ENTROPY(7)

ENTROPY(7) | Miscellaneous Information Manual | ENTROPY(7)

# NAME

`entropy` — random unpredictable secrets needed for security

# DESCRIPTION

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 random number generators based on thermal noise in silicon circuits, others may require operator intervention for security.

## Threats

- 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 adversary 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 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 /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 network 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 generators, 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 virtualization 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.

## Adding entropy

You can manually save and load seeds with the rndctl(8) tool. For example, 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 it with
`rndctl -L seed`
on the target machine; then run
`/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. `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 running:

`echo thttthhhhttththtttht... > /dev/urandom`
Then run
`/etc/rc.d/random_seed stop`
to ensure that the effort will be saved for next boot.

Inputs from the superuser (uid 0) to /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.

After adding entropy, make sure to regenerate any long-term keys that might be predictable because they were previously generated with too little 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).

# DIAGNOSTICS

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 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.

  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.

# SEE ALSO

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, https://www.usenix.org/conference/usenixsecurity12/technical-sessions/presentation/heninger, https://factorable.net/, 205-220, August 2012.

openssl — predictable random number generator, Debian Security Advisory, https://www.debian.org/security/2008/dsa-1571.html, 2008-05-13, DSA-1571-1.

Features/VirtIORNG, QEMU Wiki, https://wiki.qemu.org/Features/VirtIORNG, 2016-10-17.

June 30, 2023 | NetBSD 11.0