[Python-Dev] PEP 506 secrets module
steve at pearwood.info
Sat Oct 17 05:50:50 EDT 2015
On Sat, Oct 17, 2015 at 03:26:46AM +1100, Steven D'Aprano wrote:
> On Fri, Oct 16, 2015 at 06:35:14PM +0300, Serhiy Storchaka wrote:
> > I suggest to add only randrange(). randint() is historical artefact, we
> > shouldn't repeat this mistake in new module. The secrets module is not
> > good way to generate dice rolls. In most other cases you need to
> > generate integers in half-open interval [0; N).
> > And randbelow() is absolute redundant. Random._randbelow() is
> > implementation detail and I inclined to get rid of it (implementing
> > randrange() in C instead).
> This was discussed on Python-Ideas, and there was little consensus there
> either. (Looks like Tim Peters' prediction is coming true :-)
> I've also raised this issue on the python-list mailing list.
I've had some feedback on python-list. To summarise the various
positions expressed so far:
randbelow only: 3 in favour
randint only: 1 neutral (neither opposed nor in favour)
randrange only: 1 in favour, 1 against
both randrange and randint: 2 in favour
(Total number of comments is more than the total number of posts, as
some people expressed more than one opinion in the same post. As in, "I
prefer X, but Y would be good too".)
So you can see there is nothing even close to consensus as to which API
is best, which is an argument for keeping all three functions.
But significanly, only *one* of the commenters has claimed to have any
significant experience in crypto work, and I will quote him:
Having done quite a bit of serious crypto implementation over the
past 25 years, I don't recall ever wanting anything like randrange,
and if I did need it, I'd probably build it inline from randbelow
rather than force some hapless future code maintainer to look up the
specs on randrange.
My opinion, FWIW: I like randbelow, because in modern crypto one
very frequently works with integers in the range [0,M-1] for some
large modulus M, and there is a constant risk of asking for
something in [0,M] when one meant [0,M-1]. One can eliminate this
risk, as randbelow does, by building in the -1, which normally
introduces a risk of making a mistake that gives you [0,M-2], but
the name "randbelow" seems like a neat fix to that problem.
-- Peter Pearson
This matches what Serhiy suggests: in crypto, one normally only needs to
generate the half-open interval [0...n). It also matches the reason why
Tim Peters added randbelow in the first place.
As the author of the PEP, I'm satisfied by this argument, and will now
state that my preferred option is to drop randint and randrange, and
just keep randbelow.
My second choice is to keep all three functions.
I think it is fair to say that out of the three functions, there is
consensus that randbelow has the most useful functionality in a crypto
context. Otherwise, people seem roughly equally split between the three
functions. There doesn't seem to be any use-case for the three argument
version of randrange.
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