Re: [PATCH 2/3] crypto: X25519 core functions for ppc64le

Danny Tsen
2024-05-15T13:09:44Z

Hi Andy,

Points taken.  And much appreciate for the help.

Thanks.

-Danny

On 5/15/24 3:29 AM, Andy Polyakov wrote:
> Hi,
>
>> +static void cswap(fe51 p, fe51 q, unsigned int bit)
>> +{
>> +    u64 t, i;
>> +    u64 c = 0 - (u64) bit;
>> +
>> +    for (i = 0; i < 5; ++i) {
>> +        t = c & (p[i] ^ q[i]);
>> +        p[i] ^= t;
>> +        q[i] ^= t;
>> +    }
>> +}
>
> The "c" in cswap stands for "constant-time," and the problem is that
> contemporary compilers have exhibited the ability to produce
> non-constant-time machine code as result of compilation of the above
> kind of technique. The outcome is platform-specific and ironically
> some of PPC code generators were observed to generate "most"
> non-constant-time code. "Most" in sense that execution time variations
> would be most easy to catch. One way to work around the problem, at
> least for the time being, is to add 'asm volatile("" : "+r"(c))' after
> you calculate 'c'. But there is no guarantee that the next compiler
> version won't see through it, hence the permanent solution is to do it
> in assembly. I can put together something...
>
> Cheers.
>