Chapter 21, Problem 8PP

Program Plan Intro

To decrypt a file that was encrypted using monoalphabet cipher

Program plan:

• In the file “CaesarCipher.java”, import necessary packages and create a class “CaesarCipher”,
  • Declare a variable.
  • Define the constructor to initialize the value.
  • Define the method “decryptStream()” to decrypt the contents of a stream and reports how many words are found,
    • Execute “try” statement to check if exception occurs.
    • Find the number of words found.
    • Catch if exception occurs,
      • Handle the exception.
  • Define the method “decrypt()” to decrypt a value.
• In the file “FrequencyMap.java”, create a class “FrequencyMap” to count the frequency of letters,
  • Declare the array and variable.
  • Define the constructor to initialize the variable.
  • Define the method “getFrequency()” to get the frequency of the specified letter.
  • Define the method “add()” to add a letter to be counted.
• In the file “Decryptor.java”, import necessary packages and create a class “Decryptor”,
  • Define the method “main()”,
    • Initialize the necessary variables.
    • Execute “while” loop till the length of the argument,
      • Check, if the input file is null,
        • Get the input file.
        • • Increment count.
    • Execute “try” statement,
      • Create an object for “FrequencyMap”.
      • Execute till not done,
        • Assign the character.
        • Check if “-1”.
        • • Set the Boolean value to true.
        • Otherwise, check if there is a letter.
        • • Call the method “add()” to add a letter to be counted.
    • Close the reader.
    • Execute a loop.
      • Print the frequencies.
    • Catch if exception occurs,
      • Print the stack trace.
    • Execute another “try” statement,
      • Brute force the decryption with the Caesar cipher and output the number of words from the dictionary file found for each key.
        • Create an object for “CaesarCipher”.
        • • • Close the input stream.
    • Catch if exception occurs,
      • Handle the exception.