Cryptology Trivia Quiz

Answer: Rot3

Caesar used a rotate-by-three cipher to communicate with his military. Each letter in a message is rotated left around the alphabet three places, so that D would become A, P would become M, and A would become X. This method was effective because most people in that age were illiterate, and those who were not had not yet devised cryptanalysis (methods for breaking codes).

Answer: The same operation both encodes and decodes

Rot13 is a simple way to encode data so that it is not legible unless somebody *wants* to read it, because you only need to write one function. Rotating by thirteen places in the standard 26-letter English alphabet will encode plaintext (un-encrypted text), and decode ciphertext (encrypted text).

Whether it is unlucky is just superstition, and as for the other two choices... Somebody could probably break a Rot13 encoded message before you could finish writing it.

Answer: Multiple Caesar ciphers with different rotations

The Vigenère cipher was first invented by Giovan Batista Belaso, although it takes its name from Blaise Vigenère. The basic idea is to take a key word or phrase, and, going one letter at a time, rotate the current letter of the plaintext by the amount that is necessary to rotate A to the current letter of the key. For example, if the key is "AND", the first letter of the plaintext would not be changed (no rotation to move A to A), the second would be rotated by 13 (13 places rotate A to N), and the third letter would be rotated by 3 (3 places to rotate A to D). Once you reach the end of your key, start again. Then the fourth letter of the plaintext will remain the same, the fifth will be rotated by 13, and the sixth by 3, and so on.

Rot13 rotates the letters around the alphabet 13 places, and the Affine cipher involves multiplying and shifting each letter into a new position.

Answer: Frequency analysis

Frequency analysis involves counting the letters in a block of ciphertext and comparing them to known frequencies for letters in the language of the plaintext. For example, in English, the three most common letters are E, T, and A. If you know what the most common letter is in the ciphertext, you can make an educated guess that it corresponds to the letter E, and so on down the line.

Brute force involves testing every single possibility until the correct answer is found, Random chance is impossible to predict (basically brute-force but without any order to the choices), and a One-time pad is a method of encryption.

Answer: 56 bits

Although a DES key is 64 bits long, 8 of those bits are checksums, so the effective key length is 56 bits.

DES was developed by IBM, based on an encryption method called Lucifer. Originally it had a 64-bit key, but after IBM submitted it as a candidate for a US Federal encryption standard, the NSA played with it and reduced the key length to 56 effective bits. Many believed that this was so that the NSA could break codes if they had to, as their computing power was well ahead of anybody else.

Today, basic DES is insufficient; by using the computers in a modern University laboratory, one can brute-force his way through all possible keys within 24 hours. However, 2TDES and 3TDES are still in use, whereby the message is encrypted multiple times using DES with different keys.

Answer: Three

The three key sizes are 128-bit, 192-bit, and 256-bit. AES is the successor to DES (the Data Encryption Standard), with improvements on key sizes, flexibility, and speed. In general, AES is about six times faster than DES on the same hardware, and its smallest acceptable key size is still more than twice as large as the DES standard. And in case that doesn't seem like very much, remember that each bit that you add to a key size doubles the number of possible keys!

Answer: Two prime numbers

It is best if the two prime numbers are large (at least 256 bits apiece, which is approximately equivalent to 85 decimal digits). To generate the key pair, multiply your two prime numbers (p and q) together to get n:

n = pq

Also, compute the totient of n (the number of integers less than n that are coprime with n):

m = Φ(n) = (p - 1)(q - 1)

Now, choose a public key e such that:

1 is less than e which is less than m
lcd(e, m) = 1

That is, e is between 1 and m, and the numbers e and m are coprime.

Finally, choose a private key d such that:

de ≡ 1 (mod m)

The public key is the pair (e, n), and the private key is the pair (d, n).

Answer: Affine

The Affine cipher substitutes one alphabet for another to encode a message, which is why it is a substitution cipher. Other examples are the Caesar cipher and Vigenère cipher.

DES and AES are block ciphers. That means that they take blocks of a message one at a time, and act on them as if each is a single entity, shifting the places of the characters in the block and altering their values in accordance with the key.

RSA is a public-key system which treats portions of a message as numbers, multiplying them by the key, and then taking the modulus with a number common to both the public and private keys.

Answer: They can calculate many possibilities at once

The whole point of a quantum computer is that it takes advantage of the superposition of various states to solve a problem multiple ways at once, without any additional hardware. With a conventional, electronic computer, if you want to solve two problems at the same time, you need two chips. A quantum computer can simply (well, not all that simply) super-impose all of the possibilities into its processor and wait for them all to finish.

Unfortunately, however, there is no way to tell when the computation is complete. Heisenberg's Uncertainty Principle states that if we look at the current state of the processor, we will change it. Basically, you need to wait until you are absolutely certain that it is finished, and even then you can only read the answer once.

Answer: Perfectly secure

A one-time pad is basically like a Vigenère cipher without a repeat of the key. Used correctly, the key should be completely random and *only used once*. That is why it is called a one-time pad. If you use it a second time, it is possible to learn actual information about the message being sent. Unfortunately, both the sender and the receiver need to have copies of the same key, which poses a distribution problem.

If you see a movie or read a book that involves spies or nuclear submarines and a code book, they are probably using one-time pads. After encoding or decoding a message using the specified page in the book, that page should be completely destroyed to prevent somebody else from stealing it and being able to decipher the message. The security of one-time pads depends heavily on the physical security of the keys.