Tanya Khovanova's Math Blog

My friend Zeb, aka Zarathustra Brady, invented a new game that uses chess pieces and a chessboard. Before the game, the players put all chess pieces on white squares of the board: white pieces are placed in odd-numbered rows and black pieces are in even-numbered rows. At the beginning all white squares are occupied and all black squares are empty. As usual white starts.

On your turn, you can move your piece from any square to any empty square as long as the number of enemy neighbors doesn’t decrease. The neighbors are defined as sharing a side of a square. Before the game starts each piece has zero enemy neighbors and each empty square has at least one white and one black neighbor. That means that on the first turn the white piece you move will increase the number of neighbors from zero to something. As usual, the player who doesn’t have a move loses.

As you can immediately see, that number of pairs of enemy neighbors is not decreasing through the game. I tried to play this game making a move which minimizes the increase of the pairs of neighbors. I lost, twice. I wonder if there is a simple strategy that is helpful.

It is important that this game is played with chess pieces in order to confuse your friends who pass by. You can see how much time it takes them to figure out that this game is not chess, but rather a Chessnot. Or you can enjoy yourself when they start giving you chess advice before realizing that this is not chess, but rather a Chessnot.

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I heard this puzzle many years ago, and do not remember the origins of it. The version below is from Peter Winkler’s paper Seven Puzzles You Think You Must Not Have Heard Correctly.

Puzzle. Jan and Maria have fallen in love (via the internet) and Jan wishes to mail her a ring. Unfortunately, they live in the country of Kleptopia where anything sent through the mail will be stolen unless it is enclosed in a padlocked box. Jan and Maria each have plenty of padlocks, but none to which the other has a key. How can Jan get the ring safely into Maria’s hands?

I don’t know whether this puzzle appeared before the Diffie-Hellman key exchange was invented, but I am sure that one of them inspired the other. The official solution is that Jan sends Maria a box with the ring in it and one of his padlocks on it. Upon receipt Maria affixes her own padlock to the box and mails it back with both padlocks on it. When Jan gets it, he removes his padlock and sends the box back, locked only with Maria’s padlock. As Maria has her own key, she can now open it.

My students suggested many other solutions. I wonder if some of them can be translated to cryptography.

• Jan can send the ring in a padlock box that is made of cardboard. Maria can just cut the cardboard with a knife.
• Jan can use the magic of the Internet to send Maria schematics of the key so she can either 3d print it or get a professional to forge it. If they are afraid of the schematics getting stolen Jan can send the schematics after the package has been delivered.
• Jan can use a digital padlock and send the code using the Internet.
• Jan can send it in a secret puzzle box that can be opened without a key.
• Maria can smash the padlock with a hammer.

Now that we’ve looked at the Padlock Puzzle, let’s talk about cryptography. I have an imaginary student named Charlie who doesn’t know the Diffie-Hellman key exchange. Charlie decided that he can adapt the padlock puzzle to help Alice send a secret message to Bob. Here’s what Charlie suggests:

Suppose the message is M. Alice converts it to binary. Then she creates a random binary key A and XORs it with M. She sends the result, M XOR A, to Bob. Then Bob creates his own random key B and XORs it with what he receives and sends the result, M XOR A XOR B, back to Alice. Alice XORs the result with her key to get M XOR A XOR B XOR A = M XOR B and sends it to Bob. Bob XORs it with his key to decipher the message.

Each sent message is equivalent to a random string. Intercepting it is not useful to an evil eavesdropper. The scheme is perfect. Or is it?

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Here is a logic puzzle.

Puzzle. You are visiting an island where all people know each other. The islanders are of two types: truth-tellers who always tell the truth and liars who always lie. You meet three islanders—Alice, Bob, and Charlie—and ask each of them, “Of the two other islanders here, how many are truth-tellers?” Alice replies, “Zero.” Bob replies, “One.” What will Charlie’s reply be?

The solution proceeds as follows. Suppose Alice is a truth-teller. Then Bob and Charlie are liars. In this situation Bob’s statement is true, which is a contradiction. Hence, Alice is a liar. It follows, that there is at least one truth-teller between Bob and Charlie. Suppose Bob is a liar. Then the statement that there is one truth-teller between Alice and Charlie is wrong. It follows that Charlie is a liar. We have a contradiction again. Thus, Alice is a liar and Bob is a truth-teller. From Bob’s statement, we know that Charlie must be a truth-teller. That means, Charlie says “One.”

But here is another solution suggested by my students that uses meta considerations. A truth-teller has only one possibility for the answer, while a liar can choose between any numbers that are not true. Even if we assume that the answer is only one of three numbers—0, 1, or 2—then the liar still has two options for the answer. If Charlie is a liar, there can’t be a unique answer to this puzzle. Thus, the puzzle question implies that Charlie is a truth-teller. It follows that Alice must be lying and Bob must be telling the truth. And the answer is the same: Charlie says, “One.”

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You might have noticed that my blogging slowed down significantly in the last several months. I had mono: My brain was foggy, and I was tired all the time. Now I am feeling better, and I am writing again. What better way to get back to writing than to start with some jokes?

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The wife of a math teacher threw him out from point A to point B.

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At the job interview at Google.
—How did you hear about our company?

* * * (submitted by Sam Steingold)

50% of marriages end with divorce. The other 50% end with death.

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People say that I am illogical. This is not so, though this is true.

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Humanity invented the decimal system, because people have 10 fingers. And they invented 32-bit computers, because people have 32 teeth.

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When a person tells me, “I was never vaccinated, and, as you can see, I am fine,” I reply, “I also want to hear the opinion of those who were never vaccinated and died.”

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I will live forever. I have collected a lot of data over the years, and in all of the examples, it is always someone else who dies.

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Just got my ticket to the Fibonacci convention! I hear this year is going to be as big as the last two years put together.

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I am afraid to have children as one day I will have to help them with math.

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