Tuesday, December 9, 2014

Don't call it a comeback ... Ok, maybe it is

Has it really been four years since my last post!? Well I guess it has! So a better question is: "What has changed for me to bring this blog back?"

Well last February I  got rid of my apartment, took leave of my job, and hiked all 2185.3 miles of the Appalachian Trail from Georgia to Maine. During that time I realized how much blogging meant to me. Long story short, I've been working with my employer to figure out how I could restart this blog now that I'm back.

I don't want to spend too much time on this news post but I might as well end it on one more question: "What should you expect?" That's up in the air right now and I plan on remaining flexible, but I feel one thing I have to contribute to the research community is the fact that I really do enjoy constructing and running experiments. I may not be the best coder, l33t3st password cracker, or have a million dollar cracking setup, but I do have mad Excel skills and I love digging into algorithms. Right now I'm investigating the new PRINCE tool from At0m, creator of Hashcat, (You can get it here) so hopefully I should have a post up about it in a couple of days.


Balance, I have it

Saturday, October 30, 2010

CCS Paper Part #2: Password Entropy

Round Peg, Square Hole
This is part #2 in a (mumble, cough, mumble) part serious of posts discussing the results published in the paper I co-authored on the effectiveness of passwords security metrics. Part #1 can be found here.

I received a lot of insightful comments on the paper since my last post, (one of the benefits of having a slow update schedule), and one thing that stands out is people really like the idea of password entropy. Here’s a good example:
“As to entropy, I think it would actually be a good measure of password complexity, but unfortunately there's no way to compute it directly. We would need a password database comparable in size (or preferably much larger than) the entire password space in order to be able to do that. Since we can't possibly have that (there are not that many passwords in the world), we can't compute the entropy - we can only try to estimate it in various ways (likely poor)”
First of all I want to thank everyone for their input and support as I really appreciate it. This is one of the few cases though where I’m going to have to disagree with most of you. In fact, as conceited as it sounds, my main takeaway has been that I've done a poor job of making my argument, (or I’m wrong which is always a possibility). So the end result is another post on the exciting topic of password entropy ;)

When I first started writing this post, I began with a long description on the history of Shannon Entropy, how it’s used, and what it measures. I then proceeded to delete what I had written since it was really long, boring, and quite honestly not that helpful. All you need to know is:
  1. Claude Shannon was a smart dude.
  2. No seriously, he was amazing; He literally wrote the first book on modern code-breaking techniques.
  3. Shannon entropy is a very powerful tool used to measure information entropy/ information leakage.
  4. Another way of describing Shannon entropy is that it attempts to quantify how much information is unknown about a random variable.
  5. It’s been effectively used for many different tasks; from proving one time pads secure, to estimating the limits of data compression.
  6. Despite the similar sounding names, information entropy and guessing entropy are not the same thing.
  7. Yes, I’m actually saying that knowing how random a variable is doesn’t tell you how likely it is for someone to guess it in N number of guesses, (with the exception of the boundary cases where the variable is always known – aka the coin is always heads- or when the variable has an even distribution – aka a perfectly fair coin flip).
Ok, I’ll add one more completely unnecessary side note about Shannon Entropy. Ask a crypto guy, (or gal), if the Shannon entropy of a message encrypted with a truly random and properly applied one time pad is equal to the size of the key. If they say “yes”, point and laugh at them. The entropy is equal to that of original message silly!

Hey, do you know how hard it is to make an entropy related joke? I’m trying here…

Anyways, to calculate the entropy of a variable you need to have a fairly accurate estimate of the underlying probabilities of each possible outcome. For example a trick coin may land heads 70% of the time, and tails the other 30%. The resulting Shannon entropy is just a summation of the probability of each event multiplied by the log2 of its probability, (and then multiplied by -1 to make it a positive value). Aka:


So the Shannon entropy of the above trick coin would be -(.7 x log2(.7) + .3 x log2(.3)) which is equal to 0.8812 bits. A completely fair coin flip’s entropy would be equal to 1.0. In addition, the total entropy of different independent variables is additive. This means the entropy of flipping the trick coin and then the fair coin would be .8812 + 1.0 = 1.8812 bits worth of entropy.

I probably should have put a disclaimer above to say that you can live a perfectly happy life without understanding how entropy is calculated.

The problem is that while the Shannon entropy of a system is determined using the probability of the different outcomes, the final entropy measurement does not tell you about the underlying probability distribution. People try to pretend it does though, which is where they get into trouble. Here is a picture, (and a gratuitous South Park reference), that I used in my CCS presentation to describe NIST’s approach to using Shannon entropy in the SP800-63 document:


Basically they take a Shannon entropy value, assume the underlying probability distribution is even, and go from there. Why this is an issue is that when it comes to human generated passwords, the underlying probability distribution is most assuredly not evenly distributed. People really like picking “password1”, but there is always that one joker out there that picks a password like “WN%)vA0pnwe**”. That’s what I’m trying to say when I show this graph:



The problem is not that the Shannon value is wrong. It’s that an even probability distribution is assumed. To put it another way, unless you can figure out a method to model the success of a realistic password cracking session using just a straight line, you’re in trouble.

Let me make this point in another way. A lot of people get hung up on the fact that calculating the underlying probability distribution of a password set is a hard problem. So I want to take a step back and show you this holds true even if that is not the case.

For an experiment, I went ahead and designed a variable that has 100 possible values that occur at various probabilities, (thanks Excel). This means I know exactly what the underlying probability distribution is. This also means I’m able to calculate the exact Shannon entropy as well. The below graph shows the expected guessing success rate against one such variable compared to the expected guessing success generated by assuming the underlying Shannon entropy had an even distribution.


Now tell me again what useful information the Shannon entropy value tells the defender about the resistance of this variable to a guessing attack? What’s worse is the graph below that shows 3 different probability distributions that have approximately the same entropy, (I didn’t feel like playing around with Excel for a couple of extra hours to generate the EXACT same entropy; This is a blog and not a research paper after-all).


These three variables have very different resistance to cracking attacks, even though their entropy values are essentially the same. If I want to get really fancy, I can even design the variables in such a way that the variable with a higher Shannon entropy value is actually MORE vulnerable to a shorter cracking session.


This all comes back to my original point that the Shannon entropy doesn’t provide “actionable” information to a defender when it comes to selecting a password policy. Even if you were able to perfectly calculate the Shannon entropy of a password set, the resulting value still wouldn’t tell you how secure you were against a password cracking session. What you really want to know as a defender is the underlying probably distribution of those passwords instead. That's something I've been working on, but I’ll leave my group’s attempts to calculate that for another post, (hint, most password cracking rule-sets attempt to model the underlying probability distribution because they want to crack passwords as quickly as possible).

Thursday, October 7, 2010

New Paper on Password Security Metrics

I'm in Chicago at the ACM CCS conference, and the paper I presented there: "Testing Metrics for Password Creation Policies by Attacking Large Sets of Revealed Passwords", is now available online.
Since I had the paper and presentation approved through my company's public release office I was given permission to blog about this subject while the larger issue of my blog is still going through the proper channels. Because of that I'm going to limit my next couple of posts to this subject rather than talking about the CCS conference as a whole, but let me quickly point you to the amazing paper "The Security of Modern Password Expiration: An Algorithmic Framework and Empirical Analysis", written by Yinqian Zhang, Fabian Monrose and Michael Reiter. In short, they managed to obtain a great dataset, their techniques were innovative and sound, and there's some really good analysis on how effective password expiration policies really are, (spoiler: forcing users to change their password every six months isn't very useful).

I'd like to first start by acknowledging the other authors who contributed to the "Testing Password Creation Metrics..." paper.
  • Dr. Sudhir Aggarwal - Florida State University: My major professor, who spent I don't know how many hours helping walk me through the subtle intricacies of information entropy.
  • Michael Collins - Redjack LLC: Another data driven researcher, and much cooler than me since he uses GNUPlot instead of Excel ;)
  • Henry Stern - Cisco IronPort: He was the driving force behind getting this paper written. It was over lunch at the Microsoft Digital Crime Consortium, (it's a conference to combat cybercrime, and not a group of people from Microsoft looking to commit digital crime like the name implies...), that the framework for this paper was laid out.
As for the contents of the paper, I'm planning on breaking the discussion about it down into several different posts, with this post here being more of an overview.

When writing this paper, we really had two main goals:
  1. How does the NIST model of password entropy as defined in SP800-63 hold up when exposed to real password datasets and realistic attacks?
  2. How much security is actually provided by typical password creation policies, (aka minimum length, character requirements, blacklists)?
Based on our results, we then looked at the direction we would like password creation policies move to in the future. This ended up with us suggesting how to turn our probabilistic password cracker around, and instead use it as part of a password creation strategy that allows people to create passwords however they like, as long as the probability of the resulting password remains low.

Of all that, I feel our analysis of the NIST password entropy model is actually the most important part of the paper. I know it sounds like an esoteric inside baseball subject, but the use of NIST's password entropy model has a widespread impact on all of us. This is because it provides the theoretical underpinning for most password creation policies out there. Don't take my word for how widespread the use of it is. Check out the Wikipedia article on password strength, (or better yet, read the discussion page) for yourself.

Our findings were that the NIST model of password entropy does not match up with real world password usage or password cracking attacks. If that wasn't controversial enough, we then made the even more substantial claim that the current use of Shannon Entropy to model the security provided by human generated passwords at best provides no actionable information to the defender. At worse, it leads to a defender having an overly optimistic view of the security provided by their password creation policies while at the same time resulting in overly burdensome requirements for the end users.

Getting in front of a room full of crypto experts and telling them that Shannon Entropy wasn't useful to evaluate the security of password creation policies and "We REALLY need to STOP using it", was a bit of a gut clenching moment. That's because the idea of information entropy is fairly central to the evaluation of most cryptographic algorithms. I would have never done it except for the fact that we have a lot of data backing this assertion up. The reason we are making the broader point is because it's tempting to dismiss the flaws in the NIST model by saying that NIST just estimated the entropy of human generated passwords wrong. For example, if you juggle the constants around or perhaps look at word entropy vs character entropy, things will work out. Our point though is not that you can't come up with a fairly accurate Shannon entropy model of human generated passwords. You most assuredly can. It's just that it's not apparent how such a model can provide "actionable information". In addition, the way we currently use Shannon Entropy in evaluating password security policies is fundamentally flawed.

This subject really does require another blog post, but before I head back to Boston I wanted to leave you with one of the graphs from our paper that demonstrates what I'm talking about:


The above graph shows cracking sessions run against passwords that met different minimum length password creation requirements, (aka must be at least seven characters long). The NIST estimated cracking speed is based on the calculated NIST entropy of passwords created under a seven character minimum password creation policy. You may notice that it overestimates the security of the creation policy over shorter cracking sessions, but at the same time doesn't model longer cracking sessions either. This is what I keep on saying that it doesn't provide "actionable intelligence", (third time and counting). When we say "password entropy" what we really want to know is the Guessing Entropy of a policy. Unfortunately, as a community, we keep using Shannon entropy instead. Guessing entropy and Shannon entropy are two very different concepts, but unfortunately there doesn't exist a very good way of calculating the guessing entropy, while calculating the Shannon entropy of a set of text is well documented. This is part of the reason why people keep trying to use Shannon entropy instead.

So I guess I should end this post by saying, if any of this sounds interesting please read the paper ;)

Friday, September 10, 2010

Quick Status Update

This is just a quick post to let you know that I for once have a valid excuse for not updating this blog in a timely manner. I actually found a job! Thanks to everyone who offered help, recommendations and encouragements. The only catch is that right now it's being decided if I have to run my posts through our public release office or not. Don't worry, this blog is not going away regardless of the decision.. It might just gain a few unwilling readers ;)

As to my new company, I'm going to keep that a bit of an open secret. This blog reflects my personal views. I certainly don't speak for them, and I plan on avoiding any topics that have to do with my day job, (Don't worry, I'm not doing any password cracking there).

Once again thanks, and I'll resume posting once I get the OK and can update this blog while complying with company policies. I just want to make sure I handle this situation the right way.

Wednesday, July 28, 2010

Defcon Crack Me if You Can Competition

I'd be remiss if I didn't spend a little time talking about the "Crack Me if you Can" competition at Defcon. It's really been amazing the amount of interest that this contest is drumming up. People are excited; it seems like everyone is refining their mangling rules, putting together new wordlists, and finishing up various password cracking tools. The impact that this is having on the password cracking community as a whole is hard to overstate. Needless to say, I'm a fan of that, and I have a ton of respect for Minga and the folks at KoreLogic for putting this together.

I'll be participating, though I certainly don't plan on winning. What I'm really looking forward to though is the chance to meet with everyone else and learn what other people are doing. I'm hoping this turns into an event like the lockpicking village with the contest being almost besides the point. Of course I might be saying that because I'm going to get creamed as well...

Since I've had a few people ask me about the competition itself, here's my two cents. My biggest concern is that the passwords we will be cracking aren't real. This isn't a criticism. There's no way you could run this competition with real corporate passwords, (well, legally that is...). It's just something to keep in mind. What will be interesting though is applying the techniques learned from the winner, (part of the rules are that you have to disclose your cracking techniques), to other datasets as they become available. That's why I have this blog. I might not be the best password cracker out there, but I can certainly run other people's attacks and plot the results on Excel ;)

If I had to hazard a guess, here's some predictions of mine about the contest:

1) Most passwords will be based on relatively common dictionary words. Way more so than you would find normally.

2) Most of the cracking will center around applying the correct mangling rules. Yes there will be the 'Dictionary123' words, but I expect most 'high score' passwords will have less common rules such as 'xD1ct1onaryx'.

3)There will probably be some LANMAN passwords, so bring your rainbow tables.

4) I expect there to be so many NTLM passwords that rainbow tables for them won't be cost effective.

5) I'll be interested to see if they have any 'exotic' password hashes. WinRAR, TrueCrypt, etc.

6) It'll be a ton of fun ;)

I'll see you guys there.

Saturday, July 3, 2010

Protecting Physical Documents


The above picture is of my Subaru Baja. Sometime last night, someone broke my back window, and stole almost everything from my car, (they apparently did not like my music; btw Punk is not dead). Normally this would be a costly annoyance, but in this case I'm in the process of moving and finding a new job. While most of my stuff is sitting safely in a storage locker, I still had several boxes of various items stored in my back seat, including unfortunately my "to-go" bag.

My to-go bag contained all of my important possessions that I planned on grabbing if my house was in the process of burning down. For example, it contained two thumb drives with backups of all of my work plus assorted other documents. I'm not worried about them though since I used TrueCrypt. Good luck cracking those, which BTW, is the reason I love TrueCrypt. What I am concerned about though is that my social security card, my passport, my birth certificate, my extra banking checks, and a whole lot of other important paper documents were also taken. Visualize everything you don't want to be stolen and you have a pretty good idea of what was in that bag.

Of course the next question is "Why the Hell did you leave such important documents in your car?" My response is, laziness and a poor threat model. I needed several of those documents when getting a job. A drivers license by itself doesn't cut it, as you need at least two other forms of ID. Rather than grab those two documents out of my bag and leave the rest in the storage locker I grabbed the whole thing in case there was anything else I needed. Normally I also wouldn't leave it in my truck if I was planning on going somewhere sketchy, but I was parking in a well lit hotel parking lot, and quite simply I had my hands full hauling my bag of clothes, my suit and my computer bag into my room. Darned if I wanted to make a second trip back down to my car. What's worse though is I rationalized it away by saying that at least my car was locked, unlike my hotel room where any of the housekeeping staff could access it during my stay; Plus my car's never been broken into before... The reason that's worse is because I didn't simply forget my bag; I realized it might be a problem and then actively convinced myself that, "No, everything really is ok".

I have to confess that this is a difficult post for me to write. I was counseled by several friends never to mention this incident to anyone, (besides the cops and other officials of course). Their comments were along the lines of "You're looking for a security job, and you just had your life stolen because you left it in the back of a car. Even I wouldn't hire you now!" Of course that was said in jest, but the concern is real.

I'm willing to take that risk though for several reasons. First of all I'm really angry, both at the person who did this and myself. Actually mostly at myself which is seriously messed up. By talking about this incident hopefully I can gain a bit more control over the situation. Second, I'm a big fan of disclosure. If the above description doesn't sound like a typical computer attack, let me rewrite it for you:
The attacker performed an SQL injection attack against subaru_bajas_rule.com. After gaining access to the database they downloaded the user's social security number, banking information, and other personally identifiable information. Afterwards the attacker performed a 'drop table users' destroying the local copy of the data. When the site administrator was asked about this, he responded that knew SQL injection attacks were common, but he never expected to be targeted by one. As for the reason why the user data was accessible, the administrator admitted the site was in the process of transitioning to a new forum software, and that if the attack happened a week later when the new forum software was in place, this wouldn't have been a problem.
I've always felt that security incidents can happen to anyone, and what's important is to focus on the remediation, and use them as a learning tool to make sure the same attack doesn't happen again. That's one nice thing about having a blog, I'm on record on saying much the same thing when talking about the ZF0 attack, so at least this isn't a new found belief I came to after finding myself completely 0wned ;)

So in the spirit of full disclosure I wanted to talk about this attack in a public forum where hopefully it will benefit other people, and if someone doesn't want to hire me because I'm not perfect, well at least they found out now. So on to a more detailed analysis of the attack:
  1. Only items in the back seat were stolen. Since there was so much stuff it looks like the attacker grabbed what they could, and then left without doing a full search of the car. I just was really unlucky that everything I cared about was in the back seat.
  2. When this happened, my car was parked at the far end of the parking lot, since I'd rather walk than squeeze into a small spot. This was a serious mistake considering all of the stuff I had in my car.
  3. While there was a night watchman, he did not notice the attack. Likewise there were no sensors collecting forensically useful data, (aka cameras, and the thief did not leave any useable fingerprints).
  4. I'm much too focused on digital issues. The fact that I bothered to encrypt my electronic documents and the store them with paper documents that are way more valuable to an attacker shows a serious lack of priorities and/or threat modeling. I'm not saying don't encrypt your files, but simply that I should have taken the same care with my other documents and locked them up in my hotel room safe.
  5. The mindset, "Just because something bad hasn't happened to me in the past means it won't happen to me in the future" is very hard to avoid.
  6. I really hope all that is happening right now is some 16 year old kid is trying to use my passport to buy booze. That being said, I need to plan for much more serious scenarios, hence closing my old bank account rather than just canceling my checks, signing up to a credit check service, etc.
  7. Dealing with issues like this on a holiday weekend is extremely difficult. Canceling my old bank account and moving it to a new one was particularly stressful since I had a couple of hours to do it before the bank closed for the long weekend. Likewise, I will have to wait till Tuesday to have my car window replaced. Of course, cyberattacks never happen during a holiday...
  8. Security policies are important, but what's more important is enforcement of those policies. You really do need some force from on high telling people, "Yes, it is a pain to take a second trip down to your car, but you are going to do it anyway".
  9. Storing all of your valuables in one place has advantages and disadvantages. I still don't know if the idea of a to-go bag was a fundamentally bad idea, but I certainly should have "checked out" my two forms of id from it rather than taking the whole thing.
  10. What makes a fiasco is a cascade of multiple smaller mistakes/failures occurring together. Whether we are talking about the BP oil spill, or a horribly hilarious Peter Pan play, serious problems often are not the result of just one thing going wrong, but several poor decisions.
  11. Combined with my comments in the previous paragraphs, it's really easy to analyze all of the stuff that I did wrong after the fact. The problem is it all of my decisions seemed so reasonable at the time. On the other hand, you can't live a perfectly secure life. What I'm wrestling with right now is how to re-evaluate my personal threat models and learn from this incident without letting it ruin my life.
Well, that about does it for now. Hopefully I can get back to the focus of this blog, the academic study of password cracking techniques, soon. This whole real life security thing can be pretty annoying...

Monday, May 31, 2010

Carders.cc - General Observations and Updates - Part 3

Digging into this data is like watching an episode of Lost. Whenever it seems like one question gets answered, about ten other questions pop up.

Before I get into details, I want to start with a comment Per Thorsheim sent me as to what other password cracking programs support salted sha1 hashes:

The sha1(lowercase_username.password_guess) is at least supported by these:

Extreme GPU Bruteforcer (www.insidepro.com) hashcat and oclhashcat (cpu/gpu respectively) www.hashcat.net

I'm kicking myself for not thinking about hashcat, since it's a extremely powerful password cracker; plus it's free. Unfortunately the GPU version doesn't support the salted sha1 hash type, but even the non-gpu version is quite nice.

As for InsidePro, it also is very good, though it does cost some money. I've had a license-free version of questionable origin offered to me before, but I turned that down. Legality aside, installing pirated software given to you by shady people at a hacker conference would just be stupid...

Also, I've been talking to Ron Bowes over at the excellent skullsecurity blog, and he and some other people are hard at work cracking the passwords. It sounds like they have some serious hardware behind the effort, so expect to see something posted on his site about that in the near future.

OK, now onto the analysis:

First of all, I'm downgrading my opinion of the skill showed by the hackers in their password cracking attack. What I didn't realize before was the extent that the users of carders.cc had been compromised previously. Just about all of the non-trivial passwords that were cracked appear in publicly available input dictionaries which are based on passwords cracked from user submitted hashes - aka hashkiller, insidepro, etc. Please note, I'm not saying they were script kiddies. The attackers were able to target the salted sha1 hash, and they knew where to get some good input dictionaries. It's just that they are not some uber-l33t password crackers, and anyone else using those input dictionaries could crack the same number of passwords in a couple of hours.

What this also means is we might be able to figure out which input dictionaries the attackers were using by looking at the hashes they cracked, vs. what the input dictionary would crack. To demonstrate this, below is a Venn diagram of what an input dictionary the attackers used would look like:


For example, there is a good chance the attackers used the InsidePro Big dictionary, since it cracks the same 598 passwords from the carders.cc list that the attackers cracked. Add in a couple of the other publicly available input dictionaries, and you get real close to the 920 total passwords they managed to crack. To put this in perspective, I've so far managed to crack 62%, (compared to the 53% that the hackers cracked), of the salted Sha1 passwords on my laptop using basic dictionaries with almost no mangling rules. I fully expect other people to blow past that mark.


Next up, initial thoughts about the carders.cc database:

What I really need to do is load all of the tables into my own database so I can do quick SQL queries, vs. manually going through the data by hand. That being said there are a few things that stick out:
  1. There's a lot of userids/password hashes in the carders_smf_members table that did not appear in the write-up.
  2. The MD5 hash is salted, but at least the salt is also available in the carders_smf_members table. The hash itself is a vBulletin3 hash type, MD5(MD5(Password).Salt). Both John the Ripper and Hashcat support this hash type.
  3. I'm not sure if the IP addresses stored in the table are accurate or not, since it looks like the site admins tried to obscure it in the webserver logs, but if they are, the database stores the last two ip addresses used.
  4. Other interesting fields include date joined, number of posts, karma level, last login date, etc.
  5. The above doesn't even begin to get into all of the data contained in the actual posts themselves...
That about it for this update. Let me leave you with one last fact that Ron found out:

Another interesting factoid:

Last MD5 password: 2010-01-10 21:54:01

First SHA1 password: 2010-01-10 22:40:16

So it was on January 10, 2010, later in the evening (in CDT) that they upgraded from vBulletin 3 to SMF.

*shrug* the more you know!