Black Ops 2012

August 6, 2012 3 comments

Here’s my slides from Black Hat and Defcon for 2012.  Pile of interesting heresies — should make for interesting discussion.  Here’s what we’ve got:

1) Generic timing attack defense through network interface jitter
2) Revisiting Random Number Generation through clock drift
3) Suppressing injection attacks by altering variable scope and per-character taint
4) Deployable mechanisms for detecting censorship, content alteration, and certificate replacement
5) Stateless TCP w/ payload retrieval

I hate saying “code to be released shortly”, but I want to post the slides and the code’s pretty hairy.  Email me if you want to test anything, particularly if you’d like to try to break this stuff or wrap it up for release.  I’ll also be at Toorcamp, if you want to chat there.

Categories: Security

RDP and the Critical Server Attack Surface

March 18, 2012 18 comments

MS12-020, a use-after-free discovered by Luigi Auriemma, is roiling the Information Security community something fierce. That’s somewhat to be expected — this is a genuinely nasty bug. But if there’s one thing that’s not acceptable, it’s the victim shaming.

As people who know me, know well, nothing really gets my hackles up like blaming the victims of the cybersecurity crisis. “Who could possibly be so stupid as to put RDP on the open Internet”, they ask. Well, here’s some actual data:

On 16-Mar-2012, I initiated a scan across approximately 8.3% of the Internet (300M IPs were probed; the scan is ongoing). 415K of ~300M IP addresses showed evidence of speaking the RDP protocol (about twice as many had listeners on 3389/tcp — always be sure to speak a bit of the protocol before citing connectivity!)

Extrapolating from this sample, we can see that there’s approximately five million RDP endpoints on the Internet today.

Now, some subset of these endpoints are patched, and some (very small) subset of these endpoints aren’t actually the Microsoft Terminal Services code at all. But it’s pretty clear that, yes, RDP is actually an enormously deployed service, across most networks in the world (21767 of 57344 /16’s, at 8.3% coverage).

There’s something larger going on, and it’s the relevance of a bug on what can be possibly called the Critical Server Attack Surface. Not all bugs are equally dangerous because not all code is equally deployed. Some flaws are simply more accessible than others, and RDP — as the primary mechanism by which Windows systems are remotely administered — is a lot more accessible than a lot of people were aware of.

I think, if I had to enumerate the CSAS for the global Internet, it would look something like (in no particular order — thanks Chris Eng, The Grugq!):

  • HTTP (Apache, Apache2, IIS, maybe nginx)
  • Web Languages (ASPX, ASP, PHP, maybe some parts of Perl/Python. Maybe rails.)
  • TCP/IP (Windows 2000/2003/2008 Server, Linux, FreeBSD, IOS)
  • XML (libxml, MSXML3/4/6)
  • SSL (OpenSSL, CryptoAPI, maybe NSS)
  • SSH (OpenSSH, maybe dropbear)
  • telnet (bsd telnet, linux telnet) (not enough endpoints)
  • RDP (Terminal Services)
  • DNS (BIND9, maybe BIND8, MSDNS, Unbound, NSD)
  • SNMP
  • SMTP (Sendmail, Postfix, Exchange, Qmail, whatever GMail/Hotmail/Yahoo run)

I haven’t exactly figured out where the line is — certainly we might want to consider web frameworks like Drupal and WordPress, FTP daemons, printers, VoIP endpoints and SMB daemons, not to mention the bouillabaisse that is the pitiful state of VPNs all the way into 2012 — but the combination of “unfirewalled from the Internet” and “more than 1M endpoints” is a decent rule of thumb. Where people are maybe blind, is in the dramatic underestimation of just how many Microsoft shops there really are out there.

We actually had the opposite situation a little while ago, with this widely discussed bug in telnet. Telnet was the old way Unix servers were maintained on the Internet. SSH rather completely supplanted it, however — the actual data revealed only 180K servers left, with but 20K even potentially vulnerable.

RDP’s just on a different scale.

I’ve got more to say about this, but for now it’s important to get these numbers out there. There’s a very good chance that your network is exposing some RDP surface. If you have any sort of crisis response policy, and you aren’t completely sure you’re safe from the RDP vulnerability, I advise you to invoke it as soon as possible.

(Disclosure: I do some work with Microsoft from time to time. This is obviously being written in my personal context.)

Categories: Security

Open For Review: Web Sites That Accept Security Research

February 26, 2012 11 comments

So one of the core aspects of my mostly-kidding-but-no-really White Hat Hacker Flowchart is that, if the target is a web page, and it’s not running on your server, you kind of need permission to actively probe for vulnerabilities.

Luckily, there are actually a decent number of sites that provide this permission.

37 Signals
UPDATE 2, courtesy of Neal Poole:
Reddit (this is particularly awesome)
Constant Contact

One could make the argument that you can detect who in the marketplace has a crack security team, by who’s willing and able to commit the resources for an open vulnerability review policy.

Some smaller sites have also jumped on board (mostly absorbing and reiterating Salesforce’s policy — cool!):

Simplify, LLC
Team Unify
Modus CSR

There’s some interesting implications to all of this, but for now lets just get the list out there. Feel free to post more in the comments!

Categories: Security

White Hat Hacker Flowchart

February 20, 2012 4 comments

For your consideration. Rather obviously not to be taken as legal advice. Seems to be roughly what’s evolved over the last decade.

Categories: lulz, Security

#Nerdflix. Because LOL

February 19, 2012 9 comments

So, one of my goals for 2012 has been to write quite a bit more — more code, and more long form analysis. So far, so good.

Doesn’t mean it’s all going to be about security, or even that it’s all going to be all that serious. My whole #Protolol thing worked out pretty amusingly…and so, here’s #nerdflix.

LOL. Some highlights. I can’t even imagine the horrible punnery I’m going to wake up to.

UPDATE 1: Apparently at some point #nerdflix was trending worldwide? It’s certainly taken San Francisco by storm 😉

Anyway, here’s more.

Update 2: Also, Germany. This is not surprising.

@dakami: Memory Resident Evil
@dakami: Driving Miss Daisy Chain (h/t @wadebaker)
@dakami: Raiders of the Lost ARP (h/t @quadling)
@dakami: –wx—— (h/t @ennor)
@AtheistHacker: A Beautiful BIND
@RySub Desperate house wifi’s
@JGoldOrlando: no SALT
@johngineer: Ohm Alone
@kickfroggy: I Now Pronounce You PCI Compliant
@kickfroggy: There’s Something About Mary’s Code
@KrisPaget: Full Metal Packet
@l0qii: Beverly Hills Pcap
@marshray: Batman Returns-into-libc
@mratzlaff: Extremely pwned and Incredibly hosed
@otanistudio: ~/alone
@otanistudio: HTML5 Gordon
@plaverty24: Born on December 31, 1969
@redtwitdown: The Kaminsky Code (cc @dakami)
@wadebaker: Gone with the rm Memorable line: “frankly Scarlet, -rf”
@Walshman23: Girl, Interrupt-driven
@jadedsecurity: Apache Down
@chenb0x: The Netbook
@manzuik: Whitehats Can’t Code.
@afabmedia: Magnum, 3.1415926535897932384626433832795028841971693993751058209
@felipeLvalero: RT @Mackaber: RT @Voulnet: Despicable WindowsME / Lol
@0x17h: Gone with the Windows
@vogon: @dakami The SHA-1 Redemption
@vogon: @dakami 1U Over the Cuckoo’s Nest
@ThemsonMester: Oceans 802.11
@enriquereyes: 500 days of evaluation period
@a_r_w: The TEMPEST (staring Van Eck)
@marshray: Batman Returns-into-libc
@marshray: The Bitcoin Miner’s Daughter
@gbrunkhorst: The Little Endian in the Cupboard
@bm_: RT @_jtmelton: @dakami Lost in Machine Translation
@kickfroggy: Paul Blart: Mall CISSP << lol
@0x17h: The Deprecated
@0x17h: All About Eve Online
@afabmedia: So I Married a VAX Murderer
@infojeff: Pink Floyd The Wall of Sheep
@jdunck: Referer never dies /cc @kevinmarks
@kickfroggy: Fast Times at 802.3bg
@kickfroggy: How to Train Your User
@liambeeton: Firewall Club
@longobord: Schindler’s Linked List
@mfukar: reinterpret_cast Away
@obscuresec: Inglorious Hackers
@obscuresec: Toy Story 2++ (should be Toy Story ++2, actually. –Dan)
@pageman: The preg_replace ment Killers
@rattis: hackers on a plane (come one, someone had to do it). (HOAP, is a real thing).
@RySub: Puss in boot sector
@someara: @dakami Oh Backup Where Art Thou
@theharmonyguy: @dakami XSS in the City, with sequel XSS in the City: alert(2)
@ThemsonMester: Oceans 802.11
@Twirrim: 12 Angry Sysadmins
@redtwitdown: @Twirrim Saving Private Ryan [XX……………….] 5.7% 16h47m remaining.
@0x17h: Twelve Code Monkeys
@davehull: APT Pupil
@Mackaber: The Angry Birds
@spridel11: $ whoami Legend
@Pickering: The King’s Speech Recognition Software
@BrightApollo: Saving Private Keys
@fkorling: Apocalype System.currentTimeMillis()
@OhAxi: Teenage Mutant Logo Turtles
@andreasudo: Planet of the APIs
@Ryallcowling: #00FF00 Lantern
@Pickering: The FIOS Connection
@andreasudo: Citizen DANE
@courtneyBolton: Gone with the Meme
@danvesma: Sense and adSensibility
@fkorling: In Her Majesty’s SQL Server
@ThemsonMester: To Kill an AWKing Bird books to movie…
@eikonoklastic: I Know What You Did Last Summer of Code
@mfukar: Port Knocked Up
@mfukar: Fantastic ARCFOUR
@ennor: Top GNU
@SecurityHumor: Two Macs One CUPS
@Madrox: Harry Potter and the Order Of Operations
@hmier: The Manchurian release candidate
@rogueclown: Kernighan and Ritchie do America
@alanpdx: Death Race Condition 2000
@hmier: A beautiful BIND
@Tylos: The Ring 0
@Tylos: ulimitless
@0x17h: The Hunchback of Notre DOM
@afabmedia: Information Technology Came from Outer Space
@Voulnet: BackTrack Mountain
@scottymuse: Sopadish
@davidgropper: The Girl With The Snapdragon Tattoo (cc: @0x17h)
@liambeeton: Independence 0day
@johngineer: 2 Fast 2 Fourier
@l0qii: elements[4]
@Slanderous23: Beauty and the BSD
@dakami: The A* Team (h/t @GKokoris)
@GKokoris: Jurassic PARC
@drb0n3z: James and the Giant Peach Fuzzer
@Mackaber: Indiana Jones and the Last Cross-site scripting
@artisan002: DIMM and DIMMer
@yawnbox: Stateless in Seattle
@addelindh: Low Orbit Ion Canonball Run
@buckstwits: Sudo The Right Thing
@ma1: A Phisher Called from Rwanda
@speno: Enemy Minecraft
@Rzieher: Sheldon Island
@Rzieher: The Men Who Stare at Goatse
@damphi: The good, the bad and the code you inherited from your predecessor
@damphi: Python on a plane
@agnat: 007 – License to … read, write and execute
@b4seb4nd: A League of Their Chown
@damphi: Deep Packet Inspector Gadget
@damphi: RoboCopy
@hvcco: Terminate and stay resident evil
@Nightwolf42: rm -rf / now
@0x17h: How to Train Your Dragon NaturallySpeaking
@FnordZilla: The Breakpoint Club
@0x17h: Paypal It Forward
@ArchangelAmael: Guess Who’s coming to Audit
@aymro: Google Intentions 2.0
@alech: When Alice Met Bob.
@stronate: We Need to Talk About Kelvin
@ArchangelAmael: The Birth of an Exploit
@meznak: vi for vendetta
@j4rkiLL: Alien vs. Administrator
@ArchangelAmael: The Grapes of $PATH
@artisan002: Two and a Half Men in the Middle
@angbor3D: Wall-E The Garbagecollector
@SeveQ: The Hills have iPhones
@ThemsonMester: Zack and Miri Configure, Make, Make Install a Porno
@artisan002: Dirty Twisted Pair
@swiftruss: Cowboys and Alienware
@mechtroid: Crank IPv4: Chev must surf the internet constantly. If he doesn’t find something funny every minute, he goes into cardiac arrest.
@eikonoklastic: How Stella Got Her Algorithm Back
@Nightwolf42: It’s a wonderful runtime
@KlaasJohansen: The Big Kernel Lock Theory #ReplaceFilmTitlesWithKernel
@Megaglest: Four web browsers and an Internet
@j0sema: LDAP Confidential
@FnordZilla: Clock Generator Orange
@TeethGrind3r: Crouching Tigerteam, Hidden Night Dragon
@_7and6_: “We can’t stop here, this is bash-country!” – Fear and Loathing in /usr/bin #nerdflix
@TeethGrind3r: Citizen Cain & Abel
@damphi: Dick Tracert
@_7and6_: Sex, Lies and Avi-Files
@yooogan: Watchdogmen
@stevelord: No country for old code
@_c_v_e_n: The Seven Layer Itch
@0x17h: Annie Hall-effect sensor
@apilosov: Da Vixie Code
@ArchangelAmael: The Magnificent Seven; layers of the OSI model
@xomexx: One overflow the cuckoo’s nest
@krizzzn: SELECT American FROM London WHERE wolf = 1
@GoddamnGeek: @dakami EIP Man
@0x17h: Malcolm X11
@imlxgr: The Bucket Sort
@ArchangelAmael: Who >iFramed< Roger Rabbit
@Meihrenfacht: @wlet @BeerweasleDev Lost in compilation ROFL
@ex1up: momopeche: Weekend At Bernoulli’s
@SubwayDealer: SHA Wars
@Voulnet: Batman BEGINS; DECLARES;
@Voulnet: netcat in the Hat
@fmiffal: grep -c “Monte Cristo”
@ira_victor: Monty Python and The Advanced Persistent Grail
@HamdanD: Sharepoint must die
@mrdaiber: wget -shorty / i See Deadpeople / wag the .doc / ctrl-s ‘lastdance.txt’
@HeshamAboElMagd: Pulp Function
@EmadMokhtar: RT @Voulnet: this.IsSparta();
@Ahmad_Hadeed: The Matrix[][]
@juphoff: Tim Cook, the Thief, His Wife & Her Lover.
@Voulnet: VBScript She Wrote
@old_man_mose: Indiana Jones and the Mountain of Dew
@Ibrahimism: How I hacked your mother
@f70r1: Grave of the Firefoxes
@otac0nFluX: the /x41 team
@mrdaiber: Alice in WLANd
@jochenWolters: Family GUI
@saschaleib: 0x20, the final frontier #NotAMovieTitle
@MagnusRevang: Enemy of the Statemachine
@f70r1: American History XML – an experienced coder wants to prevent his young colleague from taking the same wrong path that he did.
@cmhscout: Rear Windows 3.1
@imlxgr: 2038
@yeahyeahyens: dude, where’s my cdr?
@deepsec: Game of Inodes.
@lnxkid: The XORcist
@f70r1: The Old Man and the C
@webtonull: Ah, forgot “NoSQL for old men”
@mrngm: gcc -Wall -E
@mrdaiber: my big fat32 Greek wedding
@mrdaiber: revenge of the sithadmin
@kinesias: Mozilla (by Roland Emmerich)
@bluehavana: Sk!diocracy
@Politik2_0: 9 1/2 leaks
@SwieSchnubb: The Hurt Logger
@brx0: I dream of JNI
@mbeison: Murder on the Object Orient Express
@robotviki: Das BOOTP
@0xerror: How I Met Your Motherboard
[redacted]: Break Point
@Bashar3A: She’s Out Of My Scope
@JarrarM: AJAX Shrugged
@brx0: Spongebob O(N^2)Pants

Categories: lulz

Primal Fear: Demuddling The Broken Moduli Bug

February 17, 2012 6 comments

There’s been a lot of talk about this supposed vulnerability in RSA, independently discovered by Arjen Lenstra and James P. Hughes et al, and Nadia Heninger et al. I wrote about the bug a few days ago, but that was before Heninger posted her data. Lets talk about what’s one of the more interesting, if misunderstood, bugs in quite some time.



  • The “weak RSA moduli” bug is almost (and possibly) exclusively found within certificates that were already insecure (i.e. expired, or not signed by a valid CA).
  • This attack almost certainly affects not a single production website.
  • The attack utilizes a property of RSA whereby if half the private key material is shared between two public keys, the private key is leaked. Researchers scaled this method to cross-compare every RSA key on the Internet against every other RSA key on the Internet.
  • The flaw has nothing to do with RSA or “multi-secret” systems. The exact same broken random number generator would play just as much havoc, if not more, with “single-secret” algorithms such as ECDSA.
  • DSA, unlike RSA, leaks the private key with every signature under conditions of faulty entropy. That is arguably worse than RSA which leaks its private key only during generation, only if a similar device emits the same key, and only if the attacker finds both devices’ keys.
  • The first major finding is that most devices offer no crypto at all, and even when they do, the crypto is easily man-in-the-middled due to a presumption that nobody cares whether the right public key is in use.
  • Cost and deployment difficulty drive the non-deployment of cryptographic keys even while almost all systems acquire enough configuration for basic connectivity.
  • DNSSEC will dramatically reduce this cost, but can do nothing if devices themselves are generating poor key material and expecting DNSSEC to publish it.
  • The second major finding is that it is very likely that these findings are only the low hanging fruit of easily discoverable bad random number generation flaws in devices. It is specifically unlikely that only a third of one particular product had bad keys, and the rest managed to call quality entropy.
  • This is a particularly nice attack in that no knowledge of the underlying hardware or software architecture is required to extract the lost key material.
  • Recommendations:
    1. Don’t panic about websites. This has very little to absolutely nothing to do with them.
    2. When possible and justifiable, generate private key material outside your embedded devices, and push the keys into them. Have their surrouding certificates signed, if feasible.
    3. Audit smartcard keys.
    4. Stop buying or building CPUs without hardware random number generators.
    5. Revisit truerand, an entropy source that only requires two desynchronized clocks, possibly integrating it into OpenSSL and libc.
    6. When doing global sweeps of the net, be sure to validate that a specific population is affected by your attack before including it in the vulnerability set.
    7. Start seriously looking into DNSSEC. You are deploying a tremendous number of systems that nobody can authenticate.

If there’s one thing to take away from this entire post, it’s the following line from Nadia Heninger’s writeup:

Only one of the factorable SSL keys was signed by a trusted certificate authority and it has already expired.

What this means, in a nutshell, is that there was never any security to be lost from crackable RSA keys; due to failures in key management, almost certainly all of the affected keys were vulnerable to being silently “swapped out” by a Man-In-The-Middle attacker. It isn’t merely the fact that all the headlines proclaiming “0.2% of websites using RSA are insecure” are straight up false, because the flaws are concentrated on devices. It’s also the case that the devices themselves were already using RSA insecurely to begin with, merely by being deployed outside a key management system.

If there’s a second thing to take away, it’s that this is important research with real actions that should be taken by the security community in response. There’s no question this research is pointing to things that are very wrong with the systems we depend on. Do not make the mistake of discounting this work as merely academic. We have a problem with random numbers, that is even larger than Lenstra and Hughes and Heninger are finding with their particular mechanism.


To grossly simplify, RSA works by:

1) Generating two random primes, p and q. These are private.
2) Multiplying p ad q together into n. This becomes public, because figuring out the exact primes used to create n is Hard.
3) Content is signed or decrypted with p and q.
4) Content is verified or encrypted with n.

It’s obvious that if p is not random, q can be calculated: n/p==q, because p*q==n. What’s slightly less obvious, however, is that if either p or q is repeated across multiple n’s, then the repeated value can be trivially extracted using an ancient algorithm by Euclid called the Greatest Common Denominator test. The trick looks something like this (you can follow along with Python, if you like):

>>> from gmpy import *
>>> p=next_prime(rand(“next”))
>>> q=next_prime(rand(“next”))
>>> p
>>> q
>>> n=p*q
>>> n

Now we have two primes, multiplied into some larger n.

>>> q2=next_prime(rand(“next”))
>>> n2=p*q2
>>> n2

Ah, now we have n2, which is the combination of a previously used p, and a newly generated q2. But look what an attacker with n and n2 can do:

>>> gcd(n,n2)
>>> p

Ta-da! p falls right out, and thus q as well (since again, n/p==q). So what these teams did was gcd every RSA key on the Internet, against every other RSA key on the Internet. This would of course be quite slow — six million keys times six million keys is thirty six trillion comparisons — and so they used a clever algorithm to scale the attack such that multiple keys could be simultaneously compared against one another. It’s not clear what Lenstra and Hughes used, but Heninger’s implementation leveraged some 2005 research from (who else?) Dan Bernstein.

That is the math. What is the impact, upon computer security? What is the actionable intelligence we can derive from these findings?

Uncertified public keys or not, there’s a lot to worry about here. But it’s not at all where Lenstra and Hughes think.


Lenstra and Hughes, in arguing that “Ron was Wrong, Whit was right”, declare:

Our conclusion is that the validity of the assumption is questionable and that generating keys in the real world for “multiple-secrets” cryptosystems such as RSA is signi cantly riskier than for “single-secret” ones such as ElGamal or (EC)DSA which are based on Die-Hellman.

The argument is essentially that, had those 12,000 keys been ECDSA instead of RSA, then they would have been secure. In my previous post on this subject (a post RSA Inc. itself is linking to!) I argued that this paper, while chock-full of useful and important data, failed to make the case that the blame could be laid at the feet of the “multiple-secret” RSA. Specifically:

  • Risk in cryptography is utterly dominated, not by cipher selection, but by key management. It does not matter the strength of your public key if nobody knows to demand it. Whatever the differential risk is that is introduced by the choice of RSA vs. ECDSA pales in comparison to whether there’s any reason to believe the public key in question is the actual key of the desired target. (As it turns out, few if any of the keys with broken moduli, had any reason to trust them in the first place.)
  • There aren’t enough samples of DSA in the same kind of deployments to compare failure rates from one asymmetric cipher to another.
  • If one is going to blame a cipher for implementation failures in the field, it’s hard to blame the maybe dozens of implementations of RSA that caused 12,000 failures, while ignoring the one implementation of ECDSA that broke millions upon millions of Playstation 3’s.

But it wasn’t until I read the (thoroughly excellent) blog post from Heninger that it became clear that, no, there’s no rational way this bug can be blamed on multi-secret systems in general or RSA in particular.

However, some implementations add additional randomness between generating the primes p and q, with the intention of increasing security:

p = prng.generate_random_prime()
q = prng.generate_random_prime()
N = p*q

If the initial seed to the pseudorandom number generator is generated with low entropy, this could result in multiple devices generating different moduli which share the prime factor p and have different second factors q. Then both moduli can be easily factored by computing their GCD: p = gcd(N1, N2).

OpenSSL’s RSA key generation functions this way: each time random bits are produced from the entropy pool to generate the primes p and q, the current time in seconds is added to the entropy pool. Many, but not all, of the vulnerable keys were generated by OpenSSL and OpenSSH, which calls OpenSSL’s RSA key generation code.

Would the above code become secure, if the asymmetric cipher selected was the single-secret ECDSA instead of the multi-secret RSA?


No. All public keys emitted would be identical (as identical as they’d be with RSA, anyway). I suppose it’s possible we could see the following construction instead:

ecdsa_private_key=concatenate(first_half, second_half);

…but I don’t think anyone would claim that ECDSA, DSA, or ElGamel is secure in the circumstance where half its key material has leaked. (Personal note: Prodded by Heninger, I’ve personally cracked a variant of RSA-1024 where all bits set to 1 were revealed. It wasn’t difficult — cryptosystems fail quickly when their fundamental assumptions are violated.)

Hughes has made the argument that RSA is unique here because the actions of someone else — publishing n composed of your p and his q — impact an innocent you. Alas, you’re no more innocent than he is; you’re republishing his p, and exposing his q2 as much as he’s exposing your q. Not to mention, in DSA, unlike RSA, you don’t need someone else’s help to expose your private key. A single signature, emitted without a strong RNG, is sufficient. As we saw from the last major RNG flaw, involving Debian:

Furthermore, all DSA keys ever used on affected Debian systems for signing or authentication purposes should be considered compromised; the Digital Signature Algorithm relies on a secret random value used during signature generation.

The fact that Lenstra and Hughes found a different vulnerability rate for DSA keys than RSA keys comes from the radically different generator for the former — PGP keys as generated by GNU Privacy Guard or PGP itself. This is not a thing commonly executed on embedded hardware.

There is a case to be made for the superiority of ECDSA over RSA. Certainly this is the conclusion of various government entities. I don’t think anyone would be surprised if the latter was broken before the former. But this situation, this particular flaw, reflects nothing about any particular asymmetric cipher. Under conditions of broken random number generation, all ciphers are suspect.

Can we dismiss these findings entirely, then?

Lets talk about that.


The headlines have indeed been grating. “Only 99.8% of the worlds PKI uses secure randomness”, said TechSnap. “EFF: Tens of thousands of websites’ SSL “offers effectively no security” from Boing Boing. And, of course, from the New York Times:

The operators of large Web sites will need to make changes to ensure the security of their systems, the researchers said.

The potential danger of the flaw is that even though the number of users affected by the flaw may be small, confidence in the security of Web transactions is reduced, the authors said.

“Some people may say that 99.8 percent security is fine,” he added. That still means that approximately as many as two out of every thousand keys would not be secure.

The operators of large web sites will not need to make any changes, as they are excluded from the population that is vulnerable to this flaw. It is not two out of every thousand keys that is insecure. Out of those keys that had any hope of being secure to begin with — those keys that participate in the (flawed, but bear with me) Certificate Authority system — approximately none of these keys are threatened by this attack.

It is not two out of every 1000 already insecure keys that are insecure. It is 1000 of 1000. But that is not changed by the existence of broken RSA moduli.

To be clear, these numbers do not come from Lenstra and Hughes, who have cautiously refused to disclose the population of certificates, nor from the EFF, who have apparently removed those particular certificates from the SSL Observatory (interesting side project: Diff against the local copy). They come from Heninger’s research, which not only discloses who isn’t affected (“Don’t worry, the key for your bank’s web site is probably safe”), but analyzes the population that is:

Firewall product X:
52,055 hosts in our SSL dataset
6,730 share public keys
12,880 have factorable keys

Consumer-grade router Y:
26,952 hosts in our SSL dataset
9,345 share public keys
4,792 have factorable keys

Enterprise remote access solution Z:
1,648 hosts in our SSL dataset
24 share public keys
0 factorable

Finally, we get to why this research matters, and what actions should be taken in response to it. Why are large numbers of devices — of security devices, even! — not even pretending to successfully manage keys? And worse, why are the keys they do generate (even if nobody checks them) so insecure?


Practically every device on a network is issued an IP address. Most of them also receive DNS names. Sometimes assignment is dynamic via DHCP, and sometimes (particularly on corporate/professionally managed networks) addressing is managed statically through various solutions, but basic addressing and connectivity across devices from multiple vendors is something of a solved problem.

Basic key management, by contrast, is a disaster.

You don’t have to like the Certificate Authority system. I certainly don’t; while I respect the companies involved, they’re pretty clearly working with a flawed technology. (I’ve been talking about this since 2009, see the slides here. Slide 22 talks about the very intermediate certificate issuance that people are now worrying about with respect to Trustwave and GeoTrust) However…

Out of hundreds of millions of servers on the Internet with globally routable IP addresses, only about six million present public keys to be authenticated against. And of those with keys, only a relatively small portion — maybe a million? — are actually enrolled into the global PKI managed by the Certificate Authorities.

The point is not that the CA’s don’t work. The point is that, for the most part, clients don’t care, nothing is checking the validity of device certificates in the first place. Most devices, even security devices, are popping these huge errors every time the user connects to their SSL ports. Because this is legitimate behavior — because there’s no reason to trust the provenance of a public key, right or wrong — users click through.

And why are so many devices, even in the rare instance that they have key material on their administrative interfaces, unenrolled in PKI? Because it requires this chain of cross organizational interaction to be followed. The manufacturer of the device could generate a key at the factory, or on first boot, but they’re not the customer so they can’t have certificates signed in a customers namespace (which might change, anyway). Meanwhile, the customer, who can easily assign IP addresses and DNS names without constantly asking for anyone’s permission, has to integrate with an external entity, extract key material from the manufacturer’s generated content, provide it to that entity, integrate the response back into the device, pay the entity, and do it all over again within a year or three. Repeat for every single device.

Or, they could just not do all that. Much easier to simply not use the encrypted interface (it’s the Internal Network, after all), or to ignore the warning prompts when they come up.

One of the hardest things to convince people of in security, is that it’s not enough to make security better. Better doesn’t mean anything, it’s a measure without a metric. No, what’s important is making security cheaper. How do we start actually delivering value, without imagining that customers have an infinite budget to react with? How do we even measure cost?

I can propose at least one metric: How many meetings need to be held, to deploy a particular technology? Money is one thing but the sheer time required to get things done is a remarkably effective barrier to project completion. And the further away somebody is from a deploying organization — another group, another company, another company that needs to get paid thus requiring a contract and Purchasing’s involvement — the worst things are.

Devices are just not being enrolled into the CA system. Heninger found 19,610 instances of a single firewall — a security device — and not one of those instances was even pretending to be secure. That’s a success rate not of 99.8%, but of 0.00%.

Whatever good we’ve done on production websites has been met with deafening silence across, “firewalls, routers, VPN devices, remote server administration devices, printers, projectors, and VOIP phones”. Believe me, this abject security failure cares not a whit about the relative merits of RSA vs. ECDSA. The key management penalty is just too damn high.

The solution, of course, is DNSSEC. It’s not entirely true that third parties aren’t involved with IP assignment; it’s just that once you’ve got your IP space, you don’t have to keep returning to the well. It’s the same with DNS — while yes, you need to maintain your registration for, you don’t need to interact with your registrar every time you add or remove a host (unless you’d like to, anyway). In the future, you’ll register keys in your own DNS just as you register IP addresses. It will not be drama. It will not be a particularly big deal. It will be a simple, straightforward, and relatively inexpensive mechanism by which devices are brought into your network, and then if you so choose, recognized as yours worldwide.

This will be very exciting, and hopefully, quite boring.

It’s not quite so simple, though. DNSSEC offers a very nice model — one that will be completely vulnerable to the findings of Lenstra, Hughes, and Heninger, unless we start dealing with these badly generated private keys.


DNSSEC will make key management scale. That is not actually a good thing, if the keys it’s spreading are in fact insecure! What we’re finding here is that a lot of small devices are generating keys insecurely. The biggest mistake we can make here is thinking that only the keys vulnerable to this particular attack, were badly generated.

There are many ways to fail at key generation, that aren’t nearly as trivially discoverable as a massive group GCD. Heninger found that Firewall Vendor Z had over a third of their keys either crackable via GCD or repeated elsewhere (as you’d expect, if both p and q were emitted from identical entropy). One would have to be delusional to expect that the code is perfectly secure two thirds of the time, and only fails in this manner every once in a while. The primary lesson from this incident is that there’s a lot more from where the Debian flaw came from. According to Heninger, much of the hardware they saw was actually running OpenSSL, pretty much the gold standard in available libraries for open cryptographic work.

It still failed.

This particular attack is nice, in that it’s independent of the particular vagaries of a device implementation. As long as any two nodes share either p or q, both p and q will be disclosed. But this is a canary in the coal mine — if this went wrong, so too must a lot more have. What I predict is that, when we crack devices open, we’re going to see mechanisms that rarely repeat, but still have an insufficient amount of entropy to “get the job done” — nodes seeding with MAC addresses (far fewer than 48 bits of entropy, when you think about it), the ever classic clock seeds, even fixed “entropic starting points” stored on the file system are going to be found.

Tens of thousands of random keys being not quite so random is one thing. But that a generic mechanism found this many issues, strongly implies a device specific mechanism will be even more effective. We should have gone looking after the Debian bug. I’m glad Lenstra, Hughes, and Heninger et al. did.


So what to do? Here is my advice.

1) Obviously, don’t panic about any website being vulnerable. This is pretty much a web interface problem.

2) When possible and justifiable, generate private key material outside your embedded devices, and push the keys into them. Have their surrouding certificates signed, if feasible.

This is cryptographic heresy, of course. Private keys should not be moved, as bits are cheap. But, you know, some bits are cheaper than others, and I’ve got a lot more faith now in a PC generating key material at this point than some random box. (No way I would have argued this a few weeks ago. Like I said from the beginning, this is excellent work.) In particular, I’d regenerate keys used to authenticate clients to servers.

3) Audit smartcard keys.

The largest population of key material that Lenstra, Hughes, and Heninger could never have looked at, are the public keys contained within the world’s smartcard population. Unless you’re the manufacturer, you have no way to “look inside” to see if the private keys are being reasonably generated. But you do have access to the public keys, and most likely you don’t have so many of them that the slow mechanism (gcd’ing each moduli against each other moduli) is infeasibly slow. Ping me if you’re interested in doing this.

4) Stop buying or building CPUs without hardware random number generators.

Alright, CPU vendors. There’s not many of you, and it’s time for you to stop being so deterministic. By 2014, if your CPU does not support cryptographically strong random number generation, it really doesn’t belong in anything trying to be secure. Since at this point, that’s everything, please start providing a simple instruction that isn’t hidden in some random chipset somewhere for developers and kernels to use to seed proper entropy. It doesn’t even need to be fast.

I hear Intel is adding a Hardware RNG to “Ivy Bridge”. One would also be nice for Atom.

5) Revisit truerand, an entropy source that only requires two desynchronized clocks, possibly integrating it into OpenSSL and libc.

This is the most controversial advice I’ve ever given, and most of you have no idea what I’m talking about. From the code, written originally seventeen years ago by D. P. Mitchell and modified by the brilliant Matt Blaze (who’s probably going to kill me):

* Truerand is a dubious, unproven hack for generating “true” random
* numbers in software. It is at best a good “method of last resort”
* for generating key material in environments where there is no (or
* only an insufficient) source of better-understood randomness. It
* can also be used to augment unreliable randomness sources (such as
* input from a human operator).
* The basic idea behind truerand is that between clock “skew” and
* various hard-to-predict OS event arrivals, counting a tight loop
* will yield a little bit (maybe one bit or so) of “good” randomness
* per interval clock tick. This seems to work well in practice even
* on unloaded machines…

* Because the randomness source is not well-understood, I’ve made
* fairly conservative assumptions about how much randomness can be
* extracted in any given interval. Based on a cursory analysis of
* the BSD kernel, there seem to be about 100-200 bits of unexposed
* “state” that changes each time a system call occurs and that affect
* the exact handling of interrupt scheduling, plus a great deal of
* slower-changing but still hard-to-model state based on, e.g., the
* process table, the VM state, etc. There is no proof or guarantee
* that some of this state couldn’t be easily reconstructed, modeled
* or influenced by an attacker, however, so we keep a large margin
* for error. The truerand API assumes only 0.3 bits of entropy per
* interval interrupt, amortized over 24 intervals and whitened with
* SHA.

Disk seek time is often unavailable on embedded hardware, because there simply is no disk. Ambient network traffic rates are often also unavailable because the system is not yet on a production network. And human interval analysis (keystrokes, mouse clicks) may not be around, because there is no human. (Remember, strong entropy is required all the time, not just during key generation.)

what’s also not around on embedded hardware, however, is a hypervisor that’s doling out time slices. And consider: What makes humans an interesting thing to build randomness off of, is that we’re operating on a completely different clock and frequency than a computer. We are a “slow oscillator” that chaotically interacts with a fast one.

Well, most every piece of hardware has at least two clocks, and they are not in sync: The CPU clock, and the Real Time Clock. You can add a third, in fact, if you include system memory. All three may be made “slow oscillators” relative to the rest, if only by running in loops. At the extreme scale, you’re going to find slight deviations that can only be explained not through deterministic processes but through the chaotic and limited tolerances of different clocks running at different frequencies and different temperatures.

On the one hand, I might be wrong. Perhaps this research path only ends in tears. On the other hand, it is the most persistent delusion in all of computing that there’s only one computer inside your computer, rather than a network of vaguely cooperative subsystems — each with their own clocks that are in no way synchronized against one another.

It wouldn’t be the first time asynchronous behavior made a system non-deterministic.

Ultimately, the truerand approach can’t make things worse. One of the very nice things about entropy sources is that you can stir all sorts of total crap in there, and as long as there’s at least 80 or so bits that are actually difficult to predict, you’ve won. I see no reason we shouldn’t investigate this path, and possibly integrate it as another entropy source to OpenSSL and maybe libc as well.

(Why am I not suggesting ‘switch your apps over to /dev/random? Because if your developers did enough to break what is already enabled by default in OpenSSL, they did so because tests showed the box freezing up, and aren’t going to appreciate some security yutz telling them to ship something that locks on boot.)

6) When doing global sweeps of the net, be sure to validate that a specific population is affected by your attack before including it in the vulnerability set.

7) Start seriously looking into DNSSEC. You are deploying a tremendous number of systems that nobody can authenticate.


It’s 2012, and we’re still having problems with Random Number Generators. Crazy, but we’re still suffering (mightily) from SQL injection and that’s a “fixed problem” too. This research, even if accompanied by some flawed analysis, is truly important. There are more dragons down this path, and it’s going to be entertaining to slay them.

Categories: Security

Survey is good. Thesis is strange.

February 14, 2012 21 comments

(UPDATE: I’ve written quite a bit more about this subject, in the wake of Nadia Heninger posting some fantastic data on the subject.)

Recently, Arjen Lenstra and James Hughes et al released some excellent survey work in their “Ron Was Wrong, Whit Was Right” paper regarding the quality of keys exposed on the global Internet. Rather than just assume proper generation of public key material, this survey looked at 11.7 million public keys in as much depth as possible.

The conclusion they reached was that RSA, because it has two secrets (the two primes, p and q), is “significantly riskier” than systems using “single-secrets” like (EC)DSA or ElGamel.


Let me be clear. This is a mostly great paper, with lots of solid data on the state of key material on the Internet. We’re an industry that really operates without data, and with this work, we see (for example) that there’s not an obvious Debian-class time bomb floating around out there.

But there’s just no way we get from this survey work, to the thesis that surrounds it.

On the most basic level, risk in cryptography is utterly dominated, not by cipher selection, but by key management. The study found 12,720 public keys. It also found approximately 2.94 million expired certificates. And while the study didn’t discuss the number of certificates that had no reason to be trusted in the first place (being self signed), it did find 5.4M PGP keys.

It does not matter the strength of your public key if nobody knows to demand it. What the data from this survey says, unambiguously, is that most keys on the Internet today have no provenance that can be trusted, not even through whatever value the CA system affords. Key Management — as Whit Diffie himself has said — is The Hard Problem now for cryptography.

Whether you use RSA or DSA or ECDSA, that differential risk is utterly dwarfed by our problems with key management.

Is all this risk out of scope? Given that public key cryptography is itself a key management technology for symmetric algorithms like AES or 3DES, and that the paper is specifically arguing for one technology over another, it’s hard to argue that. But suppose we were to say key management is orthogonal to cryptographic analysis, like buffer overflows or other implementation flaws.

This is a paper based on survey work, in which the empirically validated existence of an implementation flaw (12,720 crackable public keys) is being used to justify a design bias (don’t use a multi-secret algorithm). The argument is that multi-secret algorithms cause crackable public keys.

You don’t just get to cite implementation flaws when they’re convenient.

More importantly, correlation is not causation. That we see a small number of bad keys at the same time we see RSA does not mean the latter caused the former. Alas, this isn’t even correlation. Out of 6,185,372 X.509 certificates seen by the survey, 6,185,230 of them used RSA. That’s all of 142 certificates that didn’t. We clearly do not have a situation where the choice of RSA vs. otherwise is random. Indeed, cipher selection is driven by network effects, historical patents, and government regulation, not the roll of the dice even mere correlation would require.

Finally, if we were to say that a cipher that created 12,720 broken instances had to be suspect, could we ever touch ECDSA again? Sony’s Playstation 3 ECDSA Fixed Nonce hit millions of systems around the world. Thus the fault with this whole “multi-secret” complaint — every cipher has moving parts that can break down. “Nothing is foolproof because fools are so ingenious”, as they say. Even if one accepted the single-sample link between multi-secret and RSA, the purportedly safe “single-secret” systems have also failed in the past, in quite larger numbers when you get down to it.

I don’t mean to be too hard on this paper, which again, has some excellent data and analysis inside. I’ve been strongly advocating for the collection of data in security, as I think we operate more on assumption and rumor than we’d like to admit. The flip side is that we must take care not to fit our data to those assumptions.

Apparently this paper escaped into the popular press.

I think the most important question to be asked is — if 0.2% of the global population of RSA moduli are insecure, what about those attached to unexpired certificates with legitimate issuers? Is that also 0.2%, or less?

(There’s no security differential if there was no security to begin with.)

Categories: Security

How The WPS Bug Came To Be, And How Ugly It Actually Is

January 26, 2012 6 comments

FINDING: WPS was designed to be secure against a malicious access point. It isn’t: A malicious AP recovers the entire PIN after only two protocol exchanges. This server to client attack is substantially easier than the client to server attack, with the important caveat that the latter does not require a client to be “lured”.

SUMMARY: Stefan Viehböck’s WPS vuln was one of the bigger flaws found in all of 2011, and should be understood as an attempt to improve the usability of a security technology in the face of severe and ongoing deficiencies in our ability to manage key management (something we’re fixing with DNSSEC). WPS seems to have started as a mechanism to integrate UI-less consumer electronics into the Wi-Fi fold; it grew to be a primary mechanism for PCs to link up. WPS attempted to use an online “proof of possession” process to mutually authenticate and securely configure clients for APs, including the provisioning of WEP/WPA/WPA2 secrets. In what appears to have been a mechanism to prevent malicious “evil twin” APs from extracting PINs from rerouted clients, the already small PIN was split in half. Stefan noted that this made brute force efforts much easier, as the first half could be guessed independently of the second half. While rate limiting at the AP server is an obvious defense, it turns out to be the only defense, as both not continuing the protocol, and providing a fake message, are trivially detectable signs that the half-PIN was guessed incorrectly. Meanwhile, providing the legitimate response even in the face of an incorrect PIN guess actually leaks enough data for the attacker to brute force the PIN. I further note that, even for the purposes of mutual authentication, the split-PIN is problematic. Servers can immediately offline brute force the first half of the PIN from the M4 message, and after resetting the protocol, they can generate the appropriate messages to allow them to offline brute the second half of the PIN. To conclude, I discuss the vagaries of just how much needs to be patched, and thus how long we need to wait, before this vulnerability can actually be addressed server side. Finally, I note that we can directly trace the existence of this vulnerability to the unclear patent state of Secure Remote Passwords (SRP), which really is the canonical way to do Password Authenticated Key Exchange (PAKE).

Note: I really should be writing about DNSSEC right now. What’s interesting is, in a very weird way, I actually am.

On Usability
Attacks, Online and Off
A Simple Plan
Rate Limiting: The Only Option?
It Gets (Unexpectedly) Worse
On Patching
Right Tech, Wrong Law


Security is not usually a field with good news. Generally, all our discussions center around the latest compromise, the newest vulnerabilities, or even the old bugs that stubbornly refuse to disappear despite our “best practices”. I’m somewhat strange, in that I’m the ornery optimist that’s been convinced we can, must, and will (three different things) fix this broken and critical system we call the Internet.

That being said — fixing the Net isn’t going to be easy, and it’s important people understand why. And so, rather than jump right into DNSSEC, I’m going to write now about what it looks like when even an optimist gets backed into a corner.

Lets talk about what I think has to be considered one of the biggest finds of 2011, if not the biggest: Stefan Viehböck’s discovery of fundamental weaknesses in Wi-Fi Protected Setup, or WPS. Largest number of affected servers, largest number of affected clients, out of some of the most battle hardened engineers in all of technology. Also, hardest to patch bug in recent memory. Maybe ever.(Craig Heffner also found the bug, as did some uncountable number of unnamed parties over the years. None of us are likely to be the first to find a bug…we just have the choice to be the last. Craig did release the nicely polished Reaver for us though, which was greatly appreciated, and so I’m happy to cite his independent discovery.)

It wasn’t supposed to be this way. If any industry group had suffered the slings and arrows of the security community, it was the Wi-Fi Alliance. WEP was essentially a live-fire testing range for all the possible things that could go wrong when using RC4 as a stream cipher. It took a while, but we eventually ended up with the fairly reasonable WPA2 standard. Given either a password, or a certificate, WPA2 grants you a fairly solid encrypted link to a network.

That’s quite the given — for right there, staring us in the face once more, was the key management problem.

(Yes, the very same problem we ultimately require DNSSEC to solve. What follows is what happens when engineers don’t have the right tools to fix something, but try anyway. Please forgive me for not using the word passphrase; when I actually see such phrases in the wild we can talk.)


So it should be an obvious statement, but a system that is not usable, is not actually used. But wireless encryption in general, and WPA2 in particular, actually has been achieving a remarkable level of adoption. I’ve already written about the deployment numbers; suffice it to say, this is one of the brighter spots in real world cryptography. (The only things larger are the telnet->ssh switch, and maybe the widespread adoption of SSL by Top 100 sites in the wake of Firesheep and state level packet chicanery.)

So, if this WPA2 technology is actually being used, something had to be making it usable. That something, it turned out, was an entirely separate configuration layer: WPS, for WiFi Protected Setup. WPS was pretty clearly originally designed as a mechanism for consumer electronic devices to link to access points. This makes a lot of sense — there’s not much in the way of user interface on these devices, but there’s always room to paste a sticker. Not to mention that the alternative, Bluetooth, is fairly complex and heavy, particularly if all you want is just IP to a box.

However, a good quarter to half of all the access points exposing encrypted access today expose WPS not for random consumer devices, but for PCs themselves. After all, home users rather obviously didn’t have an Enterprise Certificate Authority around to issue them a certificate, and those passwords that WPA2 wanted are (seriously, legitimately, empirically) hard to remember.

Not to mention, how does the user deal with WPA2 passwords during “onboarding”, meaning they just pulled the device out of the box?

WPS cleaned all this up. In its most common deployment concept, “Label”, it envisioned a second password. This one would be unique to the device and stickered to the back of it. A special exchange, independent of WPA2 or WPA or even WEP, would execute. This exchange would not only authenticate the client to the server, but also the server to the client. By this mechanism, the designers of WPS would defend against so-called “evil twin” attacks, in which a malicious access point present during initial setup would pair with the client instead of the the real AP. The password would be simplified — a PIN, 8 digits, with 1 digit being a check so the user could be quickly informed they’d typed it wrong.


How could this possibly be safe? Well, one of the finer points of password security is that there is a difference between “online” and “offline” attackers. An online attacker has to interact with the defender on each password attempt — say, entering a password into a web form. Even if a server doesn’t lock you out after a certain number of failures (itself a mechanism of attack) only so many password attempts per second can be reasonably attempted. Such limits do not apply to the offline attacker, who perhaps is staring at a website’s backend database filled with hashes of many users’ passwords. This attacker can attempt as many passwords as he has computational resources to evaluate.

So there does exist a game, in which the defender forces the attacker to interact with him every time a password is attempted. This game can be made quite slow, to the point where as long as the password itself isn’t something obvious (read: chosen by the user), the attacker’s not getting in anytime soon. That was the path WPS was on, and this is what it looked like, as per the Microsoft document that Stefan Viehbock borrowed from in his WPS paper:

There’s a lot going on here, because there’s a lot of different attacks this protocol is trying to defend against. It wants to suppress replay, it wants to mutually authenticate, it wants to create an encrypted channel between the two parties, and it really wants to be invulnerable to an offline attack (remember, there’s only 10^7, or around 23 bits of entropy in the seven digit password). All of these desires combine to make a spec that hides its fatal, and more importantly, unfixable weakness.


Lets simplify. Assume for a moment that there is in fact a secure channel between the client and server. We’re also going to change the word “Registrar” to “Client” and “Enrollee” to “Server”. (Yes. This is backwards. I blame some EE somewhere.) The protocol now becomes something vaguely like:

1. Server -> Client:  Hash(Secret1 || PIN)         
2. Client -> Server:  Hash(Secret2 || PIN), Secret2
3. Server -> Client:  Secret1

The right mental image to have, is one of “lockboxes”: Neither party wants to give the other the PIN, so they give eachother hashes of the PIN combined with long secrets. Then they give eachother the long secrets. Now, even without sending the PINs, they can each combine the secrets they’ve received with the secret PINs they already know, see that the hashes computed are equal to the hashes sent, and thus know that the other has knowledge of the PIN. The transmission of Secrets “unlocks” the ability for the Hash to prove possession of the PIN.

Mutual authentication achieved, great job everyone.

Not quite. What Stefan figured out is that WPS doesn’t actually operate across the entire PIN directly; instead, it splits things up more like:

1. Server -> Client:  Hash(Secret1 || first_half_of_PIN)         
2. Client -> Server:  Hash(Secret2 || first_half_of_PIN), Secret2
3. Server -> Client:  Secret1

First_half_of_PIN is only four digits, ranging from 0000 to 9999. A client sending Hash(Secret2 |first_half_of_PIN) may actually have no idea what the real first half is. It might just be trying them all…but whenever they get it right, Server’s going to send Client Secret! It’s a PIN oracle! Whatever can the server do?


The obvious answer is that the server could rate limit the client. That was the recommended path, but most APs didn’t implement that. (Well, didn’t implement it intentionally. Apparently it’s really easy to knock over wireless access points by just flooding this WPS endpoint. It’s amazing the degree to which global Internet connectivity depends on code sold at $40/box.)

What’s not obvious is the degree to which we really have no other degrees of freedom but rate limiting. My initial assumption, upon hearing of this bug, was that we could “fake” messages — that it would be possible to leave an attacker blind to whether they’d guessed half the PIN.

Thanks to mutual authentication, no such freedom exists. Here’s the corner we’re in:

If we do as the protocol specifies — refuse to send Secret1, because the client’s brute forcing us — then the “client” can differentiate a correct guess of the first four digits from an incorrect guess. That is the basic vulnerability.

If we lie — send, say, Secret_fake — ah, now the client simply compares Hash(Secret1 || first_half_of_PIN) from the first message with this new Hash(Secret_fake || first_half_of_PIN) and it’s clear the response is a lie and the first_half_of_PIN was incorrectly guessed.

And finally, if we tell the truth — if we send the client the real Secret1, despite their incorrect guess of first_half_of_PIN — then look what he’s sitting on: Hash(secret1, first_half_of_PIN) and Secret1. Now he can run an offline attack against first_half_of_PIN.

It takes very little time for a computer to run through 10,000 possibilities.

So. Can’t shut up, can’t lie, and absolutely cannot tell the truth. Ouch.


Aside from rate limiting, can we fix the protocol? It’s surprisingly difficult. Lets remove a bit more simplification, and add second_half_of_PIN:

[M3] 1. Server -> Client:  Hash(Secret1 || first_half_of_PIN), 
                           Hash(Secret3 || second_half_of_PIN)         
[M4] 2. Client -> Server:  Hash(Secret2 || first_half_of_PIN),
                           Hash(Secret4 || second_half_of_PIN), Secret2
[M5] 3. Server -> Client:  Secret1
[M6] 4. Client -> Server:  Secret4
[M7] 5. Server -> Client:  Secret3

So, in this model, the server provides information about the entire PIN, but with Secret1 and Secret3 (really, E-S1 and E-S2) being 128 random bits, the PIN cannot be brute forced. Then the client provides the same mishmash of information about the PIN, now with Secret2 (R-S1) and Secret4 (R-S2). But immediately the client “unlocks” the first half of the PIN to the server, who replies by “unlocking” that same half. Only when the server “unlocks” the first half to the client, does the client “unlock” the second half of the PIN to the server and vice versa.

It’s always a bit tricky to divine what people where thinking when they made a protocol. Believe me, DNSSEC is way more useful than even its designers intended. But it’s pretty clear the designers were trying to make sure that a malicious server never got its hands on the full PIN from the client. That seems to be the thinking from the designer here — Secret4, unlocking second_half_of_PIN from the client to the server, is only sent if the server actually knew first_half_of_PIN back in that first (well, really M3) message.

For such a damaging flaw, we sure didn’t get much in return. Problem is, a malicious “evil twin” server is basically handed first_half_of_PIN right there in the second, M4 message. He’s got Hash(Secret2||first_half_of_PIN) and Secret2. That’s 10,000 computations away from first_half_of_PIN.

Technically, it’s too late! He can’t go back in time and fix M3 to now contain the correct first_half_of_PIN. But, in an error usually only seen among economists and freshman psychology students, there is an assumption that the protocol is “one-shot” — that the server can’t just tell the server that something went wrong, and that client should then start from scratch.

So, in the “evil twin” attack, the attacker resets the session after brute forcing the correct first_half_of_PIN. This allows him to send a correct Hash(Secret1||first_half_of_PIN) in the next iteration of M3, which causes the Secret1 in M5 to be accepted, which causes Secret4 in M6 to be sent to the server thus unlocking second_half_of_PIN.

Again, it’s possible I’m missing the “real reason” for splitting the PIN. But if this was it, not only did the split make it much easier for a malicious client to attack an AP server, but it also made it absolutely trivial for a malicious AP server to attack a client. There are rumors of this pattern elsewhere, and I’m now rather concerned.

(To be entirely honest, I was not intending to find another WPS attack. I was just trying to document Stefan’s work.) Ouch.


Fixing WPS is going to be a mess. There’s a pile of devices out there that simply expect to be able to execute the above protocol in order to discover their wireless configuration, including encryption keys. Even when a more advanced protocol becomes available, preventing the downgrade attack in which an attacker simply claims to be an unpatched client is brutal. Make no mistake, the patch treadmill developed over the last decade is impressive, and hundreds of millions of devices recieve updates on a regular basis.

There’s billions that don’t. Those billions include $40 access points, mobile phones, even large numbers of desktops and laptops. When do you get to turn off an insecure but critical function required for basic connectivity?

Good question. Hard, hard answer. Aside from rate limiting defenses, this flaw is going to be around, and enabled by default, for quite some time. years, at least.

To some extent, that’s the reality of security. It’s the long game, or none at all. Accepting this is a ray of hope. We can talk about problems that will take years to solve, because we know that’s the only scale in which change can occur. The Wi-Fi Alliance deserves credit for playing this game on the long scale. This is a lost battle, but at least this is a group of engineers that have been here before.


Now, what should have been built? Once again, we should have used SRP, and didn’t. SRP is unambigiously the right way to solve the PAKE (Password Authenticated Key Exchange) problem, quite a bit more even than my gratuitious (if entertaining) work with Phidelius which somewhat suffers from offline vulnerability. Our inability to migrate to SRP is usually ascribed to some unclear status relating to patents; that our legal system has been unable to declare SRP covered or not pretty much led to the obviously correct technology not being used.

Somebody should start lobbying or something.

Categories: Security

Salt The Fries: Some Notes On Password Complexity

January 5, 2012 12 comments

Just a small post, because Rob Graham asked.

The question is: What is the differential complexity increase offered by salting hashes in a password database?

The short answer, in bits, is: The square root base 2 log of the number of accounts the attacker is interested in cracking.

Rob wanted me to explain this in a bit more depth, and so I’m happy to. In theory, the basic properties of a cryptographic hash is that it’s infeasible to invert the hash back to the bitstream that forged it. This is used in password stores by taking the plaintext password provided by the user, hashing it, and comparing the hash from the database to the hash of the user provided plaintext. If they match, the user is authenticated. Even if the database is lost, the attack won’t be able to invert the database back to the passwords, and users are protected.

There are, of course, two attacks against this model. The first, surprisingly ignored, is that the plaintext password still passes through the web app front end before it is hash-compared against the value in the database. An attacker with sufficient access to dump the database often has sufficient access to get code running on either the server or client front ends (and you thought cross-site scripting was uninteresting!). Granted, this type of attack reduces exposed users from “everyone” to “everyone who logs in while the site is compromised”. That’d probably be more of an improvement, if we believed attackers were only interested in instantaneous smash-and-grabs and were unable to, or afraid of persisting their threats for long periods of time.


The more traditional threat against password hashes is offline brute forcing. While hashes can’t be inverted, the advent of GPUs means they can be tested at the rate of hundreds of millions to billions per second. So, instead of solving the hard problem of figuring out what input created the given output, just try all the inputs until one happens to equal your desire.

How long will this take? Suppose we can test one billion hashes per second. That’s approximately 2^30, so we can say we can handle 30 bits of entropy per second. If we’re testing eight character passwords with the character set A through Z, a through z, and 0 through 9, that’s (26+26+10)^8, or 218,340,105,584,896 attempts. That’s roughly 2^48, or 48 bits of entropy. We can handle only 30 per second, and 48 – 30 == 18, so it will take approximately 2^18 seconds to run through the entire hash space — that’s 262,144 seconds, or 72 hours.

(Yes, I’m rounding extensively. The goal is to have a general feel for complexity, not to get too focused on the precise arithmetic.)

There are two important oversimplifications going on here. First, passwords selected by humans, even those that use those all important punctuation, numbers, and other forms of l33tsp33k, do not utilize these hardening factors particularly randomly. XKCD has made fun of this in the past. One of the real consequences of all these passwords drops is that we’re getting hard data on the types of password patterns people actually use. So groups like TeamHashCat are selecting their billion-hashes-per-second not in random order, but increasingly close to the likelihood that a given input will actually be somebody’s password.

So how does salting play into this?

There’s a much older optimization for password cracking, than selecting passwords based on complex models of what people use. An unsalted hash is the direct output of hash(password). If two people use the same password, they will have the same password hash. And thus, the work effort to crack passwords amortizes — the same operation to find out if Alice’s password is “abcd1234” also discloses whether Bob’s password is “abcd1234”. Salting changes this, by adding some sort of randomizer (possibly just “Alice” or “Bob”) to the hash function. That way, Alice’s password hash is the hash for “Aliceabcd1234”, while Bob’s password hash is the hash for “Bobabcd1234”. To crack both Alice and Bob’s passwords requires twice as much work.

That’s one extra bit.

And that then is how you back into the complexity increase of salting on password cracking. When you’re cracking an unsalted database of passwords, the more accounts you have, the more “bites at the apple” there are — each attempt may match any one of n hashes. Salting just removes the optimization. If you want to crack 256 accounts instead of 1, you need to work 256 times harder — that’s 2^8, or 8 bits. If you want to crack 1M accounts, you need to work a million times harder — that’s 2^20, or 20 bits.

Note however that the question is not how many accounts there are in total, but how many accounts you’re interested in. If only 256 of the 1M accounts are interesting, salting only wins you 8 bits.

A bigger deal, I think, is memory hardness. Password hash functions can and often are run in a loop, such that the hash function is actually run thousands of times in a serial fashion. This does win ~12 bits of security. However, the loops are still viable within the practical means by which attackers are getting billions of hashing operations per second, i.e. GPUs. GPUs have an architectural limit, however — they can be run out of memory. Requiring each hash process to spend 256M+ of RAM may practically eliminate GPUs from the hash-cracking game entirely, even if the number of interesting accounts is just one.

That’s not game-over for all attackers — million node botnets really do have a couple gigs of RAM available for each cracking process, after all — but it does reduce the population of attackers who get to play. That’s always nice. The best path for memory-hard hashes is Colin Percival’s scrypt, which I’ve been playing with lately inside of my cute (if ill-advised) Phidelius project. scrypt has an odd operational limitation (no libscrypt) but hopefully that can be resolved soon, and we can further refine best practices in this space.

Edit: Ah, some of my commenters have mentioned rainbow tables. These actually do change things slightly, though from an interesting direction. Unsalted password hashes are not merely shared across all accounts in a given database; they’re also shared across all accounts for that entire implementation, at least for each account someone is trying to crack. That means it’s possible to both pregenerate hashes (there’s no need to know a salt in advance, because there isn’t one) and to calculate more hashes for more time (since the work effort can be amortized not just cross-account, but cross-database).

It’s become a bit of a thing to just take password hashes and put them into Google. If they’re MD5, and they’re unhashed, a surprising amount of time you do get an answer. Moral of the story: Salting does keep you out of a hash dataset that may be 24+ bits (16M+) “cheaper” (less expensive per account) to attack than just your own.

Peter Maxwell also noted (read the comments) that it’s important to, say, not have a 16 bit salt. Really? Do people do that?

Categories: Security

Phidelius: Constructing Asymmetric Keypairs From Mere Passwords For Fun and PAKE

January 3, 2012 1 comment

TL;DR: New toy.

$ phidelius
Phidelius 1.0: Entropy Spoofing Engine
Author: Dan Kaminsky /
Description: This code replaces most sources of an application's entropy with
a psuedorandom stream seeded from a password, a file, or a generated
sequence. This causes most cryptographic key generators (ssh-keygen,
openssl, etc) to emit apparently strong keys with a presumably memorable
backdoor. For many protocols this creates PAKE (Password Authenticated
Key Exchange) semantics without the server updating any code or even
being aware of the password stored client side. However, the cost of
blinding the server is increased exposure to offline brute force
attacks by a MITM, a risk only partially mitigatable by time/memory
hard crack resistance.
Example: phidelius -p "ax-op-nm-qw-yi" -e "ssh-keygen -f id_dsa"

Passwords are a problem. They’re constantly being lost, forgotten, and stolen. Something like 50% of compromises are associated with their loss. Somewhere along the way, websites started thinking l33tsp33k was a security technology to be enforced upon users.

So what we’re about to talk about in this post — and, in fact, what’s in the code I’m about to finally drop — is by no means a good idea. It may perhaps be an interesting idea, however.

So! First discussed in my Black Ops of 2011 talk, I’m finally releasing Phidelius 1.0. Phidelius allows a client armed with nothing but a password to generate predictable RSA/DSA/ECC keypairs that can then be used, unmodified, against real world applications such as SSH, SSL, IPsec, PGP, and even BitCoin (though I haven’t quite figured out that particular invocation yet — it’s pretty cool, you could send money to the bearer of a photograph).

Now, why would you do this? There’s been a longstanding question as to how can a server support passwords, without necessarily learning those passwords. The standard exhortations against storing unhashed passwords mean nothing against this problem; even if the server is storing hashed values, it’s still receiving them in plain text. There are hash-based challenge response protocols (“please give me the password hashed with this random value”) but they require the server to then store the password (or a password equivalent) so they can recognize valid responses.

There’s been a third class of solutions, belonging to the PAKE (Password Authenticated Key Exchange) family. These solutions generally come down to “password authenticated Diffie-Helman”. For various reasons, not least of which have been patents, these approaches haven’t gotten far. Phidelius basically “solves” PAKE, by totally cheating. Rather than authenticating an otherwise random exchange, the keypair itself is nothing but an encoding of the password. The server doesn’t even have to know. (If you are a dev, your ears just perked up. Solutions that require only one side to patch are infinitely easier to deploy.) How can this work?

Well, what was the one bug that affected asymmetric key generation universally, corrupting RSA, DSA, and ECC equally? Of course, the Debian Random Number Generator flaw. See, asymmetric keys of all sorts are all essentially built the same way. A pile of bits is drawn from some ostensibly random source (/dev/random, CryptGenRandom, etc). These bits are then accepted, massaged, or rejected, until they meet the required form for a keypair of the appropriate algorithm.

Under the Debian RNG bug, only a rather small subset of bit patterns could be drawn, so the same keys would be generated over and over. Phidelius just makes predictable key generation a feature: We know how to turn a password into a stream of random numbers. We call that “seeding a pseudorandom number generator”. So, we just make the output of a PRNG the input to /dev/random, /dev/urandom, a couple of OpenSSL functions, and some other miscellaneous noise, and — poof — same password in, same keypair out, as so:

# phidelius -p "ax-op-nm-qw-yi" -e "ssh-keygen -f $RANDOM -N ''"
...The key fingerprint is:
# phidelius -p "ax-op-nm-qw-yi" -e "ssh-keygen -f $RANDOM -N ''"
The key fingerprint is:

Now, as I mentioned earlier, it’s not necessarily a particularly good idea to use this technique, and not just because passwords themselves are inherently messy. The biggest weakness of the Phidelius approach is that the public keys it generates are also backdoored with the password. The problem here is subtle. It’s not interesting that the server can brute force the public key back to the password. This is *always* the case, even in properly beautiful protocols like SRP. (In fact, Phidelius uses the PRNG from scrypt, a time and memory hard algorithm, to make brute forcing not only hard, but probably harder than any traditional PAKE algorithm can achieve.)

What’s interesting is that public keys are supposed to be public. So, most protocols expose them at least to MITM’s, and sometimes even publish the things. Yes, there’s quite a bit of effort put into suppressing brute force attacks, but this is a scheme that really threatens you with them.

It also doesn’t help that salting Phidelius-derived keys is pretty tricky. The fact that the exact same password creates the exact same keypair isn’t necessarily ideal. The best trick I’ve found is to embed parameters for the private key in the larger context afforded to most public keys, i.e. the certificate. You’d retrieve the public key, mix in the randomized salt and some useful parameters (like, say, options for scrypt), and reconstruct the private key. This requires quite a bit of infrastructure though.

And finally, of course, this is a rather fragile solution. The precise bit-for-bit use of entropy isn’t standardized; all bits are supposed to be equal. So you’re not just generating a key with ssh-keygen, you’re generating from ssh-keygen 5.3, possibly as emitted from a particular compiler. A proper implementation would have to standardize all this.

So, there are caveats. But this is interesting, and I look forward to seeing how people play with it.

(Oh, why Phidelius? It’s a spell from Harry Potter, which properly understood is the story of the epic consequences of losing one’s password.)

Edit: Forgot! Tom Ritter saw my talk at Black Hat, and posted that he’d actually worked out how to do this sort of password-to-keypair construction for PGP keys. It’s actually a bit tricky. You can see his code here.

Categories: Security