Substitution Ciphers

Problem

Write a program to help an analyst decrypt a simple substitution cipher.Your program
should take the ciphertext as input, compute letter frequency counts, and display
these for the analyst. The program should then allow the analyst to guess the key
and display the results of “decryption” with the putative key.

Note: This is from Mark Stamp's Information Security: Principles and Practice book, chapter 2. Solution below was submitted as homework for CS265 Cryptography and Information Security class at San Jose State University.

Solution

package Cypto;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.HashMap;
import java.util.LinkedHashMap;
import java.util.Map;

public class SubstitutionCipher {

 Map letterFreqMap;
 Map key;
 String ciphertext;
 String plaintext;
 BufferedReader reader;
 

 private void createInputBuffer(){
  reader = new BufferedReader(new InputStreamReader(System.in));
 }

 private void closeInputBufer() throws Exception{
  if(reader!=null)
   try {
    reader.close();
   } catch (IOException e) {

    e.printStackTrace();
    throw new Exception("Error while trying to close the input stream");
   }
 }

 private void getCipherText() throws Exception {
  System.out.println("***** Substitution Cipher Cracker *****");
  System.out.print("Enter ciphertext: ");

  reader = new BufferedReader(new InputStreamReader(System.in));
  try {
   ciphertext = reader.readLine();
  } catch (IOException e) {
   e.printStackTrace();
   throw new Exception("Error while reading the ciphertext");
  }
  ciphertext = ciphertext.toUpperCase();
  System.out.println("Ciphertext (in upper case) entered is '" + ciphertext + "'");
 }

 private void displayLetterFreq(){
  letterFreqMap = new LinkedHashMap();

  for(int i = 0; i < ciphertext.length(); i++){
   Character c = ciphertext.charAt(i);
   if (!(c >= 'A' && c <= 'Z')) continue;
   Integer count = letterFreqMap.get(c);
   if(count == null){
    letterFreqMap.put(c, 1);
   } else {
    letterFreqMap.put(c, count+1);
   }
  }
  System.out.println(letterFreqMap);
 }


 private void guessAKey() throws Exception {

  System.out.print("Guess a key for abcdefghijklmnopqrstuvwxyz: ");
  String putativeKey = "";

  try {
   putativeKey = reader.readLine();
//   System.out.println(putativeKey);
  } catch (IOException e) {
   e.printStackTrace();
   throw new Exception("Error while reading the putative key");
  } 

  createKeyMap(putativeKey);
 }

 private void getPlaintext() {
  StringBuffer plaintextBuf = new StringBuffer();
  for(int i = 0; i < ciphertext.length(); i++) {
   Character c = key.get(ciphertext.charAt(i));
   if(c == null) {
    System.out.println("Error: Character " + ciphertext.charAt(i)
      + " in ciphertext is not mapped to any alphabet");
    plaintextBuf.append(ciphertext.charAt(i));
   }
   else plaintextBuf.append(c);
  }
  plaintext = plaintextBuf.toString();
  System.out.println("Plaintext is: " + plaintext);
 }


 private void createKeyMap(String putativeKey) {
  if(putativeKey.length() < 26) {
   System.out.println("Key input is less than 26 characters. Invalid Key!");
   return;
  }
  key = new HashMap();
  putativeKey = putativeKey.toUpperCase();

  for(int i=0; i < putativeKey.length(); i++){
   key.put(putativeKey.charAt(i), (char)(i+'A'));
  }
 }

 public void problem12() {
  try {
   createInputBuffer();
   getCipherText();
   displayLetterFreq();
   guessAKey();
   getPlaintext();
  } 
  catch (Exception e) {
   e.printStackTrace();
  } 
  finally {
   try {
    closeInputBufer();
   } catch (Exception e) {
    e.printStackTrace();
   }
  }
 }

 public static void main(String[] args) {

  SubstitutionCipher sc = new SubstitutionCipher();
  sc.problem12();
 }
}