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Bean a écrit :
> Hi,
>
> Oh, I just come up with a better way to do this:
>
> typedef char grub_password_t[1024];
>
> int
> grub_auth_strcmp (const grub_password_t s1, const grub_password_t s2)
> {
> char r1 = 0;
> char r2 = 0;
> char *p;
> int i, c;
>
> p = &r1;
> c = 0;
> for (i = 0; i < sizeof (grub_password_t); i++, s1++, s2++)
> {
> *p | = (*s1 ^ *s2);
> if ((int) *s1 == c)
> {
> p = &r2;
> c = 0x100;
> }
> }
>
> return (r1 != 0);
> }
>
> The condition (int) *s1 == c would be true exactly once.
>
Well, it seems I lost something somewhere. I don't understand the need
of doing it exactly sizeof (grub_password_t) times, except from having a
perfectly symetric function. IMHO, stopping the comparison when the
input buffer is done reading, or when the maximum size of a passphrase
is reached does not leak any information to the attacker. So I would
stick to
typedef char grub_password_t[1024];
int
auth_strcmp (const grub_password_t input, grub_password_t key)
{
int retval, it;
for (it = retval = 0; it < PASSPHRASE_MAXSIZE; it++, input++, key++)
{
retval |= (*input != *key);
if (*input == '\0')
break;
}
return !retval;
}
Also, take care that it requires to check how the function is
optimized; sometimes you have surprises ... ;)
Thomas.
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