Author: John

RSA with Pseudoprimes

RSA setup Recall the setup for RSA encryption given in the previous post. Select two very large prime numbers p and q. Compute n = pq and φ(n) = (p – 1)(q – 1). Choose an encryption key e relatively prime to φ(n). Calculate the decryption key d such that ed = 1 (mod φ(n)). Publish e and n, and keep d, p, and q secret. φ is Euler’s totient function, defined here. There’s a complication in the first […]

Can I have the last four digits of your social?

Imagine this conversation. “Could you tell me your social security number?” “Absolutely not! That’s private.” “OK, how about just the last four digits?” “Oh, OK. That’s fine.” When I was in college, professors would post grades by the last four digits of student social security numbers. Now that seems incredibly naive, but no one objected […]

RSA encryption exponents are mostly all the same

The big idea of public key cryptography is that it lets you publish an encryption key e without compromising your decryption key d. A somewhat surprising detail of RSA public key cryptography is that in practice e is nearly always the same number, specifically e = 65537. We will review RSA, explain how this default e was chosen, and discuss why […]

Revealing information by trying to suppress it

FAS posted an article yesterday explaining how blurring military installations out of satellite photos points draws attention to them, showing exactly where they are and how big they are. The Russian mapping service Yandex Maps blurred out sensitive locations in Israel and Turkey. As the article says, this is an example of the Streisand effect, […]

Numerical methods blog posts

I recently got a review copy of Scientific Computing: A Historical Perspective by Bertil Gustafsson. I thought that thumbing through the book might give me ideas for new topics to blog about. It still may, but mostly it made me think of numerical methods I’ve already blogged about. In historical order, or at least in the […]

Simulating identification by zip code, sex, and birthdate

As mentioned in the previous post, Latanya Sweeney estimated that 87% of Americans can be identified by the combination of zip code, sex, and birth date. We’ll do a quick-and-dirty estimate and a simulation to show that this result is plausible. There’s no point being too realistic with a simulation because the actual data that […]

No funding for uncomfortable results

In 1997 Latanya Sweeney dramatically demonstrated that supposedly anonymized data was not anonymous. The state of Massachusetts had released data on 135,000 state employees and their families with obvious identifiers removed. However, the data contained zip code, birth date, and sex for each individual. Sweeney was able to cross reference this data with publicly available […]

Sine of a googol

How do you evaluate the sine of a large number in floating point arithmetic? What does the result even mean? Sine of a trillion Let’s start by finding the sine of a trillion (1012) using floating point arithmetic. There are a couple ways to think about this. The floating point number t = 1.0e12 can only […]

Six degrees of Kevin Bacon, Paul Erdos, and Wikipedia

I just discovered the web site Six Degrees of Wikipedia. It lets you enter two topics and it will show you how few hops it can take to get from one to the other. Since the mathematical equivalent of Six Degrees of Kevin Bacon is Six degrees of Paul Erdős, I tried looking for the […]

Mersenne prime trend

Mersenne primes have the form 2p -1 where p is a prime. The graph below plots the trend in the size of these numbers based on the 50 51 Mersenne primes currently known. The vertical axis plots the exponents p, which are essentially the logs base 2 of the Mersenne primes. The scale is logarithmic, so […]

Spherical trig, Research Triangle, and Mathematica

This post will look at the triangle behind North Carolina’s Research Triangle using Mathematica’s geographic functions. Spherical triangles A spherical triangle is a triangle drawn on the surface of a sphere. It has three vertices, given by points on the sphere, and three sides. The sides of the triangle are portions of great circles running […]

Visualizing data breaches

The image below is a static screen shot of an interactive visualization of the world’s biggest data breaches. The site lets you filter the data by industry and type of breach. See the site for credits and the raw data.

Topping out

There’s an ancient tradition of construction workers putting a Christmas tree on top of a building when it reaches its full height. I happened to drive by a recently topped out building this morning.

Complex exponentials

Here’s something that comes up occasionally, a case where I have to tell someone “It doesn’t work that way.” I’ll write it up here so next time I can just send them a link instead of retyping my explanation. Rules for exponents The rules for manipulating expressions with real numbers carry over to complex numbers […]

Sine sum

Sam Walters posted something interesting on Twitter yesterday I hadn’t seem before: The sines of the positive integers have just the right balance of pluses and minuses to keep their sum in a fixed interval. (Not hard to show.) #math pic.twitter.com/RxeoWg6bhn — Sam Walters ☕️ (@SamuelGWalters) November 29, 2018 If for some reason your browser […]

My Twitter graveyard

I ran into The Google Cemetery the other day, a site that lists Google products that have come and gone. Google receives a lot of criticism when they discontinue a product, which is odd for a couple reasons. First, the products are free, so no one is entitled to them. Second, it’s great for a […]

Poetic description of privacy-preserving analysis

Erlingsson et al give a poetic description of privacy-preserving analysis in their RAPPOR paper [1]. They say that the goal is to … allow the forest of client data to be studied, without permitting the possibility of looking at individual trees. Related posts What is differential privacy? Data privacy consulting [1] Úlfar Erlingsson, Vasyl Pihur, and […]

Searching for Mersenne primes

The nth Mersenne number is Mn = 2n – 1. A Mersenne prime is a Mersenne number which is also prime. So far 50 51 have been found [1]. A necessary condition for Mn to be prime is that n is prime, so searches for Mersenne numbers only test prime values of n. It’s not sufficient for n to be prime […]

Searching for Fermat primes

Fermat numbers have the form Fermat numbers are prime if n = 0, 1, 2, 3, or 4. Nobody has confirmed that any other Fermat numbers are prime. Maybe there are only five Fermat primes and we’ve found all of them. But there might be infinitely many Fermat primes. Nobody knows. There’s a specialized test for […]

Geometry of an oblate spheroid

We all live on an oblate spheroid [1], so it could be handy to know a little about oblate spheroids. Eccentricity Conventional notation uses a for the equatorial radius and c for the polar radius. Oblate means a > c. The eccentricity e is defined by For a perfect sphere, a = c and so e = 0. The eccentricity for earth is […]