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From:   Ard Biesheuvel <ard.biesheuvel@linaro.org>
Date:   Mon, 22 Oct 2018 19:43:01 -0300
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Subject: Re: [RFC PATCH v2 09/12] crypto: nhpoly1305 - add NHPoly1305 support
To:     Eric Biggers <ebiggers@kernel.org>
Cc:     "open list:HARDWARE RANDOM NUMBER GENERATOR CORE" 
        <linux-crypto@vger.kernel.org>, linux-fscrypt@vger.kernel.org,
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        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        Paul Crowley <paulcrowley@google.com>,
        Greg Kaiser <gkaiser@google.com>,
        Michael Halcrow <mhalcrow@google.com>,
        "Jason A . Donenfeld" <Jason@zx2c4.com>,
        Samuel Neves <samuel.c.p.neves@gmail.com>,
        Tomer Ashur <tomer.ashur@esat.kuleuven.be>
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On 22 October 2018 at 19:40, Eric Biggers <ebiggers@kernel.org> wrote:
> 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.
>

Fair enough.