csnotes/312/hw/2/2.md

title	author
Homework 2	Alejandro Santillana

Section 1 - Symmetric Key Cryptography

1. The two approaches to attacking a given cipher are using brute force or cryptanalysis. Brute force methods are a simpler method of attack because they naively try all possible inputs for a given output. Cryptanalysis is the process of using the particular characteristics of a cipher to de-cipher the original message.

2. Chosen plain text attacks involve using a given plain text to determine a cipher text. The attacker themselves choose the plain text and have some kind of cipher text available to them. The algorithm on the other hand is not known to the attacker when performing cryptanalysis. In this way a black box approach to reversing the cipher is taken.

3. Cryptography is the study and practice of creating hidden messages from plain messages. Cryptanalysis is the practice of analyzing hidden messages to determine a plain message.

4. Unconditionally secure ciphers are ciphers which are considered secure regardless of how many resources or time are given to reversing the cipher, sometimes due to not having enough information to determine anything useful. Computationally secure ciphers exist on the premise that it is impractical to attempt to break the cipher given a limited amount of compute power.

5. Key distribution is the first major hurdle to ensure that the one time pad is used effectively. Without properly secure channels to share the key the scheme of a one time pad can not be trusted. The other issue is that the key must be longer than the message itself in order to ensure that parts of it are never reused.

Section 2

1. Encode: This is an easy problem.
   Decode: rmij'u uamu xyj.
   Plaintext letter: a b c d e f g h i j k l m n o p q r s t u v w x y z
   Ciphertext letter: m n b v c x z a s d f g h j k l p o i u y t r e w q \

Encode portion: uasi si mncmiw lokngch
Decode portion: wasn't that fun

2. Plain Text: Must see you over Cadogan West. Coming at once.
   Key: largest

Part A:

L A R G E 
S T B C D 
F H I K M
N O P Q U
V W X Y Z
Encoded: UZTB DLG ZPN NWLG TGTUERO VLDB DUHFPE RH WQSRZ

Part B: 
Encoded: UZTB DLG ZPN NWLG TGTUERO VLDB DUHFPE RH WQSRZ

3. Message: "Let's skip class and go to the beach"
   Depth = 4 \

L   S   C   S   G   T   E                                   
 E   K   L   A   O   H   A                                  
  T   I   A   N   T   E   C                                 
   S   P   S   D   O   B   H                               
Message: LSCSGTEEKLAOHATIANTECSPSDOBH