Post-12

Review all of Module 13 and 14.
Complete in-class exercises 21-24 in Module 13, and 2-5 and 15-21 in Module 14. (No need to do the other in-class exercises in Module 14.) Don't forget to upload these in BB.
In a pure substitution cipher, every letter of the alphabet is substituted for by another letter. A substitution cipher can be easily represented by a permutation of the letters 'a' through 'z'. We'll call this the substitution key.
For example, consider the substitution key
```
  c d e f g h i j k l m n o p q r s t u v w x y z a b
```
To figure out how to substitute a letter with this key, line up the alphabet with the key, as in:
```
  a b c d e f g h i j k l m n o p q r s t u v w x y z  // Regular plain text letters.
  c d e f g h i j k l m n o p q r s t u v w x y z a b  // Substitution code.
```
Thus, to find the code for 'f', look at 'f' in the top line. The code for 'f' then, is the letter directly below, which in this case is 'h'.
Here's another example.
```
  a b c d e f g h i j k l m n o p q r s t u v w x y z  // Regular plain text letters.
  q w e r t y u i o p a s d f g h j k l z x c v b n m  // Substitution code.
```
The code for 'f' in this case is 'y'.
In a Java program, we'll represent the substitution key with a string as in:
```
  String subKey = "qwertyuiopasdfghjklzxcvbnm";
  char[] key = subKey.toCharArray ();
```
Note: it's easy to convert a string to a char array as shown above.
Your goal in this problem is to write encryption and decryption methods for the above key. Start by creating a method called encode with the following signature:
```
  static char encode (char c, char[] key)
  {
      // Returns the substitute character based on the given key.
  }
```
Similarly, you'll want to write a method called decode that, when given the coded letter, returns the original.
Next, write methods called encrypt and decrypt that encrypt and decrypt a whole char array, respectively. For example, encrypt would look like this:
```
    static char[] encrypt (char[] text, char[] key)
    {
        // Return an encrypted version of the text, given the key.
    }
```
Now, test your encryption with "attackatdawn". You should get "qzzqeaqzrqvf". To test decryption, see what the following decrypts to: "rorngxitqkqwgxzzitofzvigvqsatrofzgqwqkqfrgkrtktrqrgxwst". Call your program SubstitutionCipher.java.
How many possible substitution ciphers are there (for lowercase letters)?
A substitution cipher is easy to crack when you are given enough encrypted messages. This is because you can guess at letter encodings based on knowing that some letters occur more frequently than others. To address this problem, both sides in World War II used rotation ciphers. In fact, the infamous Enigma machine, a central player in WWII, used a variety of rotation ciphers and was extremely difficult to break.
In a rotation cipher, the substituion key is slightly changed at each step. Consider the encryption of "attackatdawn" with the substitution key "qwertyuiopasdfghjklzxcvbnm". The first letter in the plain text is 'a'. To encode 'a', we use
```
  a b c d e f g h i j k l m n o p q r s t u v w x y z  // Regular plain text letters.
  q w e r t y u i o p a s d f g h j k l z x c v b n m  // Substitution code.
```
Thus, 'a' is substituted by 'q'. Now, as soon as this is done, we "rotate" the key before encoding the next letter:
```
  a b c d e f g h i j k l m n o p q r s t u v w x y z  // Regular plain text letters.
  w e r t y u i o p a s d f g h j k l z x c v b n m q  // Rotated key.
```
So, now, to encode 't', the encoded letter is 'x'. Then, the key is rotated again:
```
  a b c d e f g h i j k l m n o p q r s t u v w x y z  // Regular plain text letters.
  e r t y u i o p a s d f g h j k l z x c v b n m q w  // Rotated key.
```
Now, the next 't' (in "attackatdawn") gets translated to 'c'. Thus, at each step a rotation is performed of the key, which shifts each letter left by one spot in the array, moving the first one to the last position.
Your goal here is to implement a rotation cipher in a program called RotationCipher.java. If you do this correctly, "attackatdawn" gets encrypted to "qxcruhuqspun". To test decryption, see what the following decrypts to: "ihnhtltvmbjedmqzipsivevatrqdapsniwzabmmkvookmzsnlbhrhljthpl".
Don't forget your reflection exercise!

Submission: As usual, we'll use Blackboard (BB) for submission. Go to assignments and then to post-12. Upload all your Java programs in a single zip file.

Recommended, but not required: This week's theme: computer vision for the blind:

See this news video on an unusual device that uses computer vision to help the blind.
See this video on Google's driverless car project.
Do you know of any other such application?