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

Steven D'Aprano steve at pearwood.info
Fri Oct 16 06:04:58 EDT 2015

On Fri, Oct 16, 2015 at 08:57:24AM +0200, Victor Stinner wrote:

Hi,

I like the PEP. IMHO it's a better solution than using a CPRNG for random by default. I suggest to raise an error if tokenbytes(n) if calls with n < 16 bytes (128 bits). Well, I'm not sure that 16 is the good compromise between performance and security, but we must enforce users to use a minimum number of bits of entropy. tokenbytes(1) looks valid, even tokenbytes(0), according to the Python code in the PEP.

Youtube URLs have what looks like about 8 or 9 bytes of randomness:

https://www.youtube.com/watch?v=B3KBuQHHKx0

(assuming it is base64 encoded, 11 chars is about 8 or 9 bytes). I don't think that we should force people to use any specific length, it would be pointless since they can always just slice it:

py> secrets.token_bytes(16)[:2] b'\xd1s'

Unless anyone has a good argument for why we should do this, I would be inclined to allow any value for nbytes. At most, I would be prepared to raise a warning if the nbytes was less than 16, but I don't think an error is appropriate.

Thoughts?

I don't like the idea how having two functions doing almost the same thing: randint() and randrange(). There is a risk that these functions will be misused. I consider that I know some stuff on PRNG but I'm still confused by randint() and randrange(). Usually, I open python and type:

>>> x=[s.randrange(1,6) for n in range(100)] >>> min(x), max(x) (1, 5)

Wouldn't help(randrange) be easier? :-)

Choose a random item from range(start, stop[, step]).

This fixes the problem with randint() which includes the
endpoint; in Python this is usually not what you want.

I always find that comment amusing. While it is true that in slicing, half-open ranges are more useful than closed ranges, but when it comes to generating random numbers (say, simulating dice) I find randint much more useful and intuitive.

But I appreciate that some people think differently.

Hum, ok, it's not a good dice :-) I probably wanted to use randint(). So I suggest to only add randint() to secrets.

On the Python-Ideas list the argument was about equally split between those who wanted only randrange and those who wanted only randint. I think if we don't supply both, we'll be forever fielding feature requests to add in the missing one.

The PEP doesn't explain if secrets uses a "blocking" CPRNG (like /dev/random or getentropy() on Solaris) or a "non-blocking" CRPNG (like /dev/urandom). And it doesn't explain the rationale. Please explain, or I'm sure that the question will arise (ex: I just asked it ;-))

secrets uses os.urandom and random.SystemRandom, which also uses os.urandom (or equivalent under Windows). The implementation section of the PEP shows that.

As for the reason why urandom rather than /dev/random:

http://sockpuppet.org/blog/2014/02/25/safely-generate-random-numbers/

http://www.2uo.de/myths-about-urandom/

I'm not sure if this was explicitly discussed in Python-Ideas, if it was I have forgotten it. I seem to recall that everyone who seemed to know what they were talking about just assumed we'd be using os.urandom and nobody questioned it or argued.

You may also be a little bit more explicit on the CPRNG: it looks like secrets will always use a CRPNG implemented in the kernel. Is it a property of the secrets module, or can it be ssl.RANDbytes() for example? IMHO we must always use a CRPNG implemented in the kernel, there is still an issue with ssl.RANDbytes() and fork() (two child process can produce exactly the same random numbers after a lot of fork()...). I understood that OpenSSL developers doesn't want to fix it.

You may even be very explicit, list CPRNG that will be used on Python 3.6: * Linux: getrandom() syscall if available (Linux 3.17 or newer), or /dev/urandom * Solaris: getrandom() function if available (Solaris 11.3 or newer), or /dev/urandom * OpenBSD: getentropy() function (OpenBSD 5.6 or newer), or /dev/urandom * Windows: CryptAcquireContext(PROVRSAFULL, CRYPTVERIFYCONTEXT) and CryptGenRandom() * Other UNIXes: /dev/urandom

Does anyone else think that the secrets module should promise specific implementations?

-- Steve

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