[also adding in other people CCed on bug 71211] On 07/27/2015 06:34 PM, Laurent Georget wrote: > > -----BEGIN PGP SIGNED MESSAGE----- > Hash: SHA256 > > Hello, > > the text of this man page has been the subject of endless controversies > for ages. CSPRNGs are confusing and the page unfortunately adds a > little to the confusion. The newer getrandom(2) page is better, and I > (personally, this doesn't engage Michael nor any other author) think > that the random(4) page should be rewritten in the same spirit. > (getrandom is a system call used to get a random number, by default, > it's more or less equivalent than reading from /dev/urandom if you call > it without flags and for less than 256 bytes). > > Compare this in random(4) > >> The kernel random-number generator is designed to produce a small >> amount of high-quality seed material to seed a cryptographic pseudo- >> random number generator (CPRNG). It is designed for security, not >> speed, and is poorly suited to generating large amounts of random >> data. Users should be very economical in the amount of seed material >> that they read from /dev/urandom (and /dev/random); unnecessarily >> reading large quantities of data from this device will have a >> negative impact on other users of the device. > with this in getrandom(2) > >> *getrandom*() relies on entropy gathered from device drivers and other >> sources of environmental noise. Unnecessarily reading large >> quantities of data will have a negative impact on other users of the >> //dev/random/ and //dev/urandom/ devices. Therefore, *getrandom*() should >> not be used for Monte Carlo simulations or other programs/algorithms >> which are doing probabilistic sampling. > This says exactly the same thing, but getrandom(2) is less misleading. > First note that the man page for random says that /dev/urandom is > "poorly suited to generating large amounts of random data", not > "poorly suited to generating large amounts of *cryptographic* random data". > Basically, you can use /dev/urandom for all cryptographic purposes, > because you never need so many bits at once when doing cryptography. > Even generating several 16-bytes (128-bits) UIDs per minute if you need > them to be cryptographically secure (you may want to think about this > requirement, is it strict?) is not that much. A Monte-Carlo simulation > requires, say (I don't know exactly let's make a rough guess) around > several MB of random numbers per minute. That's 4 or 5 orders of > magnitude higher than your UIDs use case. This would be a wrong > usage of /dev/urandom for two reasons: > - - it would be awfully slow > - - you don't need cryptographically secure random numbers, so there's > no need to deplete the entropy pool. > Next, compare this in random(4): > >> If you are unsure about whether you should use /dev/random or >> /dev/urandom, then probably you want to use the latter. As a general >> rule, /dev/urandom should be used for everything except long-lived >> GPG/SSL/SSH keys. > with this in getrandom(2): > >> Unless you are doing long-term key generation (and perhaps not even >> then), you probably shouldn't be using GRND_RANDOM. The cryptographic >> algorithms used for /dev/urandom are quite conservative, and so should >> be sufficient for all purposes. The disadvantage of GRND_RANDOM is >> that it can block. Furthermore, dealing with the partially fulfilled >> getrandom() requests that can occur when using GRND_RANDOM increases >> code complexity. > Again, the two man pages say the same. getrandom(2) is more nuanced, > though. > > To answer your question about how much you can ask /dev/urandom for : > > in random(4) : > >> if any program reads more than 256 bits (32 bytes) from the kernel random >> pool per invocation, or per reasonable reseed interval (not less than >> one minute), that should be taken as a sign that its cryptography is >> not skillfully implemented. > In getrandom(2): > >> Calling getrandom() to read /dev/urandom for small values (<= 256 bytes) >> of buflen is the preferred mode of usage. > Furthermore, it's impossible to read more than 32MB from /dev/urandom > per invocation. > > So, actually, the random(4) page is very conservative about the reading > limit. > > My final conclusion: as long as you use /dev/urandom for cryptographic > purposes only, you should be ok, because you will never need *a lot* of > random data anyway in any sensible program. For non-cryptographic usage, > seed a user-space PRNG with a few bytes from /dev/urandom and you will > be good. > > Laurent > > > > Le 26/07/2015 22:20, Carl Winbäck a écrit : >> Hello Michael & Co, >> >> I would like to bring your attention to bug report 71211, ”random(4): >> clarify utility and volume”. >> >> https://bugzilla.kernel.org/show_bug.cgi?id=71211 >> >> This report was filed over a year ago but still hasn’t received any response. >> >> Michael, do you have the time to take a look? >> >> Perhaps you, or someone else on the linux-man list, are familiar with >> CSPRNGs and have some ideas on how to reword this man page? >> >> Unfortunately I’m not at all an expert in this area, so I’m afraid I >> don’t have any specific suggestions regarding how to change this man >> page. But I still think it would be helpful to the Linux community if >> it could be improved, and as a result, hopefully cause less confusion >> regarding getting random numbers on Linux. >> >> >> Looking forward to hear your thoughts on this. >> >> Best regards, >> Carl Winbäck >> -- >> To unsubscribe from this list: send the line "unsubscribe linux-man" in >> the body of a message to majordomo@xxxxxxxxxxxxxxx >> More majordomo info at http://vger.kernel.org/majordomo-info.html > > -----BEGIN PGP SIGNATURE----- > Version: GnuPG v2 > > iF4EAREIAAYFAlW2XawACgkQRTidSplJch4jJQD/d4LOrShlXmQx5RClVCdj7sKL > 6n7SQhlCIjfqvi86JDcA/28cCtYZ8HKL1RgWkgEjGmWoIH6ZA+AKJjgnmugk1wFj > =ff9U > -----END PGP SIGNATURE----- > > -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/ -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
