap # ap=borrow?tp:rp
.align 16
.Lcopy: # copy or in-place refresh
mov ($ap,$i,8),%rax
mov (%rsp,$i,8),$ap
mov ($rp,$i,8),$np
xor $np,$ap # conditional select:
and %rax,$ap # ((ap ^ np) & %rax) ^ np
xor $np,$ap # ap = borrow?tp:rp
mov $i,(%rsp,$i,8) # zap temporary vector
mov %rax,($rp,$i,8) # rp[i]=tp[i]
mov $ap,($rp,$i,8) # rp[i]=tp[i]
lea 1($i),$i
sub \$1,$j
jnz .Lcopy