.\"	$NetBSD: entropy.7,v 1.4.2.1 2023/08/11 14:35:25 martin Exp $
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.Dd June 30, 2023
.Dt ENTROPY 7
.Os
.\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.Sh NAME
.Nm entropy
.Nd random unpredictable secrets needed for security
.\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.Sh DESCRIPTION
Computers need random unpredictable secrets for the security of
software such as web browsers and
.Xr ssh 1 .
.Pp
Computers are designed to behave in highly predictable ways, so they
rely on observations of random physical phenomena around them, called
.Nm entropy sources ,
to derive unpredictable secrets for cryptography.
.Pp
While some computers have reliable entropy sources such as hardware
random number generators based on thermal noise in silicon circuits,
others may require operator intervention for security.
.\""""""""""""""""""""""""""""""""""""""
.Ss Threats
.Bl -bullet
.It
Web browsers and programs such as
.Xr ssh 1
rely on unpredictable secrets in cryptography to prevent eavesdropping
and detect tampering of sessions over the network.
.It
.Xr ssh-keygen 1
relies on unpredictable secrets to create keys that allow you to log in
but keep out malicious adversaries; if an adversary could guess the key
then they could impersonate you.
.It
.Nx
relies on unpredictable secrets to make sure that private user data
stored on nonvolatile media when memory is scarce
.Po
.Xr swapctl 8 ,
using
.Ql vm.swap_encrypt=1 ;
see
.Xr sysctl 7
.Pc
cannot be recovered by forensic tools after shutdown.
.El
.\""""""""""""""""""""""""""""""""""""""
.Ss Entropy in NetBSD
.Nx
gathers samples from various kinds of entropy sources, including:
.Bl -bullet -compact
.It
hardware random number generators
.It
network traffic timing
.It
user input (keystrokes, mouse movements, etc.)
.It
disk I/O latency
.It
environment sensors
.Pq Xr envsys 4
.El
The samples are mixed together with cryptography to yield unpredictable
secrets through
.Pa /dev/urandom
.Pq see Xr rnd 4
and related interfaces used by programs like
.Xr ssh 1 ,
Firefox, and so on.
.Pp
.Nx
also stores a random seed at
.Pa /var/db/entropy-file
to carry unpredictable 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 automatically by
.Pa /etc/rc.d/random_seed
.Pq see Xr rc.conf 5 .
.\""""""""""""""""""""""""""""""""""""""
.Ss Ensuring enough entropy
Entropy is measured in bits, and only 256 bits of entropy are needed
for security, thanks to modern cryptography.
.Pp
To detect potentially insecure systems,
.Nx
takes measures to alert the operator if there isn't definitely enough
for security:
.Bl -bullet
.It
.Nx
issues warnings on the console if there's not enough entropy when
programs need it; see
.Xr rnd 4 .
.It
The
.Xr motd 5
has a warning if there was not enough entropy when network daemons such as
.Xr sshd 8
first generated keys.
.It
The daily security report includes an alert if there's still not enough
entropy; see
.Xr security.conf 5 .
.El
.Pp
Since it is hard to know how unpredictable most physical systems are,
only devices specifically designed to be hardware random number
generators, or a seed file stored on disk, count toward these alerts.
.Pp
At boot,
.Nx
will wait, when
.Ql entropy=wait
is set in
.Xr rc.conf 5 ,
or fail to single-user mode, when
.Ql entropy=check
is set, if there is not enough entropy from
.Em 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
.Sq 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
.Li rndctl_flags
in
.Xr rc.conf 5 .
.Pp
Many new computers have hardware random number generators, such as
RDRAND/RDSEED in Intel/AMD CPUs, or ARMv8.5-RNDRRS;
.Xr virtio 4 Ns -based
virtualization platforms such as QEMU can expose entropy from the host
with
.Xr viornd 4 ;
bootloader firmware such as UEFI may also expose an underlying
platform's random number generator.
.Pp
However, many older computers have no reliable entropy sources.
Some have the hardware, but have it off by default, such as a disabled
.Xr tpm 4 .
On computers with no built-in reliable entropy source, you may wish to
transfer a seed from another computer with
.Xr rndctl 8 ,
or manually enter samples into
.Pa /dev/urandom
\(em see below.
.\""""""""""""""""""""""""""""""""""""""
.Ss Adding entropy
.Pp
You can manually save and load seeds with the
.Xr rndctl 8
tool.
For example, you might use
.Dl rndctl -S seed
to save a seed from one machine, transfer it \(em over a medium where
you are confident there are no eavesdroppers \(em to another machine,
and load it with
.Dl rndctl -L seed
on the target machine; then run
.Dl /etc/rc.d/random_seed stop
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.
.Ic rndctl -S
records the number of bits of entropy in the seed so that
.Ic rndctl -L
can count it.
.Pp
Users can write data to
.Pa /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
running:
.Dl echo thttthhhhttththtttht... > /dev/urandom
Then run
.Dl /etc/rc.d/random_seed stop
to ensure that the effort will be saved for next boot.
.Pp
Inputs from the superuser (uid 0) to
.Pa /dev/urandom
count toward the system'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.
.Pp
After adding entropy,
.Sy make sure to regenerate any long-term keys
that might be predictable because they were previously generated with
too little entropy.
For example, if
.Ql sshd=YES
is enabled in
.Pa /etc/rc.conf ,
then
.Nx
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
.Xr ssh 1 .
.\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.Sh DIAGNOSTICS
.Nx
may print the following warnings to the console:
.Bl -diag
.It WARNING: system needs entropy for security; see entropy(7)
Some process tried to draw use entropy from
.Nx ,
e.g. to generate a key for cryptography, 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.
.Pp
This message is rate-limited, so if you have added entropy and want to
verify that the problem is resolved, you should consult the
.Dv kern.entropy.needed
.Xr sysctl 7
variable to confirm it is zero, rather than just look for the absence
of this message; see
.Xr rnd 4
for details.
.El
.\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.Sh SEE ALSO
.Xr getrandom 2 ,
.Xr arc4random 3 ,
.Xr rnd 4 ,
.Xr rc.conf 5 ,
.Xr rc 8 ,
.Xr rndctl 8
.Rs
.%A Nadia Heninger
.%A Zakir Durumeric
.%A Eric Wustrow
.%A J. Alex Halderman
.%T Mining Your Ps and Qs: Detection of Widespread Weak Keys in Network Devices
.%B Proceedings of the 21st USENIX Security Symposium
.%I USENIX
.%D August 2012
.%P 205-220
.%U https://www.usenix.org/conference/usenixsecurity12/technical-sessions/presentation/heninger
.%U https://factorable.net/
.Re
.Rs
.%T openssl \(em predictable random number generator
.%I Debian Security Advisory
.%O DSA-1571-1
.%D 2008-05-13
.%U https://www.debian.org/security/2008/dsa-1571.html
.Re
.Rs
.%T Features/VirtIORNG
.%I QEMU Wiki
.%U https://wiki.qemu.org/Features/VirtIORNG
.%D 2016-10-17
.Re