COS 160: Program 5 - Message Decryption

Working with loops, strings, and characters.

Encrypting messages is important for keeping messages secret and personal information secure. Intercepting and decrypting secret messages has played an important role in many wars by allowing one government to find out what their opponents were planning to do.

You are going to be writing a program to decrypt messages encode by a substitution cypher. Substitution cyphers are popular activities often found in newspapers along with crossword and Suduko puzzles. For each letter of the alphabet, you must substitute a different letter to decrypt the message. The set of substitutions is know as the decryption key. In the newspaper puzzles, the reader has to figure out the decryption key, but for this assignment we give you the decryption key. You just need to write a program to perform the decryption.

A small example is shown below. The encoded message starts with the letter 'N'. To decode that you find 'N' in the alphabet at the top, and then the corresponding letter below it is what it decodes to. In this case an 'H'. Letter by letter you can decode the message: j -> e, g -> l, a -> p. Any non-letters are not encrypted, so the punctuation and spaces are simply copied. Try decoding the rest of the message to make sure that you understand the process. You are going to be writing a program to do this.

To begin, write a program to loop through a string character by character. If the character is a letter, print a question mark, otherwise print the character. Use the code below for the message string. This will be the first string that you will decode. Use the String class method .charAt(index) and the Character class method .isLetter(char). 

Copy and paste your code and results for Part 1 to your solutions document.

Next modify your program to use this decryption key to decode uppercase letters. For each letter you will find and print the corresponding letter in the decryption key. You can just use simple character math. Recall that characters are stored inside the computers as numbers: A=65, B=66, C=67, etc. It is considered poor programming to use these numbers directly. Instead use character literal such as 'A' with the understanding that it corresponds to a number. If c is a character, the expression (c - 'A') will calculate an integer value of: 0 when c is 'A', 1 when c is 'B', 2 when c is 'C', etc. This value can then be used as an index to get the corresponding character at that position in the decryption key.

When it is working correctly you should be able to read the decrypted message.

Note: Use the String charAt(index) method. This should only add 1 or 2 lines of code to part 1.

Copy and paste your code and results for Part 2 to your solutions document.

Next modify your program to also decode lower case letters. There are several ways to do this. When printed, the case of a decoded character should match the case of the original.

The Character class has methods: .isUpperCase(char), .isLowerCase(char), .toUpperCase(char), and .toLowerCase(char).

Below is a new message to decode. Cut and paste it into your code.

Note: This only requires simple changes and adds 2 or 3 more lines.

Copy and paste your code and results for Part 3 to your solutions document.

Here is a message encoded with a different key. Your challenge is to use your program to help you figure out the decryption key. I suggest that you start with one letter words, then two letters, then three letters. There will be some guessing. Fill in your guesses one at a time in the decryption key, run your program and check that the results seem plausible. Also look for common words in the encrypted message and punctuation for additional clues.

Copy and paste your code and results to your solutions document. What is the decrytion key?

Turn in your code and the results for each part of the assignment.