[Python-Dev] PEP 506 secrets module (original) (raw)

Steven D'Aprano 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.

-- Steve

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