Why Is Password Security So Important?

 Computing, Security  No Responses »
Dec 032013
 

People like me keep banging on about why the security of passwords is so important. We keep telling people they need strong passwords, when what people really want are easy to remember passwords. Of course we keep on saying the same message because not everyone pays any attention.

The truth is that it is possible; or at least partially possible to have both strong passwords and relatively easy to remember ones. But first why is it necessary at all?

The sad fact is that there are criminals out there; not spotty teenagers in basements having some sort of weird fun, but genuine criminals who want your account details for a variety of reasons. Organised crime has moved on from bathtub gin, bank robberies, and drugs realising that (amongst other activities) computer crime can be quite profitable with a lower risk of being caught.

The most obvious accounts targeted by criminals are bank accounts – online access to your bank. Whilst they will target such accounts, criminals will also target the most innocuous accounts as well – your ISP account, or a work account. The lowest level of usage of a stolen account is to send spam; not in vast quantities but even several hundred spams sent in your name can really ruin your day.

And will continue to have a less obvious negative effect over time – your email address will be less trusted by recipients if it has ever been used by a spammer. And of course that is the damage I know of. The criminals may use your account for other purposes.

In fact it is probable that any stolen account has a small but definite value on underground markets such as the Silk Road (or deeper and darker places).

And that is excluding the damage that criminals can more directly cause you by access to all the data contained within your account.

How Do Criminals Get Your Password?

So how do criminals get hold of account passwords? It turns out there are three main methods, and one is only useful in certain circumstances (and happens to be the most technical and so the most interesting to geeks).

Just Ask!

It may seem crazy, but probably the easiest method of obtaining account details is simply to ask for those details! The question is normally dressed up to confuse the situation so that it appears to be a legitimate organisation asking for the password. An email from your bank asking you to login via a provided link; an email from your IT support department asking for your password to increase your mail quota.

The defense against this is to never tell anyone your password. Your password is a method of demonstrating that you are yourself; if you give it away, you let other people pretend to be you.

Don’t do it.

Just Guess!

Some people use passwords so weak that they can be guessed relatively easily – or at least easily when the password guessing is scaled up. If a criminal has a 0.001% chance of guessing a password, but they try 1,000 different accounts with 10 different passwords at 1,000 different sites per day, they can expect to get 100 accounts a day!

The best defense against this sort of attack (for an individual) is to make sure you do not have a weak password – go for one that is long and strong (we’ll get to that later).

Password Cracking

The last method of getting account passwords is only possible with access to the password hashes which normally involves exploiting some kind of vulnerability. Once access to those hashes is obtained, it is possible to use a password cracking dictionary to generate a list of candidate passwords and calculate the password hash for each one. When the hash for a candidate password matches the hash of a real account, you know what the password is.

It shouldn’t be possible for a criminal to get access to password hashes, but they do get access to them on a regrettably frequent basis. In addition, it is not uncommon for password cracking to be used as the ultimate test of whether a password is “strong enough” – if it can be cracked with a reasonable level of resources, it is weak.

The best defense against this kind of attack is again to use a long and strong password.

Long And Strong (And Memorable) Passwords

The best passwords are long and random, but very definitely not memorable – as an example, a typical random password might be Y2JkOGY3OTg0YzY1NGMyNTUxMmUzZDkyNDFhZTU2OWYgIC0K. Not the sort of password anyone would want to remember, although password stores such as LastPass allow the use of such passwords. Certainly worth investigating.

However it still needs a master password and there are other circumstances where passwords you have to remember are essential. In such cases memorable becomes a requirement, but we still need strong passwords.

For most of us, a memorable password is made up of dictionary words, yet we are often told that a word-based password (no matter how cleverly transformed it might be) is a weak password. It turns out to be correct for single word passwords, but multi-word passwords are still relatively strong. A lot weaker than truly random passwords of an equivalent length, but somewhat surprisingly a lot stronger than short truly random passwords.

The mathematics of this gets a bit hairy, so take it on trust – length is the most important factor in determining password strength with certain exceptions (a very long word isn’t strong no matter how long it is).

The XKCD Password strength comic

Stringing together a whole bunch of words may not seem the most sensible way to come up with a memorable password; in fact I’ve been using a five word password for many years, and at this point I can’t forget it! I would suggest though that the XKCD method can be strengthened a wee bit by adding a symbol between every word – pick a random symbol like “@”.

Now pick three to four “random” words, and string them together with your random symbol :-

${word 1}${symbol}${word 2}${symbol}${word 3}${symbol}${word 4}${symbol}

Becomes: four@blatter@pong@zoo@

One thing to watch out for – you should have at least one “unusual” word in the list of random words, and don’t have too many short words – the password trustno1 is a weak password!

Google and Microsoft To Block Child Abuse Images?

 Computing, Security  No Responses »
Nov 182013
 

Today the news comes that Google and Microsoft have agreed to block child abuse images. Great!

Anyone reading (or watching) the news story could be forgiven for thinking that this will solve the problem of child abuse images on the Internet, but that won’t happen. What Microsoft and Google have done is a tiny increment on what they were already doing – instead of just excluding hosts given to them by the Internet Watch Foundation, they are also going to ‘clean up’ the search results for certain searches.

It isn’t blocking child abuse images. The search companies can’t do that; anything who thinks so needs to go and learn a bit more about the Internet which includes the government. Who have of course come out of their rabbit hutch spitting lettuce leaves everywhere, saying that if this action by the search companies isn’t effective they’ll legislate.

Which is just about the clearest evidence so far that the government is completely clueless when it comes to technology; obviously Eton‘s reputation is overstated when it comes to technology education.

People tend to think of child abuse images as being a little bit like anything else you browse to on the Internet – you just search for it, and up it pops. I haven’t tried, but I suspect what you would get is a large number of pages like this one – talking about child abuse images in some way, but no real images. Undoubtedly there are some really dumb child pornographers out there who stick up their filth on ordinary web servers; whereby they’ll quickly get indexed by the search engines and someone law enforcement bods will come pounding on the door.

However the biggest area of child abuse image distribution is likely to be one of the variety of ‘stealth’ Internets … the “dark nets’, or ‘deep web‘.

The later are web sites that cannot be indexed by the search engines for various reasons – password protection, links have never been published, etc. These would be the choice of the not quite so dumb child pornographer.

The former are harder to find – they are roughly analogous to peer-to-peer file sharing networks such as Bittorrent which is widely used for sharing copyrighted material (films, music, etc.). But ‘friend to friend’ file sharing networks are private and not public; you need an invitation to join one. This is where the intelligent child pornographer lurks.

And all the hot air we’ve heard from the government so far is going to do pretty much bugger all about the really serious stuff. If you are a clueless politician reading this, get a clue and ask someone with half a brain cell about this stuff. And don’t invent half-arsed measures before asking someone with a clue about whether they’re likely to be effective or not.

How Strong Are XKCD-Style Passwords? And Other Thoughts

 Computing, Security  No Responses »
Nov 082013
 

One of the odd things about telling people about password security is that you have to learn just a little bit more than what you are saying. This leads to the frustration of not being able to talk about ideas you might have – such as that perhaps xkcd-style passwords (“word1word2word3word4”) are not quite as strong as is made out.

Not that they’re weak of course, and indeed I encourage their use wherever it is inconvenient to use “line noise” style passwords such as “JyP;$u5+Q\hzrU[C”. But how was the strength of the password “correcthorsebatterystaple” calculated? And was that calculation correct?

When we want a quantitative value for the strength of a password, it is traditional to calculate the information entropy of the password for the simple reason that the number of bits of entropy is very quickly turned into the number of password guesses necessary to get the password. Simply calculate 2^(entropy bits) and you have the number of guesses necessary to exhaustively search all possible passwords; of course on average it is only necessary to search through half of the possible passwords to guess the correct password.

To calculate the information entropy of a password is a slightly tricky calculation: entropy = log2 (number of possible symbols) ^ (length of password). Or to be more general: entropy = log2 * (number of different possible passwords). Of course this only applies to truly random passwords, so is strictly speaking the maximum possible entropy. The “log2” is base 2 logarithm which doesn’t usually appear on a calculator, but can be calculated as logx(X) = log(X)/log(2).

If we calculate the information entropy of the “correcthorsebatterystaple” we get :-

$ calc
(suppressed)
; ln(26 ^ (5 + 7 + 6 + 7))/ln(2)
	~117.51099295352730400945

Which is far in excess of the 44 bits calculated in the cartoon.

But is it correct? If you were to translate the “correcthorsebatterystaple” password into Chinese you would get “正馬電池 訂” (don’t blame me for the poor translation) which isn’t quite what I was hoping for, as it is five “symbols” instead of four … because we have four words. If we think of the password as being four symbols long, we have a somewhat different calculation where we raise “something” to the power of 4.

Now if we count all the words in the file /usr/share/dict/words as symbols (and add 96 for the ASCII character set), we end up with a total of 99267 symbols which is greater by far than the minimal 26 symbols that we started with. But the calculation comes out differently :-

$ calc
(suppressed)
; ln(99267 ^ 4)/ln(2)
	~66.39610628854963984846

Which is a lot less … we’ve gone from the verging on overkill on terms of strength towards the lower end of “strong”. But that isn’t all. If we remove all words longer than 8 characters from /usr/share/dict/words we get a total of 31321 which gives a somewhat different result :-

$ egrep -e "^.{1,7}$" /usr/share/dict/words | grep -v "'"  | wc -l
31225
$ calc
(suppressed)
; 31225 + 96
	31321
; ln(31321^4)/ln(2)
	~59.73937061801244336267

Which is now dropping to the “reasonable” category. But that still has way more words in it than are likely to be used in passwords. If we restrict it to the top 5000 most common words in episodes of The Simpsons (it happened to be a good list easily obtainable) then we go down yet again :-

; ln(5096^4)/ln(2)
	~49.26059824902520150092

Which is now in the mid-range of the “reasonable” category. We are still above the xkcd calculated value of 44 bits of entropy (they may have used NIST 800-63 to calculate the entropy). And way higher than the amount of entropy in the typical weak password … which is a simple word, or perhaps a word with a symbol added to the end. The amount of entropy in that sort of password is around 18 bits (when we treat a word as a symbol).

As a result we can conclude that :-

  1. The xkcd method results in much stronger passwords than the typical password (49 > 18).
  2. The xkcd method is much weaker than a truly random password (49 < 164). The 164 comes from calculating the entropy of a random password with a choice of 96 possible symbols.
  3. Nobody could argue that the xkcd method is a lot easier to remember than a truly random password.
  4. The xkcd method can become very weak if an attacker can predict what dictionary of words are used. For instance if passwords are generated from a very restricted set of words (say 25 words), then the entropy drops to just over 18 bits which is in the insanely weak category.

At present (as far as we know), there aren’t any tools out there to attack xkcd-style passwords but there soon will be.

Linux: /dev/urandom ≠ /dev/random

 Computing, Linux, Security  No Responses »
Sep 292013
 

As some people know, the Linux device for generating random numbers (/dev/random) blocks when there isn’t sufficient entropy to safely generate random numbers. But people will still persist on advising using /dev/urandom as an alternative :-

“To sum up, under both Linux and FreeBSD, you should use /dev/urandom, not /dev/random.”

“Just go ahead and use /dev/urandom as is”

“Oracle wants us to move /dev/random and link /dev/urandom”

“You can remove /dev/random and link that to /dev/urandom to help prevent blocking”

Now it is true that /dev/urandom is usually good enough, but to advise people to use /dev/urandom without considering whether it is sufficient or not is irresponsible. True random numbers can be very important for cryptography, and without knowing it, we use cryptography every day; such as when we browse the web, make ssh connections, check PGP keys, etc. Using a weak random number generator can weaken the cryptographic process fatally.

Cameron’s Confused Plans for a “Cleaner Internet”

 Computing, Politics, Security  No Responses »
Jul 232013
 

Sign me up for the perv’s list … I won’t trust a politician to come up with a sensible method of censorship, and neither should you.

Ignoring the civil liberties thing: That politicians with a censorship weapon will tend to over use it, to the eventual detriment of legitimate debate.

How is Cameron’s censorship thing supposed to work? It appears nobody has a clear idea. Probably not even Cameron himself.

It seems to be two separate measures :-

  1. Completely block “extreme” porn: child abuse images, and “rape porn”. Oddly enough, he also claimed that “50 Shades of Grey” would not be banned although there are those who categorise it as rape porn. Interestingly this is nothing new as child abuse images have been blocked for years ineffectively.
  2. An “optional” mechanism for blocking some other mysterious category of porn – the “family filter” mechanism.

Now it all sounds quite reasonable, but firstly let’s take a look at the first measure. Blocking child abuse images sounds like a great idea … and indeed it is something that is already done by the Internet Watch Foundation. Whilst their work is undoubtedly valuable – at the very least it prevents accidental exposure to child abuse images – it probably doesn’t stop anyone who is serious about obtaining access to such porn. There are just too many ways around even a country-wide block.

Onto the second measure.

This means that anyone with an Internet connection has to decide when signing up whether they want to be “family friendly” or if they want to be added to the government’s list of perverts … or possibly the ISP’s list of perverts. Of course, how quickly do you think that list will be extracted and leaked? I’m sure the gutter press is salivating at the thought of getting hold of those lists to see what famous people opt to get all the porn; the same gutter press that won’t be blocked despite publishing pictures that some might say meet the criteria for being classified as porn (see Page 3).

And who decides what gets onto the “naughty list” of stuff that you have to sign up as a perv to see? What is the betting that there will be lots of mistakes?

As we already block access by default to “adult sites” on mobile networks, I have already encountered this problem. Not as you might imagine, but whilst away on a course I used an “app” to locate hostelries around my location. On clicking on the link to take me to a local pub’s web site to see a few more details, I was blocked. The interesting thing here is that the app had no problems telling me where the pub was, but the pub’s web site was blocked. Two standards for some reason?

And there are plenty of other examples of misclassification such as Facebook’s long running problem with blocking access to breast feeding information, hospitals having to remove censorship products so that surgeons could get to breast cancer information sites, etc. I happen to work in a field where sales critters are desperate to sell censorship products, and I’m aware that many places that do install such products have the endless fun of re-classifying sites.

And finally, given this is all for the sake of the children, who thinks that children will come up with ways to get around the “family filter” anyway? It is almost impossible to completely censor Internet access without extreme measures such as pulling the entire country off the Internet – even China with it’s Great Firewall is unable to completely censor Internet activity. Solutions such as proxies, VPN access, and Tor all make censorship impossible to make totally effective. If you are thinking that this is all too technical for children, you are sorely mistaken … for a start it does not take many children able to figure this stuff out as they will distribute their knowledge.

This not to say that a censorship mechanism that you control is not a sensible idea. You can select what to censor – prevent the children getting access to information about the Flying Spaghetti Monster, but block access to other religious sites, etc. And such a product has to be network-wide, to prevent someone plugging in an uncensored device; such as using the OpenDNS FamilyShield (although I have never used it, I believe it to be a good product from independent reports). Of course even DNS blocking can be worked around, but it’s a reasonable effort.

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