Re: Difference between a+b and bit32.band(a+b)

[Duncan Cross <duncan.cross@...>]

On Fri, Sep 9, 2011 at 3:35 PM, waspoza <waspoza@gmail.com> wrote:
> On Fri, Sep 9, 2011 at 4:29 PM, Lee Hickey <lahickey@gmail.com> wrote:
>>> >> > In digestblock function there is a line: H[1] = band(H[1] + a).
>>> >
>>> >> I'm not sure about this, but I've heard of people doing similar things
>>> >> in JavaScript to force the native Number type to a 32-bit integer,
>>> >> taking overflow into account correctly, etc.
>>> >
>>> > That's what the comment says:
>>> >        -- Add (mod 2^32) this chunk's hash to result so far:
>>>
>>> Yes, but on http://en.wikipedia.org/wiki/SHA-2 pseudocode says:
>>>
>>>  Add this chunk's hash to result so far:
>>>    h0 := h0 + a
>>>
>>> So in pseudocode this line is no different from other additions. And
>>> in lua code there is a difference. Why?
>>
>> From that same wikipedia page and in reference to the pseudocode:
>> Note 1: All variables are unsigned 32 bits and wrap modulo 232 when
>> calculating
>> So, band(H[1] + a) is achieving that.
>
> So why there is no 'and' here:
>
> local t2 = s0 + maj
>
> And not here:
>
> local t1 = h + s1 + ch + k[i] + w[i]
>
> ?
>
>

There are two possibilities. Either that was a mistake in the code
(those other cases *should* be using it), or it is not logically
necessary to do it for these temporary values in order to get the same
final result, which is all that really matters.

-Duncan