Improve entropy gathering

The Linux kernel uses multiple sources of randomness to initialize its cryptographically secure pseudo-random number generator (CSPRNG). This includes various sources with dubious quality wrt. randomness: the kernel command-line, serial numbers, MAC addresses, timing information…

This is totally fine, because most of these sources are not credited as good/reliable entropy, which means that the values are mixed into the entropy pool, but they do not increase the entropy counter. (By default, the kernel currently only credits inter-interrupt timings and inter-keyboard timings).

When the entropy counter reaches a certain threshold (currently 512 bits, but it’s currently being discussed on the kernel mailing list to reduce that to 256 bits) is the entropy pool marked as initialized.

Until the entropy pool is marked as initialized, reads from /dev/random and calls to the getrandom syscall are blocking, and reads from /dev/urandom return not-cryptographically secure random numbers.

If the entropy pool is seeded with predictable inputs, all of /dev/random, /dev/urandom, and getrandom return not-cryptographically secure random numbers.

Both Debian and Tails currently add additional sources which do increase the entropy counter. I would like to re-evaluate the use of those sources.

Related issues

• Related to #7102
• Related to #5650 (closed)
• Related to #17124 (closed)

Original created by @segfault on 17154 (Redmine)