Re: [RFC PATCH v2 09/12] crypto: nhpoly1305 - add NHPoly1305 support

[Eric Biggers <ebiggers@kernel.org>]

Hi Ard,

On Mon, Oct 22, 2018 at 07:25:27PM -0300, Ard Biesheuvel wrote:
> >
> > Hmm, I'm actually leaning towards the following instead.  Unrolling multiple
> > strides to try to reduce loads of the keys doesn't seem worthwhile in the C
> > implementation; for one, it bloats the code size a lot
> > (412 => 2332 bytes on arm32).
> >
> > static void nh_generic(const u32 *key, const u8 *message, size_t message_len,
> >                        __le64 hash[NH_NUM_PASSES])
> > {
> >         u64 sums[4] = { 0, 0, 0, 0 };
> >
> >         BUILD_BUG_ON(NH_PAIR_STRIDE != 2);
> >         BUILD_BUG_ON(NH_NUM_PASSES != 4);
> >
> >         while (message_len) {
> >                 u32 m0 = get_unaligned_le32(message + 0);
> >                 u32 m1 = get_unaligned_le32(message + 4);
> >                 u32 m2 = get_unaligned_le32(message + 8);
> >                 u32 m3 = get_unaligned_le32(message + 12);
> >
> >                 sums[0] += (u64)(u32)(m0 + key[ 0]) * (u32)(m2 + key[ 2]);
> >                 sums[1] += (u64)(u32)(m0 + key[ 4]) * (u32)(m2 + key[ 6]);
> >                 sums[2] += (u64)(u32)(m0 + key[ 8]) * (u32)(m2 + key[10]);
> >                 sums[3] += (u64)(u32)(m0 + key[12]) * (u32)(m2 + key[14]);
> >                 sums[0] += (u64)(u32)(m1 + key[ 1]) * (u32)(m3 + key[ 3]);
> >                 sums[1] += (u64)(u32)(m1 + key[ 5]) * (u32)(m3 + key[ 7]);
> >                 sums[2] += (u64)(u32)(m1 + key[ 9]) * (u32)(m3 + key[11]);
> >                 sums[3] += (u64)(u32)(m1 + key[13]) * (u32)(m3 + key[15]);
> 
> Are these (u32) casts really necessary? All the addends are u32 types,
> so I'd expect each (x + y) subexpression to have a u32 type already as
> well. Or am I missing something?
> 

The (u32) casts are only necessary when sizeof(int) > sizeof(u32), as then the
addends will be promoted to 'int'.  Of course, that's never the case for the
Linux kernel.  But I prefer it to be as robust and well-defined as possible,
since people might use this as a reference when coding other implementations,
which could end up finding their way into unusual and/or future platforms.

- Eric