[Python-ideas] Bitwise operations on bytes class

[Nathaniel McCallum]

On Mon, 2014-06-16 at 21:55 +0200, Stefan Behnel wrote:
> Nathaniel McCallum, 16.06.2014 21:43:
> > Perhaps some code will clarify what I'm proposing. Attached is a class I
> > have found continual reuse for over the last few years. It implements
> > bitwise operators on a bytes subclass. Something similar could be done
> > for bytearray.
> 
> Ok, according to your code, you don't want a SIMD type but rather an
> arbitrary size integer type. Why don't you just use the "int" ("long" in
> Py2) type for that? It has way faster operations than your multiple copy
> implementation.

Of course my attached code is slow. This is precisely why I'm proposing
native additions to the bytes class.

However, in most algorithms, there is a single operation like this on a
block of data which is otherwise not treated as an integer. This
operation often takes the form of something like:

  blocks.append(blocks[-1] ^ block)

In all the surrounding code, you are dealing with bytes *as* bytes.
Converting into alternate types breaks up the readability of the
algorithm. And given the security requirements of such algorithms,
readability is extremely important.

The above code example has both simplicity and obviousness. Currently,
in py3k, this is AFAICS the best alternative for readability:

 blocks.append([a ^ b for a, b in zip(blocks[-1], block)]

While this is infinitely better than Python 2.x, I think my proposal is
still significantly more readable. When implemented natively, my
proposal is also far more performant than this.

Nathaniel