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Comment by lvh

7 years ago

Is there a good reason this doesn't just use urandom/getrandom? This doesn't look like it's in the "we're so early in boot I haven't restored the random seed yet" case, for example.

31 comments

lvh

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jsnell  7 years ago

No. There's a bad reason though. Want to guess what? https://github.com/systemd/systemd/blob/master/src/basic/ran...

Yes, the old "draining entropy" fallacy.

  • dzdt  7 years ago

    Why do you say draining entropy is a fallacy? It is certainly true that entropy recorded from I/O sources accumulates at a very limited rate.

    • wolf550e  7 years ago

      Because after you gather ~256 bits of entropy from I/O sources (or rdrand^H^H^H rdseed [1]), using it to generate infinite output of /dev/urandom does not drain it. Adding more entropy only helps if the entropy pool inner state leaked, it is not needed to add more entropy because it drained through using it.

      Proof:

      1. Take 256 bits of entropy.

      2. Use HKDF to generate 128 bit key and 64 bit nonce.

      3. Use key and nonce for AES-CTR with all possible 2^64 counter values to produce 2^68 bytes of output.

      4. goto 2.

      If you can compute the inner state of the RNG (and thus predict future output) by just observing the output (not through side channels), using any amount of output, then all modern symmetric crypto is broken. If you can't, then using the entropy in an RNG does not drain the entropy pool.

      And here is a CCC talk about it: https://www.youtube.com/watch?v=OSfmtRc4VsE

      1 - https://software.intel.com/en-us/blogs/2012/11/17/the-differ...

      EDIT: Thanks for 'dragontamer for pointing out difference between rdrand and rdseed.

      16 replies →

    • nwallin  7 years ago

      Which one of these statements is false:

      1. When secure your TLS 1.2 connection with a 128 bit AES key, you can transmit a small amount of data (on the order of ~128 bits) before the entropy is drained from your CSPRNG state.

      2. When you generate at least ~128 bits of entropy in your kernel's CSPRNG state, you can generate a very large amount of random bits before the entropy is drained from your CSPRNG state.

jontro  7 years ago

Quoting from https://github.com/systemd/systemd/issues/11810#issuecomment...

BTW, the reason we use RDRAND in some cases instead of getrandom() [which we use in many others] is that we need to generate uuids early on (since every service we starts gets one passed, the "invocation ID", and for other stuff too), but getrandom complains in dmesg or blocks if we call it before the pool is initialized. Since systemd is one of the earliest programs that runs and thus very likely comes into contact with an uninitialized pool we attempt to avoid that by using RDRAND when generating uuids, since it should be good enough for that, as the usecase needs a "mid-quality" rng source: not crypt quality and not totally guessable either.

  • loeg  7 years ago

    Why not use getrandom() with GRND_NONBLOCK?

    Worst case, they could run RDRAND in a loop and write() it into /dev/random until getrandom() is unblocked. Then they're still using the ordinary kernel random device, more or less. They wouldn't have these catastrophic collisions due to near-zero entropy.

    • JdeBP  7 years ago

      Because systemd is not x86-64-only. The actual worst case is where there is no RDRAND, because then the systemd people have the chicken-and-egg situation of not being able to seed urandom without running a system service unit, and not being able to run a systemd service unit without being able to assign a random GUID to it as its instance ID.

      The design problem is that every service unit has an instance ID, whether meaningful and useful to it or not, the journal and other things have a machine ID, and journal files have header IDs. This requires that re-seeding urandom be pushed right to the start of the entire user-mode boot process (which begins with systemd in the initramfs on some operating systems), that something else (such as RDRAND) be used until urandom is re-seeded, or that the boot process simply stop and block running the very first service unit until urandom is re-seeded by the kernel.

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