From the Canyon Edge -- :-Dustin

Tuesday, February 18, 2014

Improving Random Seeds in Ubuntu 14.04 LTS Cloud Instances

Tomorrow, February 19, 2014, I will be giving a presentation to the Capital of Texas chapter of ISSA, which will be the first public presentation of a new security feature that has just landed in Ubuntu Trusty (14.04 LTS) in the last 2 weeks -- doing a better job of seeding the pseudo random number generator in Ubuntu cloud images.  You can view my slides here (PDF), or you can read on below.  Enjoy!


Q: Why should I care about randomness? 

A: Because entropy is important!

  • Choosing hard-to-guess random keys provide the basis for all operating system security and privacy
    • SSL keys
    • SSH keys
    • GPG keys
    • /etc/shadow salts
    • TCP sequence numbers
    • UUIDs
    • dm-crypt keys
    • eCryptfs keys
  • Entropy is how your computer creates hard-to-guess random keys, and that's essential to the security of all of the above

Q: Where does entropy come from?

A: Hardware, typically.

  • Keyboards
  • Mouses
  • Interrupt requests
  • HDD seek timing
  • Network activity
  • Microphones
  • Web cams
  • Touch interfaces
  • WiFi/RF
  • TPM chips
  • RdRand
  • Entropy Keys
  • Pricey IBM crypto cards
  • Expensive RSA cards
  • USB lava lamps
  • Geiger Counters
  • Seismographs
  • Light/temperature sensors
  • And so on

Q: But what about virtual machines, in the cloud, where we have (almost) none of those things?

A: Pseudo random number generators are our only viable alternative.

  • In Linux, /dev/random and /dev/urandom are interfaces to the kernel’s entropy pool
    • Basically, endless streams of pseudo random bytes
  • Some utilities and most programming languages implement their own PRNGs
    • But they usually seed from /dev/random or /dev/urandom
  • Sometimes, virtio-rng is available, for hosts to feed guests entropy
    • But not always

Q: Are Linux PRNGs secure enough?

A: Yes, if they are properly seeded.

  • See random(4)
  • When a Linux system starts up without much operator interaction, the entropy pool may be in a fairly predictable state
  • This reduces the actual amount of noise in the entropy pool below the estimate
  • In order to counteract this effect, it helps to carry a random seed across shutdowns and boots
  • See /etc/init.d/urandom
...
dd if=/dev/urandom of=$SAVEDFILE bs=$POOLBYTES count=1 >/dev/null 2>&1 

...

Q: And what exactly is a random seed?

A: Basically, its a small catalyst that primes the PRNG pump.

  • Let’s pretend the digits of Pi are our random number generator
  • The random seed would be a starting point, or “initialization vector”
  • e.g. Pick a number between 1 and 20
    • say, 18
  • Now start reading random numbers

  • Not bad...but if you always pick ‘18’...

XKCD on random numbers

RFC 1149.5 specifies 4 as the standard IEEE-vetted random number.

Q: So my OS generates an initial seed at first boot?

A: Yep, but computers are predictable, especially VMs.

  • Computers are inherently deterministic
    • And thus, bad at generating randomness
  • Real hardware can provide quality entropy
  • But virtual machines are basically clones of one another
    • ie, The Cloud
    • No keyboard or mouse
    • IRQ based hardware is emulated
    • Block devices are virtual and cached by hypervisor
    • RTC is shared
    • The initial random seed is sometimes part of the image, or otherwise chosen from a weak entropy pool

Dilbert on random numbers


http://j.mp/1dHAK4V


Q: Surely you're just being paranoid about this, right?

A: I’m afraid not...

Analysis of the LRNG (2006)

  • Little prior documentation on Linux’s random number generator
  • Random bits are a limited resource
  • Very little entropy in embedded environments
  • OpenWRT was the case study
  • OS start up consists of a sequence of routine, predictable processes
  • Very little demonstrable entropy shortly after boot
  • http://j.mp/McV2gT

Black Hat (2009)

  • iSec Partners designed a simple algorithm to attack cloud instance SSH keys
  • Picked up by Forbes
  • http://j.mp/1hcJMPu

Factorable.net (2012)

  • Minding Your P’s and Q’s: Detection of Widespread Weak Keys in Network Devices
  • Comprehensive, Internet wide scan of public SSH host keys and TLS certificates
  • Insecure or poorly seeded RNGs in widespread use
    • 5.57% of TLS hosts and 9.60% of SSH hosts share public keys in a vulnerable manner
    • They were able to remotely obtain the RSA private keys of 0.50% of TLS hosts and 0.03% of SSH hosts because their public keys shared nontrivial common factors due to poor randomness
    • They were able to remotely obtain the DSA private keys for 1.03% of SSH hosts due to repeated signature non-randomness
  • http://j.mp/1iPATZx

Dual_EC_DRBG Backdoor (2013)

  • Dual Elliptic Curve Deterministic Random Bit Generator
  • Ratified NIST, ANSI, and ISO standard
  • Possible backdoor discovered in 2007
  • Bruce Schneier noted that it was “rather obvious”
  • Documents leaked by Snowden and published in the New York Times in September 2013 confirm that the NSA deliberately subverted the standard
  • http://j.mp/1bJEjrB

Q: Ruh roh...so what can we do about it?

A: For starters, do a better job seeding our PRNGs.

  • Securely
  • With high quality, unpredictable data
  • More sources are better
  • As early as possible
  • And certainly before generating
  • SSH host keys
  • SSL certificates
  • Or any other critical system DNA
  • /etc/init.d/urandom “carries” a random seed across reboots, and ensures that the Linux PRNGs are seeded

Q: But how do we ensure that in cloud guests?

A: Run Ubuntu!


Sorry, shameless plug...

Q: And what is Ubuntu's solution?

A: Meet pollinate.

  • pollinate is a new security feature, that seeds the PRNG.
  • Introduced in Ubuntu 14.04 LTS cloud images
  • Upstart job
  • It automatically seeds the Linux PRNG as early as possible, and before SSH keys are generated
  • It’s GPLv3 free software
  • Simple shell script wrapper around curl
  • Fetches random seeds
  • From 1 or more entropy servers in a pool
  • Writes them into /dev/urandom
  • https://launchpad.net/pollinate

Q: What about the back end?

A: Introducing pollen.

  • pollen is an entropy-as-a-service implementation
  • Works over HTTP and/or HTTPS
  • Supports a challenge/response mechanism
  • Provides 512 bit (64 byte) random seeds
  • It’s AGPL free software
  • Implemented in golang
  • Less than 50 lines of code
  • Fast, efficient, scalable
  • Returns the (optional) challenge sha512sum
  • And 64 bytes of entropy
  • https://launchpad.net/pollen

Q: Golang, did you say?  That sounds cool!

A: Indeed. Around 50 lines of code, cool!

pollen.go

Q: Is there a public entropy service available?

A: Hello, entropy.ubuntu.com.

  • Highly available pollen cluster
  • TLS/SSL encryption
  • Multiple physical servers
  • Behind a reverse proxy
  • Deployed and scaled with Juju
  • Multiple sources of hardware entropy
  • High network traffic is always stirring the pot
  • AGPL, so source code always available
  • Supported by Canonical
  • Ubuntu 14.04 LTS cloud instances run pollinate once, at first boot, before generating SSH keys

Q: But what if I don't necessarily trust Canonical?

A: Then use a different entropy service :-)

  • Deploy your own pollen
    • bzr branch lp:pollen
    • sudo apt-get install pollen
    • juju deploy pollen
  • Add your preferred server(s) to your $POOL
    • In /etc/default/pollinate
    • In your cloud-init user data
      • In progress
  • In fact, any URL works if you disable the challenge/response with pollinate -n|--no-challenge

Q: So does this increase the overall entropy on a system?

A: No, no, no, no, no!

  • pollinate seeds your PRNG, securely and properly and as early as possible
  • This improves the quality of all random numbers generated thereafter
  • pollen provides random seeds over HTTP and/or HTTPS connections
  • This information can be fed into your PRNG
  • The Linux kernel maintains a very conservative estimate of the number of bits of entropy available, in /proc/sys/kernel/random/entropy_avail
  • Note that neither pollen nor pollinate directly affect this quantity estimate!!!

Q: Why the challenge/response in the protocol?

A: Think of it like the Heisenberg Uncertainty Principle.

  • The pollinate challenge (via an HTTP POST submission) affects the pollen's PRNG state machine
  • pollinate can verify the response and ensure that the pollen server at least “did some work”
  • From the perspective of the pollen server administrator, all communications are “stirring the pot”
  • Numerous concurrent connections ensure a computationally complex and impossible to reproduce entropy state

Q: What if pollinate gets crappy or compromised or no random seeds?

A: Functionally, it’s no better or worse than it was without pollinate in the mix.

  • In fact, you can `dd if=/dev/zero of=/dev/random` if you like, without harming your entropy quality
    • All writes to the Linux PRNG are whitened with AES and mixed into the entropy pool
    • Of course it doesn’t help, but it doesn’t hurt either
  • Your overall security is back to the same level it was when your cloud or virtual machine booted at an only slightly random initial state
  • Note the permissions on /dev/*random
    • crw-rw-rw- 1 root root 1, 8 Feb 10 15:50 /dev/random
    • crw-rw-rw- 1 root root 1, 9 Feb 10 15:50 /dev/urandom
  • It's a bummer of course, but there's no new compromise

Q: What about SSL compromises, or CA Man-in-the-Middle attacks?

A: We are mitigating that by bundling the public certificates in the client.


  • The pollinate package ships the public certificate of entropy.ubuntu.com
    • /etc/pollinate/entropy.ubuntu.com.pem
    • And curl uses this certificate exclusively by default
  • If this really is your concern (and perhaps it should be!)
    • Add more URLs to the $POOL variable in /etc/default/pollinate
    • Put one of those behind your firewall
    • You simply need to ensure that at least one of those is outside of the control of your attackers

Q: What information gets logged by the pollen server?

A: The usual web server debug info.

  • The current timestamp
  • The incoming client IP/port
    • At entropy.ubuntu.com, the client IP/port is actually filtered out by the load balancer
  • The browser user-agent string
  • Basically, the exact same information that Chrome/Firefox/Safari sends
  • You can override if you like in /etc/default/pollinate
  • The challenge/response, and the generated seed are never logged!
Feb 11 20:44:54 x230 2014-02-11T20:44:54-06:00 x230 pollen[28821] Server received challenge from [127.0.0.1:55440, pollinate/4.1-0ubuntu1 curl/7.32.0-1ubuntu1.3 Ubuntu/13.10 GNU/Linux/3.11.0-15-generic/x86_64] at [1392173094634146155]

Feb 11 20:44:54 x230 2014-02-11T20:44:54-06:00 x230 pollen[28821] Server sent response to [127.0.0.1:55440, pollinate/4.1-0ubuntu1 curl/7.32.0-1ubuntu1.3 Ubuntu/13.10 GNU/Linux/3.11.0-15-generic/x86_64] at [1392173094634191843]

Q: Have the code or design been audited?

A: Yes, but more feedback is welcome!

  • All of the source is available
  • Service design and hardware specs are available
  • The Ubuntu Security team has reviewed the design and implementation
  • All feedback has been incorporated
  • At least 3 different Linux security experts outside of Canonical have reviewed the design and/or implementation
    • All feedback has been incorporated

Q: Where can I find more information?

A: Read Up!


Stay safe out there!
:-Dustin