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Caesar Cipher (ROT-13)

ISC 2017 Computer Science Practical, Question 3 · Strings

The question

Caesar Cipher is an encryption technique which is implemented as ROT13 (‘rotate by 13 places’). It is a simple letter substitution cipher that replaces a letter with the letter 13 places after it in the alphabets, with the other characters remaining unchanged.

                                    ROT13

+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+ | A/a | B/b | C/c | D/d | E/e | F/f | G/g | H/h | I/i | J/j | K/k | L/l | M/m | +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+ | N/n | O/o | P/p | Q/q | R/r | S/s | T/t | U/v | V/v | W/w | X/x | Y/y | Z/z | +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+

Write a program to accept a plain text of length L, where L must be greater than 3 and less than 100.

Encrypt the text if valid as per the Caesar Cipher. Test your program with the sample data and some random data:

Example 1
INPUT:  Hello! How are you?
OUTPUT: The cipher text is:
        Uryyb? Ubj ner lbh?

Example 2
INPUT:  Encryption helps to secure data.
OUTPUT: The cipher text is:
        Rapelcgvba Urycf gb frpher gngn.

Example 3
INPUT:  You
OUTPUT: INVALID LENGTH

Question: ISC 2017 Computer Science Paper 2 (Practical), Question 3, set by the Council for the Indian School Certificate Examinations (CISCE). Solution and video: Vinay Singh.

Java solution

ISC2017Q3.java Download GitHub

import java.util.*;
class CaesarCipher{
    private String L,msg;
    public CaesarCipher(String L){
        this.L=L;
    }
    public void encode(){
        msg="";
        for( int position=0; position<L.length();position++){
            msg+=rot13(L.charAt(position));
        }
    }
    private static char rot13(char ch){
        if(Character.isLetter(ch)){
            if(ch>='A' && ch<='M'){
                ch= (char)(ch+13);
            }else if(ch>='N' && ch <='Z'){
                ch= (char)(ch-13);
            }else if(ch>='a' && ch<='m'){
                ch= (char)(ch+13);
            }else if(ch>='n' && ch <='z'){
                ch= (char)(ch-13);
            }
        }
        return ch;
    }
    public String toString(){
        return "\t"+msg;
    }
}
public class ISC2017Q3 {
    public static void main(String[] args) {
        Scanner in = new Scanner( System.in );
        System.out.print("INPUT:\t");
        String L=in.nextLine();
        if(L.length()>3 && L.length()<100){
            System.out.println("OUTPUT:\tThe cipher text is:");
            CaesarCipher cipher=new CaesarCipher(L);
            cipher.encode();
            System.out.println(cipher);
        }else{
        System.out.println("OUTPUT:\tINVALID LENGTH");

        }
        in.close();
    }
}

Caesar Cipher (ROT-13)

The question

Java solution

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