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RND(4) Device Drivers Manual RND(4)
NAME
rnd - random number generator
DESCRIPTION
The /dev/random and /dev/urandom devices generate bytes randomly with
uniform distribution. Every read from them is independent.
/dev/urandom Never blocks.
/dev/random Sometimes blocks. Will block early at boot if the system's
state is known to be predictable.
Applications should read from /dev/urandom, or the sysctl(7) variable
kern.arandom, when they need randomly generated data, e.g. key material
for cryptography or seeds for simulations. (The sysctl(7) variable
kern.arandom is limited to 256 bytes per read, but is otherwise
equivalent to reading from /dev/urandom and always works even in a
chroot(8) environment without requiring a populated /dev tree and without
opening a file descriptor, so kern.arandom may be preferable to use in
libraries.)
Systems should be engineered to judiciously read at least once from
/dev/random at boot before running any services that talk to the internet
or otherwise require cryptography, in order to avoid generating keys
predictably. /dev/random may block at any time, so programs that read
from it must be prepared to handle blocking. Interactive programs that
block due to reads from /dev/random can be especially frustrating.
If interrupted by a signal, reads from either /dev/random or /dev/urandom
may return short, so programs that handle signals must be prepared to
retry reads.
Writing to either /dev/random or /dev/urandom influences subsequent
output of both devices, guaranteed to take effect at next open. If you
have a coin in your pocket, you can flip it 256 times and feed the
outputs to /dev/random to guarantee your system is in a state that nobody
but you and the bored security guard watching the surveillance camera in
your office can guess:
% echo tthhhhhthhhththtthhhhthtththttth... > /dev/random
(Sequence generated from a genuine US quarter dollar, guaranteed random.)
SECURITY MODEL
The rnd subsystem provides the following security properties against two
different classes of attackers, provided that there is enough entropy
from entropy sources not seen by attackers:
⊕ An attacker who has seen some outputs and can supply some entropy
sources' inputs to the operating system cannot predict past or
future unseen outputs.
⊕ An attacker who has seen the entire state of the machine cannot
predict past outputs.
One `output' means a single read, no matter how short it is.
`Cannot predict' means it is conjectured of the cryptography in
/dev/random that any computationally bounded attacker who tries to
distinguish outputs from uniform random cannot do more than negligibly
better than uniform random guessing.
ENTROPY
The operating system continuously makes observations of hardware devices,
such as network packet timings, disk seek delays, and keystrokes. The
observations are combined into a seed for a cryptographic pseudorandom
number generator (PRNG) which is used to generate the outputs of both
/dev/random and /dev/urandom.
An attacker may be able to guess with nonnegligible chance of success
what your last keystroke was, but guessing every observation the
operating system may have made is more difficult. The difficulty of the
best strategy at guessing a random variable is analyzed as the -log_2 of
the highest probability of any outcome, measured in bits, and called its
min-entropy, or entropy for short in cryptography. For example:
⊕ A fair coin toss has one bit of entropy.
⊕ A fair (six-sided) die roll has a little over 2.5 bits of
entropy.
⊕ A string of two independent fair coin tosses has two bits of
entropy.
⊕ The toss of a pair of fair coins that are glued together has one
bit of entropy.
⊕ A uniform random distribution with n possibilities has log_2 n
bits of entropy.
⊕ An utterance from an accounting troll who always says `nine' has
zero bits of entropy.
Note that entropy is a property of an observable physical process, like a
coin toss, or of a state of knowledge about that physical process; it is
not a property of a specific sample obtained by observing it, like the
string `tthhhhht'. There are also kinds of entropy in information theory
other than min-entropy, including the more well-known Shannon entropy,
but they are not relevant here.
Hardware devices that the operating system monitors for observations are
called entropy sources, and the observations are combined into an entropy
pool. The rndctl(8) command queries information about entropy sources
and the entropy pool, and can control which entropy sources the operating
system uses or ignores.
256 bits of entropy is typically considered intractable to guess with
classical computers and with current models of the capabilities of
quantum computers.
Systems with nonvolatile storage should store a secret from /dev/urandom
on disk during installation or shutdown, and feed it back during boot, so
that the work the operating system has done to gather entropy --
including the work its operator may have done to flip a coin! -- can be
saved from one boot to the next, and so that newly installed systems are
not vulnerable to generating cryptographic keys predictably.
The boot loaders in some NetBSD ports support a command to load a seed
from disk before the kernel has started. For those that don't, the
rndctl(8) command can do it once userland has started, for example by
setting "random_seed=YES" in /etc/rc.conf, which is enabled by default;
see rc.conf(5).
LIMITATIONS
Some people worry about recovery from state compromise -- that is,
ensuring that even if an attacker sees the entire state of the operating
system, then the attacker will be unable to predict any new future
outputs as long as the operating system gathers fresh entropy quickly
enough.
But if an attacker has seen the entire state of your machine, refreshing
entropy is probably the least of your worries, so we do not address that
threat model here.
The rnd subsystem does not automatically defend against hardware
colluding with an attacker to influence entropy sources based on the
state of the operating system.
For example, a PCI device or CPU instruction for random number generation
which has no side channel to an attacker other than the /dev/urandom
device could be bugged to observe all other entropy sources, and to
carefully craft `observations' that cause a certain number of bits of
/dev/urandom output to be ciphertext that either is predictable to an
attacker or conveys a message to an attacker.
No amount of scrutiny by the system's operator could detect this. The
only way to prevent this attack would be for the operator to disable all
entropy sources that may be colluding with an attacker. If you're not
sure which ones are not, you can always disable all of them and fall back
to the coin in your pocket.
IOCTLS
The /dev/random and /dev/urandom devices support a number of ioctls,
defined in the <sys/rndio.h> header file, for querying and controlling
the entropy pool.
Since timing between hardware events contributes to the entropy pool,
statistics about the entropy pool over time may serve as a side channel
for the state of the pool, so access to such statistics is restricted to
the super-user and should be used with caution.
Several ioctls are concerned with particular entropy sources, described
by the following structure:
typedef struct {
char name[16]; /* symbolic name */
uint32_t total; /* estimate of entropy provided */
uint32_t type; /* RND_TYPE_* value */
uint32_t flags; /* RND_FLAG_* mask */
} rndsource_t;
#define RND_TYPE_UNKNOWN
#define RND_TYPE_DISK /* disk device */
#define RND_TYPE_ENV /* environment sensor (temp, fan, &c.) */
#define RND_TYPE_NET /* network device */
#define RND_TYPE_POWER /* power events */
#define RND_TYPE_RNG /* hardware RNG */
#define RND_TYPE_SKEW /* clock skew */
#define RND_TYPE_TAPE /* tape drive */
#define RND_TYPE_TTY /* tty device */
#define RND_TYPE_VM /* virtual memory faults */
#define RND_TYPE_MAX /* value of highest-numbered type */
#define RND_FLAG_COLLECT_TIME /* use timings of samples */
#define RND_FLAG_COLLECT_VALUE /* use values of samples */
#define RND_FLAG_ESTIMATE_TIME /* estimate entropy of timings */
#define RND_FLAG_ESTIMATE_VALUE /* estimate entropy of values */
#define RND_FLAG_NO_COLLECT /* ignore samples from this */
#define RND_FLAG_NO_ESTIMATE /* do not estimate entropy */
The following ioctls are supported:
RNDGETENTCNT (uint32_t)
Return the number of bits of entropy the system is estimated to
have.
RNDGETSRCNUM (rndstat_t)
typedef struct {
uint32_t start;
uint32_t count;
rndsource_t source[RND_MAXSTATCOUNT];
} rndstat_t;
Fill the sources array with information about up to count entropy
sources, starting at start. The actual number of sources described
is returned in count. At most RND_MAXSTATCOUNT sources may be
requested at once.
RNDGETSRCNAME (rndstat_name_t)
typedef struct {
char name[16];
rndsource_t source;
} rndstat_name_t;
Fill source with information about the entropy source named name,
or fail with ENOENT if there is none.
RNDCTL (rndctl_t)
typedef struct {
char name[16];
uint32_t type;
uint32_t flags;
uint32_t mask;
} rndctl_t;
For each entropy source of the type type, or if type is 0xff then
for the entropy source named name, replace the flags in mask by
flags.
RNDADDDATA (rnddata_t)
typedef struct {
uint32_t len;
uint32_t entropy;
unsigned char data[RND_SAVEWORDS * sizeof(uint32_t)];
} rnddata_t;
Feed len bytes of data to the entropy pool. The sample is expected
to have been drawn with at least entropy bits of entropy.
This ioctl can be used only once per boot. It is intended for a
system that saves entropy to disk on shutdown and restores it on
boot, so that the system can immediately be unpredictable without
having to wait to gather entropy.
RNDGETPOOLSTAT (rndpoolstat_t)
typedef struct {
uint32_t poolsize; /* size of each LFSR in pool */
uint32_t threshold; /* no. bytes of pool hash returned */
uint32_t maxentropy; /* total size of pool in bits */
uint32_t added; /* no. bits of entropy ever added */
uint32_t curentropy; /* current entropy `balance' */
uint32_t discarded; /* no. bits dropped when pool full */
uint32_t generated; /* no. bits yielded by pool while
curentropy is zero */
} rndpoolstat_t;
Return various statistics about entropy.
SYSCTLS
The following sysctl(8) variables provided by rnd can be set by
privileged users:
kern.entropy.collection (bool)
(Default on.) Enables entering data into the entropy pool. If
disabled, no new data can be entered into the entropy pool, whether
by device drivers, by writes to /dev/random or /dev/urandom, or by
the RNDADDDATA ioctl.
kern.entropy.depletion (bool)
(Default off.) Enables `entropy depletion', meaning that even
after attaining full entropy, the kernel subtracts the number of
bits read out of the entropy pool from its estimate of the system
entropy. This is not justified by modern cryptography -- an
adversary will never guess the 256-bit secret in a Keccak sponge no
matter how much output from the sponge they see -- but may be
useful for testing.
kern.entropy.consolidate (int)
Trigger for entropy consolidation: executing
# sysctl -w kern.entropy.consolidate=1
causes the system to consolidate pending entropy from per-CPU pools
into the global pool, and waits until done.
The following read-only sysctl(8) variables provide information to
privileged users about the state of the entropy pool:
kern.entropy.needed (unsigned int)
Number of bits of entropy the system is waiting for in the global
pool before reads from /dev/random will return without blocking.
When zero, the system is considered to have full entropy.
kern.entropy.pending (unsigned int)
Number of bits of entropy pending in per-CPU pools. This is the
amount of entropy that will be contributed to the global pool at
the next consolidation, such as from triggering
kern.entropy.consolidate.
kern.entropy.epoch (unsigned int)
Number of times system has reached full entropy, or entropy has
been consolidated with kern.entropy.consolidate, as an unsigned
32-bit integer. Consulted inside the kernel by subsystems such as
cprng(9) to decide whether to reseed. Initially set to 2^32 - 1
(i.e., (unsigned)-1) meaning the system has never reached full
entropy and the entropy has never been consolidated; never again
set to 2^32 - 1. Never zero, so applications can initialize a
cache of the epoch to zero to ensure they reseed the next time they
check whether it is different from the stored epoch.
IMPLEMENTATION NOTES
(This section describes the current implementation of the rnd subsystem
at the time of writing. It may be out-of-date by the time you read it,
and nothing in here should be construed as a guarantee about the
behaviour of the /dev/random and /dev/urandom devices.)
Device drivers gather samples from entropy sources and absorb them into a
collection of per-CPU Keccak sponges called `entropy pools' using the
rnd(9) kernel API. The device driver furnishes an estimate for the
entropy of the sampling process, under the assumption that each sample is
independent. When the estimate of entropy pending among the per-CPU
entropy pools reaches a threshold of 256 bits, the entropy is drawn from
the per-CPU pools and consolidated into a global pool. Keys for
/dev/random, /dev/urandom, kern.arandom, and the in-kernel cprng(9)
subsystem are extracted from the global pool.
Early after boot, before CPUs have been detected, device drivers instead
enter directly into the global pool. If anything in the system extracts
data from the pool before the threshold has been reached at least once,
the system will print a warning to the console and reset the entropy
estimate to zero. The reason for resetting the entropy estimate to zero
in this case is that an adversary who can witness output from the pool
with partial entropy -- say, 32 bits -- can undergo a feasible brute
force search to ascertain the complete state of the pool; as such, the
entropy of the adversary's state of knowledge about the pool is zero.
If the operator is confident that the drivers' estimates of the entropy
of the sampling processes are too conservative, the operator can issue
# sysctl -w kern.entropy.consolidate=1
to force consolidation into the global pool. The operator can also fool
the system into thinking it has more entropy than it does by feeding data
from /dev/urandom into /dev/random, but this voids the security model and
should be limited to testing purposes.
Short reads from /dev/urandom are served by a persistent per-CPU
Hash_DRBG instance that is reseeded from the entropy pool after any
entropy consolidation. Reads from /dev/random and long reads from
/dev/urandom are served by a temporary Hash_DRBG seeded from the entropy
pool on each read.
When `entropy depletion' is enabled by setting the sysctl variable
kern.entropy.depletion=1, every read from /dev/random is limited to 256
bits, since reading more than that would nearly always block again.
FILES
/dev/random Uniform random byte source. May block.
/dev/urandom Uniform random byte source. Never blocks.
DIAGNOSTICS
The rnd subsystem may print the following warnings to the console likely
indicating security issues:
WARNING: system needs entropy for security; see entropy(7) A
process tried to draw from the entropy pool before enough inputs
from reliable entropy sources have been entered.
The entropy may be low enough that an adversary who sees the output
could guess the state of the pool by brute force, so in this event
the system resets its estimate of entropy to none.
This message is rate-limited to happen no more often than once per
minute, so if you want to make sure it is gone you should consult
kern.entropy.needed to confirm it is zero.
The rnd subsystem may print any of various messages about obtaining an
entropy seed from the bootloader to diagnose saving and loading seeds on
disk:
entropy: entering seed from bootloader with N bits of entropy The
bootloader provided an entropy seed to the kernel, which recorded
an estimate of N bits of entropy in the process that generated it.
entropy: no seed from bootloader The bootloader did not provide an
entropy seed to the kernel before starting the kernel. This does
not necessarily indicate a problem; not all bootloaders support the
option, and the rc.conf(5) setting random_seed=YES can serve
instead.
entropy: invalid seed length N, expected sizeof(rndsave_t) = M The
bootloader provided an entropy seed of the wrong size to the
kernel. This may indicate a bug in rndctl(8). The seed will be
ignored.
entropy: invalid seed checksum The entropy seed provided by the
bootloader was malformed. The seed will be entered into the
entropy pool, but it will be considered to contribute no entropy.
entropy: double-seeded by bootloader A buggy bootloader tried to
provide an entropy seed more than once to the kernel. Subsequent
seeds will be entered into the entropy pool, but they will be
considered to contribute no entropy.
entropy: best effort The system has gathered enough samples from
interrupt timings and other non-confident sources of entropy for
the first time to unblock /dev/random, but it may not have full
entropy from a seed or hardware random number generator.
entropy: ready The system has full entropy for the first time.
SEE ALSO
arc4random(3), entropy(7), rndctl(8), cprng(9), rnd(9)
Elaine Barker and John Kelsey, Recommendation for Random Number
Generation Using Deterministic Random Bit Generators, National Institute
of Standards and Technology,
https://csrc.nist.gov/publications/detail/sp/800-90a/rev-1/final, United
States Department of Commerce, June 2015, NIST Special Publication
800-90A, Revision 1.
Meltem S"onmez Turan, Elaine Barker, John Kelsey, Kerry A. McKay, Mary L.
Baish, and Mike Boyle, Recommendations for the Entropy Sources Used for
Random Bit Generation, National Institute of Standards and Technology,
https://csrc.nist.gov/publications/detail/sp/800-90b/final, United States
Department of Commerce, January 2018, NIST Special Publication 800-90B.
Daniel J. Bernstein, Entropy Attacks!,
http://blog.cr.yp.to/20140205-entropy.html, 2014-02-05.
Nadia Heninger, Zakir Durumeric, Eric Wustrow, and J. Alex Halderman,
"Mining Your Ps and Qs: Detection of Widespread Weak Keys in Network
Devices", Proceedings of the 21st USENIX Security Symposium, USENIX,
https://www.usenix.org/conference/usenixsecurity12/technical-sessions/presentation/heninger,
https://factorable.net/, 205-220, August 2012.
Edwin T. Jaynes, Probability Theory: The Logic of Science, Cambridge
University Press, https://bayes.wustl.edu/, 2003.
HISTORY
The /dev/random and /dev/urandom devices first appeared in NetBSD 1.3.
AUTHORS
The rnd subsystem was first implemented by Michael Graff
<explorer@flame.org>, was then largely rewritten by Thor Lancelot Simon
<tls@NetBSD.org>, and was most recently largely rewritten by Taylor R.
Campbell <riastradh@NetBSD.org>.
BUGS
Many people are confused about what /dev/random and /dev/urandom mean.
Unfortunately, no amount of software engineering can fix that.
NetBSD 10.99 August 7, 2023 NetBSD 10.99