Tag Archives: true

Lets Learn About XOR Encryption

One of the more common things about this generation is the constant desire to write up (type) their thoughts. So many of the conversations from my high school days were long lasting, but quickly forgotten. Today’s generation is much more likely to blog, tweet, write status updates or simply open up a notepad file and write up their thoughts after such a conversation.

When we feel that our thoughts are not ready for public eyes (maybe you want to run your idea by the Patent and Trademark Office before speaking about it) we may seek some form of security to ensure that they stay private. An old fashioned way of doing this was to write in a diary and enclosed it within a lock and key. The mathematical field of encryption also tries to grant privacy by encoding messages so that only people with the necessary information can read them.

The type of encryption I want to speak about today is called XOR encryption. It is based on the logical operation called “exclusive or” (hence the name XOR). The exclusive or operation is true between two logical statements if exactly one of the two statements is true, but not both statement. This can be represented with the following truth table

Input 1 Input2 XOR Result
T T F
T F T
F T T
F F F

XOR Encryption is particularly useful in this day and age because we every character we type is understood by the computer as a sequence of zeros and ones. The current standard encoding that is used is Unicode (also known as UTF-8). Under this encoding the letter ‘a’ is represented as the binary string ‘01100001’. Similarly every letter, number and special character can be represented as its own binary string. These binary strings are just an assignment of numbers to these characters so that we can to help represent them in the computer. The numbers can the be thought of in base 10, which is how we generally think about numbers, or in base 2 which is how computers generally work with numbers (or a number of other ways). The way we would use these binary strings in encoding is first by translating a text from human-readable text to machine readable text via its binary string. For example, the word “Invincible”, we would get the following binary strings:

Letter Unicode in base 10 Unicode in base 2
I 73 01001001
n 110 01101110
v 118 01110110
i 105 01101001
n 110 01101110
c 99 01100011
i 105 01101001
b 98 01100010
l 108 01101100
e 101 01100101

To encrypt the message we need a key to encode the message and will simply perform an XOR operation on the key and every character in the string. Similarly, do decrypt the message we perform XOR operation on the key and every character in the encoded message. This means that the key (much like a normal key to a diary) must be kept private and only those whom the message is to be shared between have access to it.

Here is a link to the script where you can check out XOR Encrpytio. Try it out and let me know what you think.

Learning About Truth Tables

Here is a link to my sample Truth Table Generator.

Truth Tables Generator Image

An important part of mathematics and computer programming is understanding conditional expressions. These are statements that generally read like “if [condition is true], then [execute a sequence of statements]”. A simple example of this is that if we wanted to print out every even number, our conditional would be if (2 divides into x with remainder 0) then print x, or in JavaScript

if (x % 2 == 0)
{
document.write(“x”);
}

Conditional expressions belong to the world of Boolean logic. These are expressions that evaluate to true or false, depending on the values of the variables involved in this expression. When we are dealing with real world examples, this is generally a statement like “X is an even number” (for some number X) or “The element x is in the set Y” (for some element x and some set Y). Notice that both the statements can be evaluated as true or false statements. We are interested in understanding the Boolean logic behind combining a number of these expressions, and understanding how the evaluation of the simpler expressions help determine the values of the more complex formulas.

One way of doing this in mathematics is by constructing a truth table. A truth table is a table that shows how a Boolean expression’s value can be computed. The procedure in constructing a truth table is to first add a column to the table for each variable involved in the expression. Then we compute the value of each sub-expression of the expression in its own column until we have computed the entire expression in the final column.

There are four logical operators that we will be working with

– The negation operator (¬P), which returns true if the variable P is false, and returns false otherwise.
– the or operator (P ? Q), which returns true if P is true or Q is true, or if both are true, and returns false otherwise.
– the and operator (P ? Q), which returns true if both P and Q are true, and returns false otherwise.
– the implies operator (P ? Q), which returns false if P is true and Q is false, and returns true otherwise.

An example of this is below:

Suppose we have the following formula:
((Q ? P) ? (¬ (Q ? R)))

The truth table would then be:

P Q R (Q ? R) (¬ (Q ? R)) (Q ? P) ((Q ? P) ? (¬ (Q ? R)))
F F F F T F T
F F T F T F T
F T F F T F T
F T T T F F T
T F F F T F F
T F T F T F F
T T F F T T T
T T T T F T T

First, notice that the truth table has 8 rows. This corresponds to the 8 distinct possible combinations of values for the three variables. The number 8 is also = 23, which is not a mistake. In general, if an expression has n variables, its corresponding truth table will contain 2n rows. In this truth table, column 4 represents the sub-expression (Q ? R). Notice that the only times this expression evaluates to true is when both Q nd R are true. The next column is (¬ (Q ? R)), the negation of (Q ? R), so the values in this column are the opposite of those in the previous column. We follow that up with the column for the sub-expression (Q ? P), which has a value of true only when the variables Q and P both have values of true. And the final column is the expression we started with ((Q ? P) ? (¬ (Q ? R))), but we can now evaluate the expression based on the two previous columns. Notice that the values of true in this column only correspond to when at least one of the previous two columns evaluated to true.

Check out my script on truth tables to see more examples and learn more about truth tables.

How Could You Possibly Love/Hate Math?

Growing up, I never really liked math. I saw it as one of those necessary evils of school. People always told me that if I wanted to do well and get into college, I needed to do well in math. So I took the courses required of a high school student, but I remember feeling utter confusion from being in those classes. My key problem was my inquisitive nature. I really didn’t like being “told” that certain things were true in math (I felt this way in most classes). I hated just memorizing stuff, or memorizing it incorrectly, and getting poor grades because I couldn’t regurgitate information precise enough. If this stuff was in fact “true”, I wanted to understand why. It seemed like so much was told to us without any explanation, that its hard to expect anybody to just buy into it. But that’s what teachers expected. And I was sent to the principal’s office a number of times for what they called “disturbing class”, but I’d just call it asking questions.

At the same time, I was taking a debate class. This class was quite the opposite of my math classes, or really any other class I’d ever had. We were introduced to philosophers like Immanuel Kant, John Stuart Mill, Thomas Hobbes, John Rawls, etc. The list goes on and on. We discussed theories, and spoke of how these concepts could be used to support or reject various propositions. Although these philosophies were quite complex, what I loved was the inquiries we were allowed to make into understanding the various positions. Several classmates and I would sit and point out apparent paradoxes in the theories. We’d ask about them and sometimes find that others (more famous than us) had pointed out the same paradoxes and other things that seemed like paradoxes could be resolved with a deeper understanding of the philosophy.

Hate is a strong word, but I remember feeling that mathematicians were inferior to computer programmers because “all math could be programmed”. This was based on the number of formulas I had learned through high school and I remember having a similar feeling through my early years of college. But things changed when I took a course called Set Theory. Last year, I wrote a piece that somewhat describes this change:

They Do Exist!

Let me tell you a story about when I was a kid
See, I was confused and here's what I did.
I said "irrational number, what’s that supposed to mean?
Infinite decimal, no pattern? Nah, can't be what it seems."
So I dismissed them and called the teacher wrong.
Said they can't exist, so let’s move along.
The sad thing is that nobody seemed to mind.
Or maybe they thought showing me was a waste of time.

Then one teacher said "I can prove they exist to you.
Let me tell you about my friend, the square root of two."
I figured it'd be the same ol' same ol', so I said,
"Trying to show me infinity is like making gold from lead"
So he replies, "Suppose you're right, what would that imply?"
And immediately I thought of calling all my teachers lies.
"What if it can be written in lowest terms, say p over q.
Then if we square both sides we get a fraction for two."

He did a little math and showed that p must be even.
Then he asked, "if q is even, will you start believing?"
I stood, amazed by what he was about to do.
But I responded, "but we don't know anything about q"
He says, "but we do know that p squared is a factor of 4.
And that is equal to 2 q squared, like we said before."
Then he divided by two and suddenly we knew something about q.
He had just shown that q must be even too.

Knowing now that the fraction couldn't be in lowest terms
a rational expression for this number cannot be confirmed.
So I shook his hand and called him a good man.
Because for once I yould finally understand
a concept that I had denied all my life,
a concept that had caused me such strife.
And as I walked away from the teacher's midst,
Excited, I called him an alchemist and exhaled "THEY DO EXIST!"

Aside from its lack of poetic content, I think that many mathematicians can relate to this poem, particularly the ones who go into the field for its theoretic principles. For many of us, Set Theory is somewhat of a “back to the basics” course where we learn what math is really about. The focus is no longer on how well you can memorize a formula. Instead, its more of a philosophy course on mathematics – like an introduction to the theory of mathematics, hence the name Set Theory.

The poem above focuses on a particular frustration of mine, irrational numbers. Early on, we’re asked to believe that these numbers exist, but we’re not given any answers as to why they should exist. The same could be said for a number of similar concepts though – basically, whenever a new concept is introduced, there is a reasonable question of how do we know this is true. This is not just a matter of practicality, but a necessity of mathematics. I mean I could say “lets now consider the set of all numbers for which X + 1 = X + 2”, but if this is true for any X, then it means that 1 equals 2, which we know is not true. So the set I’d be referring to is the empty set. We can still talk about it, but that’s the set I’d be talking about.

So why is this concept of answering the why’s of mathematics ignored, sometimes until a student’s college years? This gives students a false impression of what math really is, which leads to people making statements like “I hate math”, not really knowing what math is about.