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s3_cbc.c: get rid of expensive divisions.

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This commit is contained in:
Andy Polyakov 2013-02-08 16:36:09 +01:00
parent 0e4f7045ea
commit c98a45452b
1 changed files with 20 additions and 13 deletions
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33
ssl/s3_cbc.c
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@ -76,6 +76,13 @@
#define DUPLICATE_MSB_TO_ALL(x) ( (unsigned)( (int)(x) >> (sizeof(int)*8-1) ) )
#define DUPLICATE_MSB_TO_ALL_8(x) ((unsigned char)(DUPLICATE_MSB_TO_ALL(x)))
/* constant_time_lt returns 0xff if a<b and 0x00 otherwise. */
static unsigned constant_time_lt(unsigned a, unsigned b)
{
a -= b;
return DUPLICATE_MSB_TO_ALL(a);
}
/* constant_time_ge returns 0xff if a>=b and 0x00 otherwise. */
static unsigned constant_time_ge(unsigned a, unsigned b)
{
@ -285,16 +292,13 @@ void ssl3_cbc_copy_mac(unsigned char* out,
rotate_offset = (div_spoiler + mac_start - scan_start) % md_size;
memset(rotated_mac, 0, md_size);
for (i = scan_start; i < rec->orig_len;)
for (i = scan_start, j = 0; i < rec->orig_len; i++)
{
for (j = 0; j < md_size && i < rec->orig_len; i++, j++)
{
unsigned char mac_started = constant_time_ge(i, mac_start);
unsigned char mac_ended = constant_time_ge(i, mac_end);
unsigned char b = 0;
b = rec->data[i];
rotated_mac[j] |= b & mac_started & ~mac_ended;
}
unsigned char mac_started = constant_time_ge(i, mac_start);
unsigned char mac_ended = constant_time_ge(i, mac_end);
unsigned char b = rec->data[i];
rotated_mac[j++] |= b & mac_started & ~mac_ended;
j &= constant_time_lt(j,md_size);
}
/* Now rotate the MAC */
@ -302,16 +306,19 @@ void ssl3_cbc_copy_mac(unsigned char* out,
j = 0;
for (i = 0; i < md_size; i++)
{
unsigned offset = (div_spoiler + rotate_offset + i) % md_size;
out[j++] = rotated_mac[offset];
out[j++] = rotated_mac[rotate_offset++];
rotate_offset &= constant_time_lt(rotate_offset,md_size);
}
#else
memset(out, 0, md_size);
rotate_offset = md_size - rotate_offset;
rotate_offset &= constant_time_lt(rotate_offset,md_size);
for (i = 0; i < md_size; i++)
{
unsigned offset = (div_spoiler + md_size - rotate_offset + i) % md_size;
for (j = 0; j < md_size; j++)
out[j] |= rotated_mac[i] & constant_time_eq_8(j, offset);
out[j] |= rotated_mac[i] & constant_time_eq_8(j, rotate_offset);
rotate_offset++;
rotate_offset &= constant_time_lt(rotate_offset,md_size);
}
#endif
}