- #[[CT255 - Next Generation Technologies II]]
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- **Next Topic:** [[Human Security & Passwords]]
- **Relevant Slides:** ![ct255_02.pdf](../assets/ct255_02_1663458790357_0.pdf)
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- ## SQL Injections
	- What is an **SQL Injection**? #card
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		- An **SQL Injection** is a ***code injection technique*** used to attack data-driven applications, in which malicious SQL statements are inserted for execution.
		- It is a way of exploiting user input & SQL statements to compromise the database & retrieve sensitive data.
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- ## Basic Terminology
	- What is **Cryptography**? #card
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		- **Cryptography** is the art of encompassing the principles & methods of transforming an intelligible message into one that is unintelligible, and then retransforming that message back into its original form.
	- What is **Plaintext**?
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		- **Plaintext** is the ^^original, intelligible message.^^
	- What is **Ciphertext**?
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		- **Ciphertext** is the encrypted messsage.
	- What is a **Cipher**? #card
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		- A **Cipher** is an algorithm for transforming an intelligible message into one that is unintelligible.
	- What is a **Key**? #card
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		- A **Key** is some critical information used by the cipher, known only to the sender & receiver, selected from a **keyspace** (the set of all possible keys).
	- What does **Encipher** mean?
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		- **Enciphering** is the process of converting plaintext into ciphertext using a cipher & a key.
	- What does **Decipher** mean?
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		- **Deciphering** is the process of converting ciphertext back into plaintext using a cipher & a key.
	- What is **Encryption**? #card
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		- **Encryption** is some mathematical function $E_K()$ mapping plaintext $P$ to ciphertext $C$ using the specified key $K$.
		- $$E_K(P) = C$$
	- What is **Decryption**? #card
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		- **Decryption** is some mathematical function ${E_K}^{-1}()$ mapping the ciphertext $C$ to plaintext $P$ using the specified key $K$.
		- $$P={E_K}^{-1}(C)$$
	- What is **Cryptanalysis**? #card
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		- **Cryptanalysis** is the study of principles & methods of transforming an unintelligible message into an intelligible message without knowledge of the key.
	- What is **Cryptology**?
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		- **Cryptology** is the field encompassing both cryptography & cryptanalysis.
	-
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- ## Model of Conventional Cryptosystem
	- ![image.png](../assets/image_1663459919021_0.png){:height 304, :width 610}
- ## Cryptanalysis via Letter Frequency Distribution
	- Human languages are **redundant** - letters are not equally commonly used.
	- In the **English** language:
		- **E** is by far the most common letter followed by T, R, N, I, O, A, and S.
		- Other letters like Z, J, K, Q, and X are fairly rare.
		- Certain letter combinations like **TH** are quite common.
		- ![image.png](../assets/image_1663488626792_0.png)
	-
	- ### C Program for Frequency Analysis of single Characters
		- ```c
		  #include <stdio.h>
		  #include <string.h>
		  #include <ctype.h>
		  
		  int main(int argc, char* argv[]) {
		    FILE* fp;
		    int data[26];
		    char c;
		    
		    memset(data, 0, siezof(data));
		    
		    if (argc != 2) {
		      return(-1);
		    }
		    if (fp = fopen(argv[1], "r" == NULL)) {
		      return(-2);
		    }
		    
		    while(!feof(fp)) {
		      c = toupper(fgetc(fp));
		      
		      if ((c >= 'A') && (c <= 'Z')) {
		        data[c-65]++;
		      }
		    }
		    
		    for (int i = 0; i < 26; i++) {
		      printf("%c:%i\n", i+65, data[i]);
		    }
		    
		    fclose(fp);
		    return(0);
		  }
		  ```
- ## Known Plaintext Attacks (KPA)
	- What is a **Known Plaintext Attack (KPA)**? #card
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		- The **Known Plaintext Attack (KPA)** is an attack model for cryptanalysis where the attacker has access to both:
			- some of, or all of, the plaintext (called a **crib**)
			- the ciphertext
		-
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- ## Caesar Cipher
	- What is a **Caesar Cipher**? #card
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		- A **Caesar Cipher** involves using an offset alphabet to encrypt a message.
		- We can use any shift from 1 to 25 to replace each plaintext letter with a letter a fixed distance away.
		- The **key letter** represents the start of this offset alphabet.
			- For example, a key letter of F means that A -> F, B -> G, and so on.
- ## Playfair Cipher
	- Not even the large number of keys in a monoalphabetic cipher provides security.
		- What is a **monoalphabetic cipher**? #card
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			- A **monoalphabetic cipher** is any cipher in which the letters of the plaintext are mapped to ciphertext letters based on a single alphabetic key.
	- One approach to improving security over monoalphabetic ciphers is to to encrypt ^^multiple letters.^^
		- The **Playfair Cipher** is one example of such an approach.
		- The algorithm was invented by Charles Wheatstone in 1854, but named after his friend Baron Playfair.
	- ### How does the Playfair Cipher work? 
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		- ![image.png](../assets/image_1663491286810_0.png)
		- 1. Create a 5x5 grid of letters; insert the keyword as shown, with each letter only considered once; fill the grid with the remaining letters in alphabetic order.
		- 2. The letters are then encrypted in pairs.
		- 3. Repeats have an "X" inserted.
			- BALLOON -> BA LX LO ON
		- 4. Letters that fall in the same row are replaced with the letter on the right.
			- OK -> GM
		- 5. Letters in the same column are replaced with the letter below.
			- FO -> OU
		- 6. Otherwise, each letter gets replaced by the letter in its row but in the other letters column.
			- QM -> TH
	- ### Security of the Playfair Cipher
		- The security is much improved over simple monoalphabetic ciphers, as the Playfair Cipher has $26^2 = 676$ combinations.
			- This requires a 676 entry frequency table to analyse (as compared to a 26 entry frequency table for a monoalphabetic cipher) and correspondingly, more ciphertext.
			- However, the Playfair Cipher *can* be cracked through frequency analysis of letter pairs, given a few hundred letters.
			-
- ## Vigenère Cipher
	- [Blaise de Vigenère](https://en.wikipedia.org/wiki/Blaise_de_Vigen%C3%A8re) is generally credited as the inventor of the **Polyalphabetic Substitution Cipher**.
		- What is a **Polyalphabetic Substitution Cipher**? #card
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			- A **Polyalphabetic Substitution Cipher** uses multiple substitution alphabets, as opposed to a monoalphabetic cipher which uses a single alphabetic key.
	- The Vigenère Cipher improves security by using many monoalphabetic substitution alphabets, so each letter can be replaced by many others.
	- You use a **key** to select which alphabet is used for each letter of the message.
		- The $i^{th}$ letter of the key specifies the $i^{th}$ alphabet to use.
		- Use each alphabet in turn.
		- Repeat from the start after the end of the key is reached.
	-
	- ### Vigenère Steps
		- ![image.png](../assets/image_1663494147352_0.png)
		- 1. Write the plaintext out, and write the keyword underneath it, repeated, for the length of the plaintext.
		- 2. Use each key letter in turn as a Caesar cipher key.
		- 3. Encrypt the corresponding plaintext letter.
		- In this example, we use the keyword "CIPHER". Hence, we have the following translation alphabets:
			- ![image.png](../assets/image_1663494236099_0.png)
	- ### How to crack the Vigenère Cipher
		- 1. Search the ciphertext for repeated strings of letters - the longer the string, the better.
		- 2. For each occurrence of a repeated string, count how many letters are between the first letters in the string, and add one.
		- 3. Factorise that number.
		- 4. Repeat this process with each repeated string you find and make a table of common factors. The most common factor, $n$ is most likely the length of the keyword used to encipher the ciphertext.
		- 5. Do a frequency count on the ciphertext, on every $n^{th}$ letter. You should end up with $n$ different frequency counts.
		- 6. Compare these counts to standard frequency tables to figure out how much each letter was shifted by.
		- 7. Undo the shifts and read the message.
- ## Enigma (Rotor Ciphers)
	- ### Rotor Ciphers
		- The mechanisation / automation of encryption.
		- An $\text{N}$-stage polyalphabetic algorithm modulo 26.
		- $26^N$ steps before a repetition, where $N$ is the number of cylinders.
			- The Enigma machine had 5 cylinders, so:
				- $$26^{N=5}=11,881,376 \text{ steps}$$
		-
	- ### Breaking Enigma using **Cribs**
		- The starting point for breaking Enigma was based on the following:
			- Plaintext messages were likely to contain certain phrases.
				- Weather reports contained the term "WETTER VORHERSAGE".
				- Military units often sent messages containing "KEINE BESONDEREN EREIGNISSE" ("nothing to report").
			- A plaintext letter was never mapped onto the same ciphertext letter.
		- While the cryptanalysts in Bletchely Park did not know exactly where these cribs were placed in an intercepted message, they could exclude certain positions.
			- ![image.png](../assets/image_1663500888551_0.png)
			- From here, possible rotor start positions & rotor wiring would be systematically examined using the "bombe" - an electromechanical device designed by Turing that replicated the action of several Enigma machines wired together.
			-
- ## Transposition Ciphers
	- What are **Transposition Ciphers**? #card
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		- **Transposition** or **Permutation Ciphers**  hide the message by rearranging the letter order ^^without altering the actual letters used.^^
		- This can be recognised since the ciphertext has the same frequency distribution as the original text.
	- ### Rail Fence Cipher
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		- Write plaintext letters out diagonally over a number of rows, then read off the cipher row by row.
			- ![image.png](../assets/image_1663501467907_0.png)
	- ### Row Transposition Cipher
		- What are **Row Transposition Ciphers**? #card
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			- **Row Transposition Ciphers** are a more complex kind of transposition cipher than ((6344093b-2f4f-4c58-95e4-39a8b30d16c3))s.
			- Plaintext letters are written out in rows over a specified number of columns.
			- The columns are then re-ordered according to some key before reading off the columns
		- ![image.png](../assets/image_1663501773385_0.png)
-
- ## Product Ciphers
	- Ciphers using just substitutions or transpositions are not secure because of language characteristics.
	- Consider using several ciphers in succession to make it harder to crack:
		- Two substitutions make a more complex substitution.
		- Two transpositions make a more complex transposition.
		- However, a substitution followed by a transposition makes a much harder cipher.
	-
-
- # Steganography
	- What is **Steganography**? #card
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		- **Steganography** is an alternative to encryption that hides the existence of the message.
		- For example:
			- Using only a subset of letters / words in a message marked in some way.
			- Using invisible ink.
			- Hiding in LSB in graphic image or sound file.
	- The drawback of steganography is that it's not very economical in terms of overheads to hide a message.
	-