SubCipher.java

import java.io.Console;


/*
        Cryptography and Internet Security  - Worksheet 1
	Seb Jacobs

        This class encrypts and decrypts a plaintext message using a substitution cipher.
	The key for this substitution cipher is derived from a keyphrase provided by the user. 
	All the spaces and duplicate letters are removed from the keyphrase. 
	The remaining letters of the alphabet are appended to the end of the string in decreasing order.

*/
public class SubCipher{

	private String keyPhrase,keyCipher;
	public String alphabet="";
	public SubCipher(String key){
		
		keyCipher="";
		keyPhrase=key;
		this.generateAlphabet();
		this.generateCipher();
	}

	public void generateAlphabet(){
		//used to generate string of alphabet a to z
		for(char c = 'a'; c <= 'z'; c++){
				
 			alphabet+=c;

		}	
		
	}
	private void generateCipher(){
		
			// remove spaces
			String [] phraseNoSpaces = keyPhrase.split(" ");
			
			for(int i=0;i<phraseNoSpaces.length;i++){
				keyCipher+=phraseNoSpaces[i];
					
			}
		
			
			//remove duplicate characters
			
			
			KeyCipher=this.removeDuplicates(KeyCipher);
			

			for(char c = 'z'; c >= 'a'; c--){ //loop through alphabet, adding characters which aren't in the cipherKey
				
				if(keyCipher.indexOf(c)==-1){ 
					
 					keyCipher+=c;
					
				}
 			   
			}
			
			
	}	
	
	public String removeDuplicates(String cipher){
		
			String [] cipherCharRemoved;
			
			for(int i=0;i<cipher.length();i++){
				
				String firstChar=cipher.substring(i,(i+1));
				
				
				String subString1= cipher.substring(0,(i+1));
				
				
				String subString2 = cipher.substring((i+1),cipher.length());
				
						
				cipherCharRemoved = subString2.split(firstChar); 
				
				cipher=subString1;
				
				for(int j=0;j<cipherCharRemoved.length;j++){
				
					cipher+=cipherCharRemoved[j];
					
				}
				
			}
			
			return cipher;
		
		
	}
	public String encrypt(String plainText){ // encrypts plaintext with generated cipher
		
		String cipherText="";
		for(int i=0;i<plainText.length();i++){
			
			char currentChar=plainText.charAt(i);
			
			if(Character.isLetter(currentChar)){ 
				if(Character.isUpperCase(currentChar)){
				
					cipherText+=String.valueOf(Character.toUpperCase(keyCipher.charAt(alphabet.indexOf(Character.toLowerCase(currentChar)))));
			
				}
				
				else{
					cipherText+=String.valueOf(keyCipher.charAt(alphabet.indexOf(currentChar)));
				}
			}
			else{
			
				cipherText+=String.valueOf(currentChar);	
				
			}
			
		}
		return cipherText;
	}
	
	public String decrypt(String cipherText){ // decrypts ciphertext with generated cipher
		
		String plainText="";
		for(int i=0;i<cipherText.length();i++){
			
			char currentChar=cipherText.charAt(i);
			
			if(Character.isLetter(currentChar)){ 
			
				if(Character.isUpperCase(currentChar)){
				
					plainText+=String.valueOf((Character.toUpperCase(alphabet.charAt(keyCipher.indexOf(Character.toLowerCase(currentChar))))));
			
				}
				else{
				
					plainText+=String.valueOf(alphabet.charAt(keyCipher.indexOf(currentChar)));
			
				}
			}
			else{
				
				plainText+=String.valueOf(currentChar);	
					
			}
		}
		return plainText;
	}
	

		
	public static void main(String[]args){
	
		if(args.length>0){ 
			
			Console c = System.console(); //declare and assign console which will be used to get input from user
			String keyPhrase = c.readLine("input a key phrase: "); 
			SubCipher sb=new SubCipher(keyPhrase); //create subcipher with keyphrase provided by user
			
			if(args[0].equals("-e")){ // if user wishes to perform encryption
				
				String plainText = c.readLine("input plaintext: "); //prompt user to input plaintext to be encrypted
				System.out.println("Encrypted text: "+sb.encrypt(plainText)); //output ciphertext generated from plaintext
				
			}
			
			else if(args[0].equals("-d")){ if user wishes to perform decryption
				
				String cipherText = c.readLine("input ciphertext: "); //prompt user to input ciphertext to be decrypted
				System.out.println("Decrypted text: "+sb.decrypt(cipherText)); //output plaintext generated from ciphertext
				
			}
		}
	
	}


}