Lecture 1: 18th September 2019

Question 1

What is cryptography?

Answer 1

Cryptography refers to secure information and communication techniques derived from mathematical concepts and a set of rule-based calculations called algorithms to transform messages in ways that are hard to decipher.

Question 2

What is plaintext?

Answer 2

Unencrypted information prior to encryption

Question 3

What is ciphertext?

Answer 3

The result of encrypting a plaintext input

Question 4

What is encryption?

Answer 4

Methods of encoding messages or information in such a way that only authorized parties can access it and those who are not authorized cannot. Encryption turns plaintext into ciphertext.

Question 5

What is decryption?

Answer 5

The methods by which encrypted ciphertext is converted back to its original source plaintext.

Question 6

What is a cryptosystem?

Answer 6

A group of cryptographic algorithms needed to implement a particular security service, usually encryption: key generation, encryption, decryption. They usually share a common key.

Question 7

What is an encyption algorithm?

Answer 7

A mathematical procedure in order to apply a predefined code to information to make it illegible to parties unauthorised to read it.

Question 8

What is an encryption key?

Answer 8

A pseudo-random set of data used with encryption algorithms to encrypt data. They are designed to be unique and unpredictable. Only parties with the necessary key can read encrypted data, so their distribution can be used to dictate who is authorised to read certain information.

Question 9

How do you notate an encryption operation? Include ciphertext, the encryption algorithm, key, and plaintext.

Answer 9

C = E(K,P)

where
C = ciphertext
E = encryption algorithm/s
K = encryption key
P = plaintext

Question 10

What is a ciphertext only attack? What are its aims?

Answer 10

Attempts to circumvent and surpass the security of a cryptographic system in which the malevolent party only has access to one or more ciphertexts.

Successful attempts manage to deduce the corresponding plaintexts and/or the encryption key/s being used.

Question 11

What is a known-plaintext attack? What are its aims?

Answer 11

Attempts to circumvent and surpass the security of a cryptographic system in which the malevolent party has one or more pairs of corresponding plaintexts and ciphertexts.

Attackers aim to deduce the cryptographic key/s being used.

Question 12

What is a chosen plaintext attack?

Answer 12

Attempts to circumvent and surpass the security of a cryptographic system in which the malevolent party can obtain an arbitrary number of plaintexts corresponding to chosen plaintext inputs.

Attackers aim to gain any information that allows them to deduce more about the encryption scheme.

Question 13

What is cryptanalysis?

Answer 13

The analysis of cryptographic systems in an attempt to breach their confidentiality and authorisation mechanisms. They aim to gain an algorithm to decrypt all encrypted messages and information.

Question 14

Why are people often the weak points of encryption systems?

Answer 14

Encryption algorithms are often too strong for cryptanalysts to break in reasonable amounts of time with brute force methods. The way people use them often allows them to do so more easily. Pattern recognition and frequency analysis can be performed on frequently recurring words, patterns, or letters, to allow information regarding the encryption mechanisms to be deduced.

Question 15

What is frequency analysis?

Answer 15

The study of letters or groups of letters contained in a ciphertext in an attempt to reveal the message. Letters and groups of letters are not evenly distributed in natural human languages. Mappings can be built from ciphertext frequencies to the language in use to reveal its contents.

Question 16

How can you break the Caesar cipher?

Answer 16

Find the frequencies of each letter, and try to find a grammatically valid mapping in which the frequencies of both letters are equal in both the ciphertext and language as a whole.

Question 17

What is a substitution cipher?

Answer 17

An algorithm for encryption and/or decryption in which letters are swapped to generate a mapping which can be used to switch (substitute) ciphertext and plaintext letters.

Question 18

What is a one time pad?

Answer 18

An encryption algorithm that uses one-time use keys, traditionally on paper “pads”, to encrypt plaintext information. Each key can only be used once and must be equal to or greater than the information in length.

Question 19

How do one time pads work?

Answer 19

Have plaintext input and current key. Sum the ith characters in the plaintext and key to produce the ith character in the ciphertext. Do this for all the characters in the plaintext.

Question 20

Why are one time pads hard to break?

Answer 20

Pseudo-randomness of key (and use of one at all) breaks normal frequencies of letters in ciphertext and removes any direct mapping between plaintext and ciphertext letters. Different key each time makes it even more difficult to decrypt.

Question 21

What is the Vernam Cipher?

Answer 21

OTP with numbers; key a sequence of pseudo-random numbers. Input text characters (all letters) converted to numeric equivalent (a - 1, b - 2, c - 3, etc) and then added to corresponding (ith) key number. The resulting ciphertext number is this sum modulo 26.

Question 22

What is a Vigenere Tableau?

Answer 22

A matrix of the letters a-z as the rows and columns that stand in for the value of key and plaintext letters. To find a ciphertext, find the row corresponding to the current plaintext letter, and then the column corresponding to the key letter within that row.

Question 23

What is a Vigenere Cipher?

Answer 23

An algorithm for encryption and/or decryption which uses Vigenere Tableaus. These are a matrix of the letters a-z as the rows and columns that stand in for the value of key and plaintext letters. To find a ciphertext, find the row corresponding to the current plaintext letter, and then the column corresponding to the key letter within that row.

This cipher cannot be broken by frequency analysis as there is no direct mapping from plaintext to ciphertext characters but can be broken with transposition, but can be by deducing the key length from repeated mappings of words, by chance, if the key is shorter than the ciphertext, and the ciphertext is long enough. Once the key length is known it is straightforward to obtain the plaintext, and afterwards the key.

Question 24

What is a Book Cipher?

Answer 24

An algorithm for encryption and/or decryption which uses a book or a certain section thereof as the encryption key. They will be decided prior to communications and accessible by both parties. OTPs often uses books for keys in this manner.

Question 25

What is a message transposition?

Answer 25

Methods of shifting the position of characters in plaintext by amounts determined by some regular system or algorithm. This produces some permutation (different ordering) of the message.

Question 26

What is a message permutation?

Answer 26

Methods of rearranging the characters in plaintext by some regular system or algorithm.

Question 27

What is pattern analysis?

Answer 27

The study of patterns in one or more letters in repeated sequences of ciphertext in the process of cryptanalysis.

Question 28

How can pattern analysis extend past lone letters?

Answer 28

Natural languages has sequences of commonly repeated groups of multiple letters (n-grams) that can also be analysed in a manner similar to the frequency analysis on sole letters. For example, “rh” and “ent” are common digrams and trigrams in English.

Question 29

What is a column transposition?

Answer 29

When you scramble text by putting it into a grid of n columns and reading output as letters top to bottom and then left to right - i.e. vertically then horizontally rather than horizontally then vertically as in the original.

Question 30

What is the foremost aim of attacks on cryptographic systems?

Answer 30

To find the encryption key/s being used.

Question 31

What is a digram?

Answer 31

A pair of letters.

Question 32

What is a trigram?

Answer 32

A group of three letters.

Question 33

What is n-gram analysis? What can it be used to attack?

Answer 33

The analysis of the frequencies of groups of letters in ciphertexts as part of cryptanalysis.

It can be used to attack ciphertexts which have undergone transpositions. This can be inferred by the frequencies of individual letters being normal but n-grams being scrambled. It can be attacked by finding a n-gram split across two columns by comparing it to the n-gram seen in full within a column elsewhere to find the number of columns to reverse the transposition.

Question 34

What is pigeon code?

Answer 34

Messages carried by pigeons in the National Pigeon Service in WW2 that were encrypted with different encryption algorithms, including OTPs.