/dev/random

File:Rngtest FIPS-140-2 screenshot.png

The Linux kernel provides the separate device files {{nowrap|{{mono|/dev/random}}}} and {{nowrap|{{mono|/dev/urandom}}}}. Since kernel version 5.6 of 2020, {{nowrap|{{mono|/dev/random}}}} only blocks when the CSPRNG hasn't initialized. Once initialized, {{nowrap|{{mono|/dev/random}}}} and {{nowrap|{{mono|/dev/urandom}}}} behave the same.{{Cite web |title=/dev/random Is More Like /dev/urandom With Linux 5.6 - Phoronix |url=https://www.phoronix.com/scan.php?page=news_item&px=Linux-5.6-Random-Rework |website=www.phoronix.com}}

In October 2016, with the release of Linux kernel version 4.8, the kernel's {{nowrap|{{mono|/dev/urandom}}}} was switched over to a ChaCha20-based cryptographic pseudorandom number generator (CPRNG) implementation{{Cite web |date=2016-07-27 |title=kernel/git/torvalds/linux.git - Linux kernel source tree |url=https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=818e607b57c94ade9824dad63a96c2ea6b21baf3 |access-date=2016-11-23 |publisher=kernel.org}} by Theodore Ts'o, based on Bernstein's well-regarded stream cipher ChaCha20.

Since version 5.17 of the Linux kernel, the random number generator switched from using the SHA-1 cryptographic hash function in the entropy collector to BLAKE2s, a newer, faster and more secure hash function.{{Cite web |title=Linux 5.17 Random Number Generator Seeing Speed-Ups, Switching From SHA1 To BLAKE2s - Phoronix |url=https://www.phoronix.com/news/Linux-5.17-RNG |website=www.phoronix.com}}

Random number generation in kernel space was implemented for the first time for Linux{{cite web

|url = https://randombit.net/bitbashing/posts/syllable_dev_random.html

|title = On Syllable's /dev/random

|first = Jack

|last = Lloyd

|date = 2008-12-09

|access-date = 2019-08-21

}} in 1994 by Theodore Ts'o.{{cite web

|url = http://everything2.com/title/%252Fdev%252Frandom

|title = /dev/random

|date = 2003-06-08

|publisher = Everything2

|access-date = 2013-07-03

|archive-url = https://web.archive.org/web/20091117215406/http://everything2.com/title/%252Fdev%252Frandom

|archive-date = 2009-11-17

|url-status = live

}}

The implementation used secure hashes rather than ciphers,{{clarify|date=February 2017}} to avoid cryptography export restrictions that were in place when the generator was originally designed. The implementation was also designed with the assumption that any given hash or cipher might eventually be found to be weak, and so the design is durable in the face of any such weaknesses. Fast recovery from pool compromise is not considered a requirement, because the requirements for pool compromise are sufficient for much easier and more direct attacks on unrelated parts of the operating system.

In Ts'o's implementation, the generator keeps an estimate of the number of bits of noise in the entropy pool. From this entropy pool random numbers are created. When read, the {{nowrap|{{mono|/dev/random}}}} device will only return random bytes within the estimated number of bits of noise in the entropy pool. When the entropy pool is empty, reads from {{nowrap|{{mono|/dev/random}}}} will block until additional environmental noise is gathered.{{man|4|random|Linux}} The intent is to serve as a cryptographically secure pseudorandom number generator, delivering output with entropy as large as possible. This is suggested by the authors for use in generating cryptographic keys for high-value or long-term protection.

A counterpart to {{nowrap|{{mono|/dev/random}}}} is {{nowrap|{{mono|/dev/urandom}}}} ("unlimited"{{cite web | url=https://repo.or.cz/w/davej-history.git/blob/d0562c8dc:/drivers/char/random.c#l682

| title=/dev/random and /dev/urandom implementation in Linux 1.3.39, function random_read_unlimited

| date=1995-11-04

| access-date=2013-11-21}}/non-blocking random source) which reuses the internal pool to produce more pseudo-random bits. This means that the call will not block, but the output may contain less entropy than the corresponding read from {{nowrap|{{mono|/dev/random}}}}. While {{nowrap|{{mono|/dev/urandom}}}} is still intended as a pseudorandom number generator suitable for most cryptographic purposes, the authors of the corresponding man page note that, theoretically, there may exist an as-yet-unpublished attack on the algorithm used by {{nowrap|{{mono|/dev/urandom}}}}, and that users concerned about such an attack should use {{nowrap|{{mono|/dev/random}}}} instead. However such an attack is unlikely to come into existence, because once the entropy pool is unpredictable it doesn't leak security by a reduced number of bits.{{cite AV media|url=https://media.ccc.de/v/32c3-7441-the_plain_simple_reality_of_entropy#video&t=1262|title=The plain simple reality of entropy|author=Filippo Valsorda|date=2015-12-29}}

It is also possible to write to {{nowrap|{{mono|/dev/random}}}}. This allows any user to mix random data into the pool. Non-random data is harmless, because only a privileged user can issue the ioctl needed to increase the entropy estimate.{{Dubious|date=December 2020|reason=This isn't the reason that writing non-random data is harmless. 1. The non-random data mixes with the random data in a way that keeps only the randomness, like XOR? That would make it harmless? 2. The entropy estimate is not increased when you write to it, according to https://linux.die.net/man/4/urandom, but it DOES affect the output, so that isn't what makes it harmless. 3. Only privileged users can update the entropy estimate, which doesn't make it harmless?}} The current amount of entropy and the size of the Linux kernel entropy pool, both measured in bits, are available in {{mono|/proc/sys/kernel/random/}} and can be displayed by the command {{code|cat /proc/sys/kernel/random/entropy_avail}} and {{code|cat /proc/sys/kernel/random/poolsize}} respectively.

Gutterman, Pinkas, & Reinman in March 2006 published a detailed cryptographic analysis of the Linux random number generator{{cite web |last1=Gutterman |first1=Zvi |last2=Pinkas |first2=Benny |last3=Reinman |first3=Tzachy |date=2006-03-06 |title=Analysis of the Linux Random Number Generator |url=http://www.pinkas.net/PAPERS/gpr06.pdf |url-status=live |archive-url=https://web.archive.org/web/20081003041432/http://www.pinkas.net/PAPERS/gpr06.pdf |archive-date=2008-10-03 |access-date=2013-07-03}} in which they describe several weaknesses. Perhaps the most severe issue they report is with embedded or Live CD systems, such as routers and diskless clients, for which the bootup state is predictable and the available supply of entropy from the environment may be limited. For a system with non-volatile memory, they recommend saving some state from the RNG at shutdown so that it can be included in the RNG state on the next reboot. In the case of a router for which network traffic represents the primary available source of entropy, they note that saving state across reboots "would require potential attackers to either eavesdrop on all network traffic" from when the router is first put into service, or obtain direct access to the router's internal state. This issue, they note, is particularly critical in the case of a wireless router whose network traffic can be captured from a distance, and which may be using the RNG to generate keys for data encryption.

The Linux kernel provides support for several hardware random number generators, should they be installed. The raw output of such a device may be obtained from {{mono|/dev/hwrng}}.{{cite web

| url=http://processors.wiki.ti.com/index.php/Cryptography_Users_Guide

| title=Cryptography Users Guide

| date=2013-06-04

| publisher=Texas Instruments

| access-date=2013-07-03

| archive-date=2018-04-16

| archive-url=https://web.archive.org/web/20180416073524/http://processors.wiki.ti.com/index.php/Cryptography_Users_Guide

| url-status=dead

}}

With Linux kernel 3.16 and newer,{{cite web|url=https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=be4000bc4644d027c519b6361f5ae3bbfc52c347|title=kernel/git/torvalds/linux.git - Linux kernel source tree @ be4000bc4644d027c519b6361f5ae3bbfc52c347 "hwrng: create filler thread"|website=Git.kernel.org|access-date=18 October 2016}} the kernel itself mixes data from hardware random number generators into {{nowrap|{{mono|/dev/random}}}} on a sliding scale based on the definable entropy estimation quality of the HWRNG. This means that no userspace daemon, such as {{mono|rngd}} from {{mono|rng-tools}}, is needed to do that job. With Linux kernel 3.17+, the VirtIO RNG was modified to have a default quality defined above 0,{{cite web|url=https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=34679ec7a0c45da8161507e1f2e1f72749dfd85c|title=kernel/git/torvalds/linux.git - Linux kernel source tree @ 34679ec7a0c45da8161507e1f2e1f72749dfd85c "virtio: rng: add derating factor for use by hwrng core"|website=Git.kernel.org|access-date=18 October 2016}} and as such, is currently the only HWRNG mixed into {{nowrap|{{mono|/dev/random}}}} by default.

The entropy pool can be improved by programs like {{mono|timer_entropyd}}, {{mono|haveged}}, {{mono|randomsound}} etc. With {{mono|rng-tools}}, hardware random number generators like Entropy Key, etc. can write to {{nowrap|{{mono|/dev/random}}}}. The diehard tests programs {{mono|diehard}}, {{mono|dieharder}} and {{mono|ent}} can test these random number generators.{{cite web |url=http://www.vanheusden.com/te/timer_entropyd-0.1.tgz |title=?? |website=Vanheusden.com |access-date=2016-10-23 |archive-url=https://web.archive.org/web/20130921054659/http://www.vanheusden.com/te/timer_entropyd-0.1.tgz |archive-date=2013-09-21 |url-status=dead }}{{cite web|url=https://code.google.com/p/dieharder/|title=Google Code Archive for dieharder|website=Code.google.com|access-date=18 October 2016}}{{cite web|url=http://stat.fsu.edu/pub/diehard/|title=The Marsaglia Random Number CDROM including the Diehard Battery of Tests of Randomness|website=Stat.fsu.edu|access-date=2016-10-23|archive-url=https://web.archive.org/web/20160125103112/http://stat.fsu.edu/pub/diehard/|archive-date=2016-01-25|url-status=dead}}{{cite web|url=https://www.gnu.org/software/hurd/user/tlecarrour/rng-tools.html|title=rng-tools|website=Gnu.org|access-date=2016-10-23}}

In January 2014, Daniel J. Bernstein published a critique{{cite web |author=Daniel J. Bernstein |author-link=Daniel J. Bernstein |date=2014-02-05 |title=cr.yp.to: 2014.02.05: Entropy Attacks! |url=https://blog.cr.yp.to/20140205-entropy.html |quote=Is there any serious argument that adding new entropy all the time is a good thing? The Linux /dev/urandom manual page claims that without new entropy the user is "theoretically vulnerable to a cryptographic attack", but (as I've mentioned in various venues) this is a ludicrous argument}} of how Linux mixes different sources of entropy. He outlines an attack in which one source of entropy capable of monitoring the other sources of entropy could modify its output to nullify the randomness of the other sources of entropy. Consider the function {{tmath|H(x,y,z)}} where H is a hash function and x, y, and z are sources of entropy with z being the output of a CPU-based malicious HRNG Z:

1. Z generates a random value of r.
2. Z computes {{tmath|H(x,y,r)}}.
3. If the output of {{tmath|H(x,y,r)}} is equal to the desired value, output r as z.
4. Else, repeat starting at 1.

Bernstein estimated that an attacker would need to repeat {{tmath|H(x,y,r)}} 16 times to compromise DSA and ECDSA, by causing the first four bits of the RNG output to be 0. This is possible because Linux reseeds H on an ongoing basis instead of using a single high quality seed.

Bernstein also argues that entropy injection is pointless once the CSPRNG has been initialized.

In kernel 5.17 (backported to kernel 5.10.119), Jason A. Donenfeld offered a new design of the Linux entropy pool infrastructure. Donenfeld reported that the old pool, consisting of a single 4096-bit LFSR is vulnerable to two attacks: (1) an attacker can undo the effect of a known input; (2) if the whole pool's state is leaked, an attacker can set all bits in the pool to zero. His new design, which is faster and safer, uses the blake2s hash function for mixing a 256-bit pool.{{cite web |title=[PATCH 5.15 038/145] random: use computational hash for entropy extraction|url=https://lore.kernel.org/lkml/20220527084855.501642285@linuxfoundation.org/|website=lore.kernel.org}}

The FreeBSD operating system provides a {{nowrap|{{mono|/dev/urandom}}}} link to {{nowrap|{{mono|/dev/random}}}}. Both block only until properly seeded. FreeBSD's PRNG (Fortuna) reseeds regularly, and does not attempt to estimate entropy. On a system with small amount of network and disk activity, reseeding is done after a fraction of a second.{{man|4|random|FreeBSD}}

DragonFly BSD inherited FreeBSD's random device files when it was forked.{{Cite web |title=random(4) |url=https://man.dragonflybsd.org/?command=random§ion=4 |access-date=2024-06-15 |website=DragonFly On-Line Manual Pages}}{{Non-primary source needed|date=June 2024}}{{Cite web |title=A comparison of /dev/random speed on Linux and BSD |url=https://ianix.com/pub/comparing-dev-random-speed-linux-bsd.html |access-date=2024-06-15 |website=ianix.com}}

Since OpenBSD 5.1 (May 1, 2012) {{nowrap|{{mono|/dev/random}}}} and {{mono|/dev/arandom}} uses arc4random, a CSPRNG function based on RC4. The function was changed to use the stronger ChaCha20 with OpenBSD 5.5 (May 1, 2014). The system automatically uses hardware random number generators (such as those provided on some Intel PCI hubs) if they are available, through the OpenBSD Cryptographic Framework.{{man|4|random|OpenBSD}}{{cite web

|url=http://bxr.su/OpenBSD/lib/libc/crypt/arc4random.c

|title=libc/crypt/arc4random.c

|website=BSD Cross Reference, OpenBSD src/lib/

|editor=deraadt |editor-link=Theo de Raadt |date=2014-07-21 |access-date=2015-01-13

|quote=ChaCha based random number generator for OpenBSD.}} {{nowrap|{{mono|/dev/arandom}}}} was removed in OpenBSD 6.3 (April 15, 2018).{{cite web

|url=https://github.com/openbsd/src/commit/0d60993c06101c0a9e6fb6c3a3302133f1b65f98

|title=src/etc/MAKEDEV.common

|website=GitHub OpenBSD source code mirror src/etc/

|editor=naddy |date=2017-11-14 |access-date=2017-11-14

|quote=/dev/arandom removed from OpenBSD.}}

NetBSD's implementation of the legacy {{code|arc4random()}} API has been switched over to ChaCha20 as well.{{cite web |url=http://bxr.su/NetBSD/lib/libc/gen/arc4random.c |title=libc/gen/arc4random.c |website=BSD Cross Reference, NetBSD src/lib/ |editor=riastradh |date=2014-11-16 |access-date=2015-01-13 |quote=Legacy arc4random(3) API from OpenBSD reimplemented using the ChaCha20 PRF, with per-thread state.}}

All Apple OSes have moved to Fortuna since at least December 2019, possibly earlier.{{cite web|url=https://support.apple.com/en-ie/guide/security/seca0c73a75b/web|title=Apple Platform Security|publisher=Apple Inc.}} It is based on SHA-256. Multiple entropy sources such as the secure enclave RNG, boot phase timing jitter, hardware interrupt (timing assumed) are used. RDSEED/RDRAND is used on Intel-based Macs that support it. Seed (entropy) data is also stored for subsequent reboots.

Prior to the change, macOS and iOS used 160-bit Yarrow based on SHA-1.{{cite web|url=https://opensource.apple.com/source/xnu/xnu-1456.1.26/bsd/dev/random/|title=xnu-1456.1.26/bsd/dev/random|publisher=Apple Inc.|access-date=18 October 2016}}

There is no difference between {{nowrap|{{mono|/dev/random}}}} and {{nowrap|{{mono|/dev/urandom}}}}; both behave identically.{{man|4|random|Darwin}}{{cite web|url=https://www.apple.com/ipad/business/docs/iOS_Security_Oct12.pdf|title=iOS Security|date=October 2012|publisher=Apple Inc.|access-date=May 27, 2015|archive-url=https://web.archive.org/web/20140405001141/https://www.apple.com/ipad/business/docs/iOS_Security_Oct12.pdf|archive-date=April 5, 2014|url-status=dead}}

{{nowrap|{{mono|/dev/random}}}} and {{nowrap|{{mono|/dev/urandom}}}} are also available on Solaris,{{cite web

| url=https://blogs.oracle.com/solaris/post/solaris-random-number-generation

| title=Solaris Random Number Generation

| first=Darren

| last=Moffat

| work=Oracle Solaris Blog

| date=2013-09-12

| access-date=2022-04-30

}}

NetBSD,{{man|4|rnd|NetBSD}}

Tru64 UNIX 5.1B,{{cite web

| url=http://h30097.www3.hp.com/docs/base_doc/DOCUMENTATION/V51B_HTML/MAN/MAN4/0199____.HTM

| title=random(4)

| date=1999-09-19

| access-date=2013-07-03}} AIX 5.2{{cite web

|url = http://publib.boulder.ibm.com/infocenter/pseries/v5r3/index.jsp?topic=/com.ibm.aix.files/doc/aixfiles/random.htm

|title = random and urandom Devices

|work = pSeries and AIX Information Center

|date = 2010-03-15

|access-date = 2013-07-03

|archive-url = https://web.archive.org/web/20210303075907/http://publib.boulder.ibm.com/infocenter/pseries/v5r3/index.jsp?topic=%2Fcom.ibm.aix.files%2Fdoc%2Faixfiles%2Frandom.htm

|archive-date = 2021-03-03

|url-status = dead

}} and HP-UX 11i v2.{{cite web

| url=http://software.hp.com/portal/swdepot/displayProductInfo.do?productNumber=KRNG11I

| title=HP-UX Strong Random Number Generator

| date=2004-07-23

| access-date=2013-07-03}} As with FreeBSD, AIX implements its own Yarrow-based design, however AIX uses considerably fewer entropy sources than the standard {{nowrap|{{mono|/dev/random}}}} implementation and stops refilling the pool when it thinks it contains enough entropy.{{cite web

| url=http://lists.gnupg.org/pipermail/gnupg-devel/2003-April/019954.html

| title=AIX 5.2 /dev/random and /dev/urandom devices

| first=Iain | last=Roberts

| date=2003-04-25

| publisher=Lists.gnupg.org

| access-date=2013-07-03

| archive-url=https://web.archive.org/web/20120222144110/http://lists.gnupg.org/pipermail/gnupg-devel/2003-April/019954.html

| archive-date=2012-02-22

| url-status=live}}

In Windows NT, similar functionality is delivered by {{nowrap|{{mono|ksecdd.sys}}}}, but reading the special file {{nowrap|{{mono|\Device\KsecDD}}}} does not work as in UNIX. The documented methods to generate cryptographically random bytes are CryptGenRandom and RtlGenRandom. Windows PowerShell provides access to a cryptographically secure pseudorandom number generator via the {{mono|Get-SecureRandom}} cmdlet.{{Cite web |last= |title=Get-SecureRandom (Microsoft.PowerShell.Utility) - PowerShell |url=https://learn.microsoft.com/en-us/powershell/module/microsoft.powershell.utility/get-securerandom?view=powershell-7.4 |access-date=2024-06-16 |website=learn.microsoft.com |language=en-us}}

Cygwin on Windows provides implementations of both {{nowrap|{{mono|/dev/random}}}} and {{nowrap|{{mono|/dev/urandom}}}}, which can be used in scripts and programs.{{Cite web|url=https://www.linuxquestions.org/questions/general-10/how-does-cygwin's-dev-random-and-urandom-work-903054/#post4493834|title=How does Cygwin's /dev/random and urandom work?|website=www.linuxquestions.org|language=en|access-date=2018-03-09}}

• CryptGenRandom – The Microsoft Windows API's CSPRNG
• {{mono|:/dev}}
• Entropy-supplying system calls
• Fortuna algorithm
• Hardware random number generator
• Standard streams

{{Reflist|30em}}

• {{cite web

|url = https://github.com/thomasbiege/papers/raw/master/random-device-analysis.pdf

|title = Analysis of a strong Pseudo Random Number Generator by anatomizing Linux' Random Number Device

|first = Thomas

|last = Biege

|website = GitHub

|date = 2006-11-06

}}

• {{cite web

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

| title=Myths about /dev/urandom

| year=2014

| first=Thomas

| last=Hühn

}} – describes the Linux 4.8 /dev/random infrastructure and the futility of counting "spent" entropy. Includes quotes from cryptographers. (Counting of "spent" entropy is removed in kernel 5.17.)

{{DEFAULTSORT:Dev Random}}

Category:Unix file system technology

Category:Device file

Category:Random number generation