Lecture 1

Question 1

How are the sets m and C related when it comes to cryptography? What is cipher space?

Answer 1

There is a set m of all possible messages, and a set C of all possible “transmitted forms” of messages(known as cipher space). These will be related by a key, a shared secret the sender and receiver have.

Question 2

What is a symmetric cryptosystem?

Answer 2

A pair of functions, encoding takes as input the key and message to produce cipher text, Decoding takes a key and cipher text to produce the original message.

Question 3

What are the four common types of attacks on messages to find the key?

Answer 3

• Ciphertext only.
• Known plaintext(some of a message with known cipher text is known).
• Chosen plaintext(Eavesdropper can send their own message, and find out the associated cipher text).
• Brute force(every possible key is tried, with a garbage detector to find wrong messages).

Question 4

How does a substitution cipher work? What are they weak to?

Answer 4

Replaces every possible key with another key. Some of these are vulnerable to brute force, such as the caesar cipher. But for the most part they re mainly vulnerable to ciphertext only(languages have known letter frequencies and letter combinations), known plaintext(can find most letters), and chosen plaintext attacks(send alphabet).

Question 5

How does a Vigenere cipher work? What is it vulnerable to?

Answer 5

Uses a key word of some length, this key is then looped and added to the mssage to determine the cipher text. If the key is the same length or greater than the message then the eavesdropper cannot gain any information. However if the key repeats it is vulnerable to chosen plaintext(send all A’s to find key), known plaintext(can subtract down to A’s), and ciphertext(work out key length with brute force(key is not likely to be super short or super long and break down into columns, then checking letter frequencies.)