Chapter 4, Problem 11PP

Your country is at war and your enemies are using a secret code to communicate with one another. You have managed to intercept a message that reads as follows:

:mmZ\dxZmx] Zpgy

The message is obviously encrypted using the enemy’s secret code. You have just learned that their encryption method is based upon the ASCII code (see Appendix 7). Individual characters in a string are encoded using this system. For example, the character ‘A’ is encoded using the number 65 and ‘B’ is encoded using the number 66. Your enemy’s secret code takes each letter of the message and encrypts it as follows:

if (OriginalChar + Key > 126) then

EncryptedChar = 32 + ((OriginalChar +Key) − 127)

else

EncryptedChar = (OriginalChar + Key)

For example, if the enemy uses Key = 10 then the message “Hey” would initially be represented as:

Character    ASCII code

H    72.

e    101

y    121

And “Hey” would be encrypted as:

Encrypted H − (72 + 10) − 02 − R in ASCII

Encrypted e − (101 + 10) – 111 − o in ASCII

Encrypted y = 32 + ((121 + 10) − 127) = 36 = $ in ASCII

Consequently, “Hey” would be transmitted as “Ro$.” Write a Java program that decrypts the intercepted message. You only know that the key used is a number between 1 and 100. You can assume that the original message consists entirely of ASCII codes that represent only printable characters. Your program should try to decode the message using all possible keys between 1 and 100. When you try the valid key, the message will make sense. For all other keys, the message will appear as gibberish. Since there are only 100 keys this would obviously be a pretty crummy encryption system. This Programming Project will require you to explore a bit on your own how to convert from a char to a number, process the number, then convert it back to a char. See Chapter 2 for String methods.

You will want to use charAt ().

Important: Note that the secret code has a \ so you will need to escape encode it by using \\ if you hard-code it in your program.