If I'm understanding the problem correctly, you essentially want to build a simple encryption/decryption program that will encrypt a string with a key and also be able to get the original string back with the same key - is that right?
If so, there are probably a lot of ways to do this, but this is the way I would go about it:
At the core of the program you should have two main functions:
encrypt and
decrypt. The purpose of these functions would be to accept a string and a key and return a new encrypted/decrypted string.
Here is what an encryption function could like in Java (I'll explain it line-by-line below)
Code:
public String encrypt(String inputString, int key)
{
String encryptedString = "";
for(int i=0; i<inputString.length(); i++)
{
encryptedString += (char)(((int)inputString.charAt(i) + key) % 256);
}
return encryptedString;
}
So here, the basic idea is that we are going through each character of the inputString, altering it with the key and then adding it to the encryptedString.
The first line (after the function header) declares the variable that we will use to store the final encrypted string and gives it an initial value of null.
The next section is the main for-loop. This loop goes through each character of inputString and keeps track of the current index with the variable i. Inside the loop is where the real work begins.
Code:
encryptedString += (char)(((int)inputString.charAt(i) + key) % 256);
We'll sort've work this one from the inside-out:
1) We get the character at index i of inputString
2) Cast that character to an integer so we can get the numerical
ASCII value (between 0 and 255)
- (int)inputString.charAt(i)
3) Add the value of the key to it
- (int)inputString.charAt(i) + key
4) Then take that result and use the % operator - otherwise known as the
MOD function - to force the final value to be between 0 and 255. (I can explain this process further if you would like)
- ((int)inputString.charAt(i) + key) % 256
5) Finally, we cast that result back to a character
- (char)(((int)inputString.charAt(i) + key) % 256)
After we convert it back into a character, we add that value to the encryptedString and continue the loop until all the characters have been encrypted. After the entire string as been converted, it will return the encrypted string so you can use it in your main program (print it out, etc...).
Again, if you're having any troubles with the any aspect of the code, just let me know.
Also, if you know and understand how this code works then writing the decrypt function will be incredibly easy.
I hope that this helps!
Tyler
