Trivial Passwords Are Worse Than Useless: A Simple Case Study in Entropy

April 7th, 2011 by Patrick Thomas

Apparently an email address I own is similar enough to an Indian surname that I get a fair amount of misdirected business correspondence. Despite protestations that they have the wrong address, one large financial institution however continues to send me account updates (including account numbers, balances and addresses). The documents are sent as password protected PDFs, which might be fine, except that they state in the text of the email that the password is the user’s date of birth in the format DDMMYYYY.

Complexity Fail

Those of you passingly familiar with the concept of entropy no doubt let out a groan there. For the rest, here’s why: using a date of birth reduces the complexity of the password into the realm of “trivially weak”. Entropy is a common measurement of information complexity; how “surprising” a piece of information is, or how “unknown” it is (…stick with me on this). Simply knowing that the password is a date reduces the unknown-ness of that password from a reasonably-secure level to an entirely unacceptable level.

For comparison, if we assume an 8-character password with the 94 standard keyboard symbols, we have an entropy of (8 log2(94) ) = 52.44 bits (or equivalently, just over 6 quadrillion possibilities), which is reasonable for most purposes.

On the other hand, a date isn’t just an 8 character password. It’s not even an 8 character numeric password (with obviously 99,999,999 options, or 26.8 bits of entropy), which would be weak but not laughable. In fact, it’s really a 3 character password: a month, a day, and a year. Those are respectively ~30.44 possibilities  (days per month), 12 possibilities, and 60 possibilities (assuming our account holder was born between 1940 and 2000). In bits, that’s approximate 4.93 + 3.58 + 5.91 = 14.42 bits. An analogous password described in characters we are familiar with would be a three character password made up of: a single number, followed by a single lower-case letter, followed by a single alphanumeric. So, your password options are no different (entropy-wise) than “1aA” or “8q3”, and you didn’t even get to pick your wussy three characters.

Solving 14 bits of Entropy

Let’s put this to work. First, a list of every date between Jan 1, 1940 and Jan 1, 2000. Python is my sketchpad of choice:

from datetime import datetime, timedelta
max_date = datetime(1999, 01, 01)
date = datetime(1940, 01, 01)
day = timedelta(1)
f = open("datelist.txt", "w")
while(date < max_date):
    date = date + day

Now datelist has a properly formatted date for each day in our range. How many possibilities is that?

$ head -n 2 datelist.txt
$ wc -l datelist.txt
21550 datelist.txt

That’s in line with our estimate above. Cool, let’s use that list to break a PDF created with this password scheme. Pdfcrack is a simple open-source password bruteforcing tool that helpfully takes a wordlist.

$ pdfcrack -f SensitiveDoc.pdf -w datelist.txt
PDF version 1.4
Security Handler: Standard
V: 2
R: 3
P: -1028
Length: 128
Encrypted Metadata: True
FileID: 9f86e55a12672dcd9b9a9cd3423303da
U: b89fd170770d5b802423d0ec2ae7ec6d00000000000000000000000000000000
O: 301981f88c00ebdafde32360d24b7ae0f6b8a3e1865ac314cbaec4f7cc7a3f49
found user-password: '13051959'

How long did that take?

$ /usr/bin/time -p pdfcrack -f SensitiveDoc.pdf -w datelist.txt cmd 2>&1  | grep user
found user-password: '13051959'
user 0.20

One fifth of a second. Super secure!

General Advice

So, to wrap up. Less complex passwords are reasonable in a security context where a system can monitor password guessing: web based systems, network logins, etc. Then you can respond with enforced guessing intervals, CAPTCHAs or secondary validation. However, when the attacker can take the data for offline cracking, the required strength of passwords goes way up. Using and trusting weak passwords in this instance caused this company to broadcast sensitive information that it wouldn’t intentionally expose.

The company would be much better off providing users a random 10 character code that they can write down and use to decrypt the account statements (yes, seriously, write down your passwords), or simply asking users to log in for the statement information.

Charlie Brown’s Nightmare Before Christmas

January 26th, 2011 by Patrick Thomas

I always enjoy reading the Christmas Challenges created by Ed Skoudis and Yori Kvitchko over at This year’s puzzle was “The Nightmare Before Charlie Brown’s Christmas” and offered a chance to play around with VoIP, which I don’t get to do much of normally.

The winners were just posted, and my entry got the nod for Best Creative Entry. This is particularly awesome for me since the original Counter Hack (by Skoudis) was one of the first security books I ever bought.

I highly recommend reading through the contest and the answers; as always, the technical walkthrough is hugely informative, and they cover a massive toychest of wicked VoIP hacking utilities. There’s also some pretty nice command line kung foo (hat tip) that makes me remember the power of the Unix philosophy of small tools.

‘Miranda Rights’ for the Internet

October 20th, 2010 by Patrick Thomas

I posted this some time ago in a different forum and was recently asked to repost it here, and I’m happy to get it a wider audience. I think this is important for everyone and equally relevant for any internet user, be they high school students, parents, or yes, we software professionals. There’s plenty more to be said on everything contained below, but I hope a degree of succinctness will set off the core ideas.



I. You have the right to remain silent.

You do not need to blog. You do not need to “Reply to this post.” You
do not need to Get MySpace, and you do not need to Facebook Me. If you
say nothing, the blogosphere will not deflate and strangers on
message boards will not miss your advice. If you say nothing, the
internet will not notice.

II. Anything you say can and will be used against you in the court of public opinion.

Nothing on the internet is private. Your real name, your AIM handle,
your livejournal, and the email address you had in high school are all
out there for anyone who cares to look. Just because you don’t know
how to find it doesn’t mean it can’t be found. The internet has a very
long memory. You should be willing to bet that it’s longer than yours.
Anyone you meet could know things about you that you have forgotten
you ever said. Speak slowly and carefully… there are a lot of people

III. You have the responsibility to be skeptical about everything; if you lack the ability to do so, find someone who will do so on your behalf.

The internet is not a library. The internet is not a newspaper. The
internet is a cacophonous bazaar of peddlers, kooks, and unruly
children sharing the same advertisement littered street corner as
politicians, scientists and parents. There are no signposts that
announce when you’re in the wrong part of town, and no one is going to
tell you when you’re being lied to or misled. An open and  skeptical
mind and a sense of personal responsibility are the rules of the road;
no shirt, no shoes, no service.