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Generate Random Numbers and Improve Entropy in Linux Systems
- 1. Random
- 2. Random numbers in AWK ● awk 'BEGIN{srand();print int(rand()*(65000))}'
- 3. 1204
- 4. 21
- 5. 21
- 6. Random numbers and shuf ● shuf -i 2000-65000 -n 1
- 7. 7
- 8. 7
- 9. 7
- 10. True Randomness
- 11. True Randomness ● counters that detect radioactive decay ● the timing of actual movements of a hard disk read/write head ● webcam/audio static noise
- 12. dev/random ● od -A n -N 2 -t u2 /dev/random ● od -A n -N 2 -t u2 /dev/urandom
- 13. man urandom A read from the /dev/urandom device will not block waiting for more entropy. As a result, if there is not sufficient entropy in the entropy pool, the returned values are theoretically vulnerable to a cryptographic attack on the algorithms used by the driver. Knowledge of how to do this is not available in the current unclassified literature, but it is theoretically possible that such an attack may exist. If this is a concern in your application, use /dev/random instead.
- 14. Random vs URandom ● URandom is a pseudo random number generator ● Random is a true random number generator
- 17. What about entropy running low? ● It doesn't matter. ● The underlying cryptographic building blocks are designed such that an attacker cannot predict the outcome, as long as there was enough randomness (256 bits).
- 18. Why is Re-seeding so important? ● If you've got more randomness just lying around, by all means use it!
- 19. man urandom If you are unsure about whether you should use /dev/random or /dev/urandom, then probably you want to use the latter. As a general rule, /dev/urandom should be used for everything except long-lived GPG/SSL/SSH keys.
- 20. Not everything is perfect ● On Linux, unlike FreeBSD, /dev/urandom never blocks. ● Linux's /dev/urandom happily gives you not- so-random numbers before the kernel even had the chance to gather entropy. ● FreeBSD doesn't have the distinction between /dev/random and /dev/urandom, both are the same device.
- 21. Not everything is perfect ● Linux distributions save some random numbers when booting up the system (but after they have gathered some entropy, since the startup script doesn't run immediately after switching on the machine) into a seed file that is read next time the machine is booting. ● Linux has implemented a new syscall, originally introduced by OpenBSD as getentropy(2): getrandom(2). This syscall does the right thing: blocking until it has gathered enough initial entropy, and never blocking after that point.
- 22. When is low entropy a problem? ● It used to be a problem for devices with no interface for human interaction when the entropy collector relied solely on than ● Today, virtual machines can have this issue if they are being cloned or restored to a checkpoint.
- 23. The end. 42