What is entropy pool in linux and how to generate and see the entropy pool data ?
I. Introduction
Entropy pool is a large number (typically around 4096 bits) stored in a memory location which can be read by programs.
This large number is a random number generated by the actual hardware noise received by various hardware devices like mouse, keyboard, etc.
Interesting feature is that the randomness of this pool varies each time when its read w.r.t. different hardware devices.
/dev/random uses so-called entropy pool and hence takes a bit more time, also sometimes no random data is available.
/dev/urandom returns any number of random data quickly as requested by the user and is not dependent on entropy pool, hence it is less random than /dev/random.
II. How to use this entropy pool to get random data ?
-bash-4.1# date; dd if=/dev/random of=random_test count=4 bs=512; date Tue Jun 25 04:15:40 PDT 2019 ^C0+0 records in 0+0 records out 0 bytes (0 B) copied, 17.4556 s, 0.0 kB/s -bash-4.1# du -sh random_test 0 random_test
We can see above no data in the random file as the entropy pool is empty.
To quickly generate data, lets see the usage of urandom.
-bash-4.1# date; dd if=/dev/urandom of=random_test count=4 bs=512; date
Tue Jun 25 04:16:13 PDT 2019
4+0 records in
4+0 records out
2048 bytes (2.0 kB) copied, 0.00039823 s, 5.1 MB/s
Tue Jun 25 04:16:13 PDT 2019
-bash-4.1#
-bash-4.1# head random_test
??M\??6AX???????+?,?F???fI?R?E
?e?`Y:(?wB?;- '?
III. See entropy pool configuration
### Pool data size -bash-4.1# cat /proc/sys/kernel/random/poolsize 4096 ### Input pool entropy count -bash-4.1# cat /proc/sys/kernel/random/entropy_avail 2531 -bash-4.1#
IV. Read random number data from /dev/random and /dev/urandom files.
od generates the octal, decimal, hex, ASCII dump, “-d” is to get only signed decimal data.
-bash-4.1# od -d /dev/urandom | head 0000000 40451 59532 29653 61171 50133 4667 40269 39364 0000020 2582 61849 22404 21982 733 38884 33145 50113 0000040 31066 38620 36771 41443 48151 30907 11008 39348 0000060 21674 55378 26910 16485 56464 17535 42537 58337 0000100 51681 24647 64184 61225 32217 18833 43709 15156 0000120 28104 25262 20084 9476 61609 46824 1700 32209 0000140 1449 17641 1065 13468 19949 12327 40879 64325 0000160 45299 46966 55111 16023 3964 43137 1450 50320 0000200 19060 3414 4812 56252 38964 18704 33070 59492 0000220 39810 27690 12853 45599 30668 29172 35796 16358 -bash-4.1# od -d /dev/random | head CPU Time Jitter
We can still see the entropy data generated is none, and is very slow.