When students write exams, they want to know their score as fast as possible. However, the system that is supposed to deliver this information to the students is usually slow, for a lot of reasons that I will not go into. Where I work, it has become quite usual to publish a pdf on the course website that lists the student ID together with the number of points they scored. Since student IDs are usually anonymous, most people don't see it as a problem. However, the more this practice is used, the less anonymous a student ID actually is: Once there are enough of these lists public, this data might theoretically be used to connect a student to his or her student ID only by knowing which courses he or she attended.
I propose a much better solution. I did not come up with this myself by the way, this is due to a brillian colleague of mine, but I still wanted to share it with the world. (more…)
I recently broke the display of my smartphone and wanted to sell it on eBay. However, I am paranoid about my data. So this is what I did:
* Get a new phone and set it up.
* Encrypt the Phone with Android's builtin encryption feature:
* Set a lock screen password (not a PIN) with a random 16 letter string (at the time of writing, Android won't let you choose longer passwords).
* Go to security in your settings and find the option to encrypt the phone, confirming your password.
* After this is done, open an ADB shell on your phone. Because the phone's internal memory is flash, there might still be areas that contain sensible and unencrypted information. We will overwrite them with random data.
cat /dev/random > /data/junkfile
* If you have a stock Android installed, perform a factory reset. If you do not have a stock Android installed:
* Go to the Google Developers' Factory Images for Nexus Devices page and grab your image.
* Extract that file and find a batch file that will flash this stock distribution on your phone automatically.
Warning. You might really have to try several different USB cables and several different USB ports on your computer before fastboot succeeds. For me it worked on the 3rd port and the 2nd cable, that's 7 failed attempts.
* Take pictures of your old phone with your new phone and make sure to remove all the metadata from those pictures:
for img in $(ls *.jpg); do mogrify -strip $img; done
* Now you're good to go, put that old heap of junk up for sale!
If you have any comments, go ahead.
So you want to have some more control over your android phone? Sure you do. For any, just slightly above userlevel stuff you might want to do with it, you require the tools ADB (the Android Developer Bridge) and Fastboot (Fastboot). Since I am still quite ignorant to all of this, I decided to write a small reminder blagpost for myself on how to get those tools. Oh yea, this is all on Windows. Linux users have package managers and stuff like this just works.
You will need the Java Development Kit. After you got that, you need to get the Android SDK Tools. Download "SDK Tools Only". Once you have installed it, open the SDK Manager. It will want to install a lot of stuff, but unless you actually want to do development, you might not even have to install anything. I installed the following only:
* Android SDK Tools
* Android SDK Platform-tools
* Google USB Drivers
Assuming that %GSDK% is the path where you installed the SDK tools, you will find the applications adb.exe and fastboot.exe in
%GSDK%\Android\android-sdk\platform-tools
You might want to add that to your path, or not. Fun fact. If you use cygwin, you can call adb shell from a cygwin terminal and then invoke bash on the phone, and the cygwin terminal will interpret all the color codes sent back from the phone's bash correctly, so you can have a really comfortable shell open on your phone:
MinTTY on LG G3 sporting CM12
TrueCrypt is pretty dead. We need some options here, and as far as I can see, there are only two three:
* CipherShed. Currently a vanilla fork of TrueCrypt.
* VeraCrypt. A fork of TrueCrypt with some fixes and improvements.
* Keep using TrueCrypt.
Neither of the two alternatives has had an official source code audit or anything. They are both open source. I will give a quick summary of the facts on both forks, concluding that I have no clue and will probably flip a coin roll a D3. Whether these facts are pro or con is up to your discretion.
According to the author in this thread, VeraCrypt was first published on June 22nd 2013, so it has already aged a bit.
In fixing some of the security flaws in TC, they break backwards-compatibility. There is a conversion tool available.
They are on CodePlex and the software is under Microsoft Public License.
Binaries are available for download, cross-platform.
Most relevant longterm plan is the ability to encrypt Windows system partitions/drives on UEFI-based computers (GPT).
So. If you have additional information, let me know in comments or by eMail. I am rattled beyond my usual level of confusion as to what I should do. Currently, I will probably give the VeraCrypt binaries a test ride on some machine.
I was promised (and am paying for) a certain bandwidth $X$. But sites like http://www.speedtest.net/ indicated a bandwidth $Y$ (much smaller than $X$). To gather more empirical data over time and to make sure, that I am not hallucinating, I installed a Zabbix server on a Raspberry Pi ((https://www.zabbix.org/wiki/Zabbix_on_the_Raspberry_Pi_%28OS_Raspbian%29)) and set up a monitoring for my Fritz!Box ((http://znil.net/index.php?title=FritzBox_mit_Zabbix_%C3%BCberwachen_HowTo_mit_Template)).
The data for the item "Fritz!Box DSL-Downstream" clearly indicated that $Y$ was around 6 Mbps. So I called my ISP and they ultimately sent the tech-guy to the rescue: He was at my home around 8:30 in the morning and measured the bandwidth with a small magical device. First at the basement where the cable enters the house and then at my office on the first floor. To my surprise, both measurements indicated a much bigger number $Y'$ (which was bigger than $Y$ and looks much more like $X$). He also left me under the impression, that everything was fine with my internet connection the whole time, which I might have believed. But:
Note the time, when something changed! I rest my case. He doesn't have magic hands. He cheated!
Let $R$ be a ring and $f\in R[X]$ a polynomial with infinitely many zeros in $R$. You might think that $f$ is the zero polynomial, but that is not true if $R$ is not commutative, as this example of the quaternions shows.
What about if $R$ is commutative? I didn't find a counterexample online, but it's easy to give one, and I found this somewhat enlightening. Consider the field $\mathbb{F}_2=\{0,1\}$ with two elements. The polynomial $f=X^2-X\in\mathbb{F}_2[X]$ has two zeros, namely $0$ and $1$. Now consider the Ring $R=\mathbb{F}_2^{\mathbb{N}}=\{ \mathbb{N}\to\mathbb{F}_2\}$. We can think of elements of $R$ as sequences $(0,1,1,0,1,\ldots)$. Now clearly, any such sequence is also a zero of $f$. So $f$ actually vanishes everywhere on $R$, which has infinite size, but $f$ is not the zero polynomial.
The statement is of course true if $R$ is a commutative integral domain.
Sitting on the ICE to Munich, I'm using my Nexus 5 to open a WiFi Hotspot for my laptop. However, I'd like to use USB tethering instead; my mobile is plugged in to charge anyway, and it would also allow me to buy Telekom WiFi for my phone for a day and use it for my laptop, too.
Sadly, it didn't work so easily, and I had to write a small patch for my kernel.
(more…)
It has come to my attention that Mathematicians don't like to ask questions. The main reason seems to be that they are afraid of looking stupid. It has annoyed me for quite some time and it has several severe disadvantages for the field of Mathematics:
* Students don't ask questions in class because it's very important to appear as though you have completely grasped and understood everything immediately. As a result, students learn less and get educated more slowly. Because in the end, it won't help you to only pretend you understood math.
* Many fellow PhD students do not use their real name on pages like math.se and mathoverflow because they fear intellectual persecution if people were to associate their name with completely stupid questions. As a result, it's harder to get in touch and find out who is working on similar problems. Anonymity is fine, I am a big fan. However - if it's out of shame for intellectual curiosity that you remain anonymous, then something in that society needs changing.
I believe that these are symptoms of a severe sickness that has spread in Mathematics and which is causing a chronical occlusion to our metaphorical vascular system of information flow. (more…)
Hello, fellow applied mathematicians and computer scientists, hello also to all the brave physicists who use the [arXiv](http://www.arxiv.org/). Did you know that you can [publish source code and other ancillary files on the arXiv](http://arxiv.org/help/ancillary_files), along with your preprint? If you didn't, this must be great news for you. However, if you ever tried to actually do this, you *might* have been just as confused as me. It's actually quite likely that you were, because as soon as this blogpost has vanished from the front page, I am pretty sure that a google search is what led you here.
> Ancillary files are included with an arXiv submission by placing them in a directory anc at the root of the submission package.
If you are a novice to uploading files to the arXiv, like me, this might be confusing. What is the *submission package*? I only ever submitted a single $\KaTeX$ file! Well, let me put it straight for you.
- In the directory with your .tex file(s), make a directory called anc.
- Place all your source code and stuff in that directory.
- Make a zip file containing all your LaTeX sources and the folder anc.
- Upload that zip file to the arXiv.
Trust me - everything will be fine.
As simple as that. I registered at the website g2play.de some time back to buy a computer game cheap, and since I use fresh eMail addresses for each service where I register, I can say without a doubt that somehow, g2play leaked the eMail address I provided. I received targeted spam to that particular address today and there is no way this eMail address could have gotten into the hands of that spammer by any other means. Shame on you, g2play.
My good friend and colleague Christian Ikenmeyer and I wrote this cute preprint about polynomials and how they can be written as the determinant of a matrix with entries equal to zero, one and indeterminantes. Go ahead and read it if you know even just a little math, it's quite straightforward. The algorithm described in section 3 has been implemented and you can download the code from my website at the TU Berlin. Compilation instructions are in ptest.c, but you will need to get nauty to perform the entire computerized proof.
I recently implemented an algorithm that has to perform checks on all subsets of some large set. A subset of an $n$-sized set can be understood as a binary string of length $n$ where bit $i$ is set if and only if the $i$-th element is in the subset. During my search for code to enumerate such bitstrings, I found the greatest page in the entire internet. If anyone can explain to me how computing the next bit permutation (the last version) works, please do.
Note the time, when something changed! I rest my case. He doesn't have magic hands. He cheated!
Note the time, when something changed! I rest my case. He doesn't have magic hands. He cheated!