Question

My decipher program is not working correctly…

  public static void decipherMessage() { 

    Scanner sc = new Scanner(in);

    System.out.println("Enter keyword: "); 
    String decipher = sc.nextLine(); 

    String plain = ("abcdefghijklmnopqrstuvwxyz"); 
    String cipher = decipher + "abcdefghijklmnopqrstuvwxyz";

    System.out.println("Enter enciphered message: ");
    String ciphMsg = sc.nextLine(); 

    String decipherMessage = ciphMsg.toLowerCase();

    char[] chars = cipher.toCharArray();
    Set<Character> charSet = new LinkedHashSet<Character>(); 

    for (char c : chars) {
        charSet.add(c);
    }

    StringBuilder cMap = new StringBuilder();
    for (Character character : charSet) {
        cMap.append(character);
        StringBuilder original = cMap;

    }
    for (int i = 0; i < 26; i++)
    {
        char cipherTextChar = cMap.charAt(i);
        char plainTextChar = plain.charAt(i);

        decipherMessage = decipherMessage.replace(cipherTextChar,plainTextChar); 

        System.out.println("Deciphered message");  
        System.out.println(decipherMessage);


    }

plain text letters are supposed to replace ciphered letters

but when I run it, it just gives the ciphered message again without deciphering?

example.

keyword: stack

plain : abcdefghijklmnopqrstuvwxyz

cipher: stackbdefghijlmnopqruvwxyz

message: overflow

ciphered message: mvhpbimw

except in reverse.
using the code I showed above it just gives the ciphered message again without deciphering anything.

Answer 1

Your code does replace characters, just not correctly (assuming that the usage of deciphMsg and decipherMessage above is a typo. They should be the same variable.)

Well one problem is that you’re doing string replacement one cypher character at a time.

For example, if my cypher is “b”, it is supposed to switch all the a’s and the b’s, but if I give it the string

abababababab

first it will replace all the a’s by b’s giving

bbbbbbbbbbbb

and then replace all the b’s by a’s:

aaaaaaaaaaaa

which is not the intended outcome.

The solution basically boils down to doing the replacement one message character at a time. You’ll have to loop over the message for that. You could also use a map to make it efficient.