![]() ![]() So, this is extremely low, and smaller values will block everyone who are reading from /dev/random /* ![]() If the available_entropy is lower than 64 bits, no one will read from /dev/random. Static int random_read_wakeup_thresh = 64 Should be enough to do a significant reseed. * The minimum number of bits of entropy before we wake up a read on #define EXTRACT_SIZE 10 - 10 bytes to be extracted from secondary pools There is source code of /dev/random and /dev/urandom driver: (you can turn on DEBUG_ENT to see the actual working of the random driver) /* The maths behind the entropy estimation in Linux is analyzed here (nothing useful for software engineer, just adding and subtracting numbers). But 251.pdf (part 4.3, page 16) says "Consequently, the Linux PRNG without entropy input is assumed to be secure if the internal state is not compromised and if the three pools are initialized by an unknown value." When entropy is high, urandom gives high-quality bits, but when entropy is zero, it gives (probably) cryptographic pseudo-random number sequence (there are better examples of such CSPRNG generators in almost every other popular OS: FreeBsd/OpenBSD,MacOS,MS). dev/random gives only high-quality bits and blocks when there is no entropy left, while /dev/urandom gives bits of any quality. I would like to consider the entropy_avail as estimation of "how many high-quality random bits may I extract from random source". When Secondary pool has low entropy, some portion of entropy from Primary pool is extracted (again with using of cryptographic one-way functions), then part of it is remixed back into primary pool and part of it mixed to the consuming pool.ġ) How low does entropy have to be for it to be dangerous? If I run cat /proc/sys/kernel/random/entropy_avail i get anything from about 130 to a couple thousand. To monitor entropy_avail you should use some daemon or simple C program which will reread pool several times without process restarting. There is interesting example when doing several cat /proc/sys/kernel/random/entropy_avail will stole most entropy, may be this is the your case too, proof at StackOverflow. kernel uses API to random driver for starting processes when ASLR is enabled (it is enabled in most distros). Both user and kernel consume entropy, e.g. Part of extracted data is given to the consumer of random data. ![]() When you asks for random data, portion of entropy is extracted from the corresponding secondary pool (using one-way functions involving SHA1), then it is partly added (mixed into) to the pool. ![]() The 'available' counter is from primary pool. Separate entropy counters are maintained for all three pools, but usually the counter of primary pool is mean. Urandom pool (or another secondary pool) 128 bytes for /dev/urandom.Secondary pool of 128 bytes to generate random for /dev/random.Primary pool of 512 bytes (128 4-byte words) incoming entropy is added to it.Then you can check The Linux Pseudorandom Number Generator Revisited, Lacharme 2012, 251.pdf paper. And the other is paper: Analysis of the Linux Random Number Generator, GuttermanĢ006, 086.pdf - check pages 4-6 and Figure 2.1 ( slides). If you need other OS, please, inform me.Īlso I will answer using source code of random.c driver from Linux 3.3.3 Kernel, because it is one of best documentation of /dev/random mechanics. I will answer considering Linux OS, as being one of most popular Unix-like OS (between OSes which have urandom). Sorry for multiple questions, but they seemed related so wanted to post it as one. Given a public/private key you've already generated - is there any way you can prove it was generated with low entropy? On a shared box, is there any way a neighbor could deplete all the randomness on the system, and force you to generate a weak key? It seems like blocking until there was sufficient entropy would solve this (#3) above. I assume /dev/random is not the default for performance/consistency reasons? Is there a flag or option in openssl where you can choose to use /dev/random instead in case you want to be sure? Is the attack just theoretical or how much work would it take to break it in practice?ĭoes openssl block until there is sufficient entropy to generate a key? My understanding is that /dev/urandom does not block and is the default, but /dev/random does. Is an attack feasible at 130, or how low would it have to go (50, 10, 1, 0)? I haven't seen any hard numbers on this. How low does entropy have to be for it to be dangerous? If I run cat /proc/sys/kernel/random/entropy_avail i get anything from about 130 to a couple thousand. I am concerned about generating weak keys on a shared linux box with openssl. (Disclaimer: I am regular software engineer with only basic crypto knowledge, so helpful if can be explained for a layman.) ![]()
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