As mentioned in the PELD spec, it is not so easy to get good pseudo-random numbers in the context of a Live system. This document describes how Tails behaves in this area, and the work that is left to be done.

Entropy sources

In addition to the Linux kernel's own entropy gathering facilities, Tails uses auxiliary entropy sources, that we describe below.

haveged

Tails ships HAVEGE, that fills /dev/random whenever the supply of random bits in /dev/random falls below the low water mark of the device. Quoting its homepage, HAVEGE "exploits [these] modifications of the internal volatile hardware states as a source of uncertainty".

The default configuration shipped with the Debian package passes -w 1024 to the haveged daemon. That is, it sets /proc/sys/kernel/random/write_wakeup_threshold to 1024. We modify that to use the same watermark as rngd, i.e. 2048 bits. The goal here is to avoid the situation when rngd starts first and always keeps the entropy pool between 1024 and 2048 bits, thus dominating the entropy pool.

rngd

rngd gets entropy from a hardware RNG, if available. Otherwise, it does not start.

rngd fills up the pool using an ioctl on /dev/random to add entropy. It does that unless fill-watermark bits are available. The fill-watermark defaults to 50% of the size of the entropy pool, which itself defaults to 4096 bits on Linux 3.14, so basically rngd feeds the entropy pool unless there are already 2048 bits in it. The Debian package does not override this default configuration, and neither does Tails.

Note: rngd (2-unofficial-mt.14-1) does not modify any parameter in /proc/sys/kernel/random/.

Entropy pool initialization

The Linux kernel initializes its RNG with various sources, including:

• Hardware RNGs (if available)
• CPU instructions like RDSEED, RDRAND, or RDTSC
• interrupt timings, including input events (keyboard, mouse, etc.)

If the entropy pool is not initialized the first time random numbers are requested, the kernel will block and generate entropy from cycle counter jitter (that's not supported on all architectures, but it is on x86), which takes about a second. There was some scepticism about the quality of this jitter entropy in the past (for example by Theodore Ts'o and Linus Torvalds) but there hasn't been much discussion about it since it was introduced in Linux 5.4 (a publication by the German Federal Office for Information Security validates the jitter entropy as a good source of entropy).

Some sources, like interrupt timings, are continuously mixed into the entropy pool, which is used to periodically reseed the kernel's RNG. This reseeding is done more often during boot, because it's unclear how good the entropy sources are during early boot, and mixing in more entropy is a good way to ensure that the entropy pool is well-seeded.

The userland can also mix in some entropy by writing to /dev/urandom or /dev/random. In Debian, systemd-random-seed.service is used to write a random seed to /var/lib/systemd/random-seed during shutdown and load it into the kernel entropy pool during boot.

That doesn't work in Tails, because of the read-only root filesystem. Instead of systemd-random-seed.service, Tails uses custom scripts executed in the initramfs during boot and before shutdown. The random seed is not stored on the filesystem, but in an otherwise unused sector on the device, at logical block address (LBA) 34 (that's the first sector after the GPT)).

This solves the problem that the root filesystem is read-only in Tails and has multiple advantages:

• The filesystem is not changed, therefore easier to verify.
• It doesn't require persistence to be set up.
• The entropy pool is seeded earlier during the boot process (in the initramfs init-top stage).

The scripts do basically the same systemd-random-seed.service does (see the source code).

During boot:

• Read the seed from the location it was stored at.
• Write the seed to /dev/urandom, which causes it to be mixed in with the existing bytes in the entropy pool.
• Update the seed with random bytes from /dev/urandom to ensure that some entropy is saved for the next boot, even if the system is not shutting down properly.

During shutdown:

• Update the seed with random bytes from /dev/urandom to save the entropy gathered during the session.

Sources to learn more about the Linux kernel's RNG and its recent changes:

• All articles in this LWN kernel index entry, especially:
  • The article on the jitter entropy introduced in Linux 5.4.
  • The article on uniting /dev/random and /dev/urandom.
• A summary of recent changes to the kernel's RNG by co-maintainer Jason A. Donenfeld.
• An analysis of the Linux RNG by the German Federal Office for Information Security.
• The section headers of the kernel's random.c.

Initialization during installation

When creating a new Tails device, Tails Cloner writes a random seed to LBA 34 to initialize the seed for the first boot.

Possible disadvantages

On #11897, the concern was raised that writing to the Tails device on each boot might allow a sophisticated attacker with physical access to the device to determine how many times it has been booted due to the wear leveling of the flash memory.

We try to mitigate this by writing the random seed a random number of times (1 to 500) during first boot. This should make it harder to determine the number of boots by looking at the wear leveling of the flash memory. Concerns that this might not be enough if the device uses static wear leveling have been raised on #11897.

Remaining concerns

HAVEGE reliability

haveged relies on the RDTSC instruction, that apparently is useless in some virtualized environments. Also, the quality of random numbers output by HAVEGE is unclear, and the topic of many discussions.

Further research on this topic is left to be done.

This is why Tails also ships rngd. Still, it is not clear how these two daemons act together.

Hardware RNG trustworthiness and availability

It is not clear how much one can trust a hardware RNG, that is hard, if not impossible, to audit. Also, not all computers include a hardware RNG.

This is why Tails also ships HAVEGE. Still, it is not clear how these two daemons act together.

Interaction between haveged and rngd

This discussion only makes sense whenever a hardware RNG supported by rngd is available. Otherwise, only haveged is used.

The way it is configured in Debian, haveged sets /proc/sys/kernel/random/write_wakeup_threshold to 2048, so that processes that are waiting to write to /dev/random are woken up whenever less than 2048 bits of entropy is available. In practice, this probably means that Linux wakes up both haveged and rngd more or less at the same time.

In such a case, haveged tries to write as many bytes as needed to fill the pool via a single ioctl, while rngd tries to write 512 bits (the default value of random_step being 64 bytes) at a time, until the pool contains 2048 bits (default value of the pool water mark). It's unclear which one wins the race. Let's discuss the possible cases:

• If haveged always wins the case, then it is actually useless to run rngd at all.

• If rngd always wins the race, then it dominates the entropy pool, but shipping haveged is still useful when no hardware RNG is available.

• If the one that wins the race may change depending on the context, then it's still useful to ship both rngd and haveged: it achieves our goal of not relying purely on either one.

Interaction between haveged and rngd

This area is left to be researched.

Early boot time

One should audit random numbers availability at early boot time: #6116.