- NetBSD Manual Pages
ENTROPY(7) NetBSD Miscellaneous Information Manual ENTROPY(7)
Powered by man-cgi (2021-06-01).
Maintained for NetBSD
by Kimmo Suominen.
Based on man-cgi by Panagiotis Christias.
entropy -- random unpredictable secrets needed for security
Computers need random unpredictable secrets for the security of software
such as web browsers and ssh(1).
Computers are designed to behave in highly predictable ways, so they rely
on observations of random physical phenomena around them, called entropy
sources, to derive unpredictable secrets for cryptography.
While some computers have reliable entropy sources such as hardware ran-
dom number generators based on thermal noise in silicon circuits, others
may require operator intervention for security.
· Web browsers and programs such as ssh(1) rely on unpredictable
secrets in cryptography to prevent eavesdropping and detect tampering
of sessions over the network.
· ssh-keygen(1) relies on unpredictable secrets to create keys that
allow you to log in but keep out malicious adversaries; if an adver-
sary could guess the key then they could impersonate you.
· NetBSD relies on unpredictable secrets to make sure that private user
data stored on nonvolatile media when memory is scarce (swapctl(8),
using `vm.swap_encrypt=1'; see sysctl(7)) cannot be recovered by
forensic tools after shutdown.
Entropy in NetBSD
NetBSD gathers samples from various kinds of entropy sources, including:
· hardware random number generators
· network traffic timing
· user input (keystrokes, mouse movements, etc.)
· disk I/O latency
· environment sensors (envsys(4))
The samples are mixed together with cryptography to yield unpredictable
secrets through /dev/urandom (see rnd(4)) and related interfaces used by
programs like ssh(1), Firefox, and so on.
NetBSD also stores a random seed at /var/db/entropy-file to carry unpre-
dictable secrets over from one boot to the next, as long as the medium
remains secret and can be updated on boot. The seed is maintained auto-
matically by /etc/rc.d/random_seed (see rc.conf(5)).
Ensuring enough entropy
Entropy is measured in bits, and only 256 bits of entropy are needed for
security, thanks to modern cryptography.
To detect potentially insecure systems, NetBSD takes measures to alert
the operator if there isn't definitely enough for security:
· NetBSD issues warnings on the console if there's not enough entropy
when programs need it; see rnd(4).
· The motd(5) has a warning if there was not enough entropy when net-
work daemons such as sshd(8) first generated keys.
· The daily security report includes an alert if there's still not
enough entropy; see security.conf(5).
Since it is hard to know how unpredictable most physical systems are,
only devices specifically designed to be hardware random number genera-
tors, or a seed file stored on disk, count toward these alerts.
At boot, NetBSD will wait, when `entropy=wait' is set in rc.conf(5), or
fail to single-user mode, when `entropy=check' is set, if there is not
enough entropy from any sources, including devices not designed to be
unpredictable, such as the CPU cycle counter sampled by a periodic timer,
provided the samples pass a simple filter called the `entropy estimator',
like other operating systems. Sources known to be predictable, which
could give a false sense of security, can be disabled from unblocking
boot by setting rndctl_flags in rc.conf(5).
Many new computers have hardware random number generators, such as
RDRAND/RDSEED in Intel/AMD CPUs, or ARMv8.5-RNDRRS; virtio(4)-based vir-
tualization platforms such as QEMU can expose entropy from the host with
viornd(4); bootloader firmware such as UEFI may also expose an underlying
platform's random number generator.
However, many older computers have no reliable entropy sources. Some
have the hardware, but have it off by default, such as a disabled tpm(4).
On computers with no built-in reliable entropy source, you may wish to
transfer a seed from another computer with rndctl(8), or manually enter
samples into /dev/urandom -- see below.
You can manually save and load seeds with the rndctl(8) tool. For exam-
ple, you might use
rndctl -S seed
to save a seed from one machine, transfer it -- over a medium where you
are confident there are no eavesdroppers -- to another machine, and load
rndctl -L seed
on the target machine; then run
on the target machine to ensure that the entropy will be saved for next
boot, even if the system later crashes or otherwise shuts down uncleanly.
rndctl -S records the number of bits of entropy in the seed so that
rndctl -L can count it.
Users can write data to /dev/urandom to be mixed together with all other
samples. For example, no matter what entropy sources are built into a
computer, you can ensure it has enough entropy (as long as there are no
surveillance cameras watching you) by flipping a coin 256 times and run-
echo thttthhhhttththtttht... > /dev/urandom
to ensure that the effort will be saved for next boot.
Inputs from the superuser (uid 0) to /dev/urandom count toward the sys-
tem's entropy estimate, at the maximum rate of one bit of entropy per bit
of data; inputs from unprivileged users will affect subsequent outputs
but will be counted as having zero entropy.
After adding entropy, make sure to regenerate any long-term keys that
might be predictable because they were previously generated with too lit-
tle entropy. For example, if `sshd=YES' is enabled in /etc/rc.conf, then
NetBSD will automatically generate ssh host keys on boot; if they were
generated with too little entropy, then you may wish to delete them and
create new ones before allowing anyone to log in via ssh(1).
NetBSD may print the following warnings to the console:
WARNING: system needs entropy for security; see entropy(7) Some process
tried to draw use entropy from NetBSD, e.g. to generate a key for cryp-
tography, before enough inputs from reliable entropy sources have been
obtained. The entropy may be low enough that an adversary could guess
keys by brute force.
This message is rate-limited, so if you have added entropy and want to
verify that the problem is resolved, you should consult the
kern.entropy.needed sysctl(7) variable to confirm it is zero, rather than
just look for the absence of this message; see rnd(4) for details.
getrandom(2), arc4random(3), rnd(4), rc.conf(5), rc(8), rndctl(8)
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,
sessions/presentation/heninger https://factorable.net/, 205-220, August
openssl -- predictable random number generator, Debian Security Advisory,
Features/VirtIORNG, QEMU Wiki, https://wiki.qemu.org/Features/VirtIORNG,
NetBSD 10.99 June 30, 2023 NetBSD 10.99