dWdgNNN0

1/*
2 * Write a program that asks the user for an encrypted message 
3 * and Caesar cipher key, and decrypts the message.
4 *
5 * Use the function caesarEncrypt! What key value do you need to 
6 * pass this function to decrypt your message?
7 *
8 */
9 
10
11var ALPHABET = "abcdefghijklmnopqrstuvwxyz";
12
13function start(){
14    var originalMessage = readLine("Enter the message you would like to encrypt: ");
15    originalMessage = originalMessage.toLowerCase();
16    
17    var secretKey = readInt("Enter the number you'd like to shift each character by: ");
18    
19    var encryptedMessage = caesarEncrypt(originalMessage, secretKey);
20    println("");
21    println("Encrypted message: " + encryptedMessage);
22    
23    println("");
24    println("Decrypting message...");
25    println("");
26    
27    // You will need to write caesarDecrypt below
28    var decrypted = caesarDecrypt(encryptedMessage, secretKey);
29    println("Done:");
30    
31    println(decrypted);
32}
33
34
35function caesarDecrypt(encryptedMessage, key){
36    // Write a function that returns the decrypted message
37    // Hint--use the caesarEncrypt method!
38    // Decrypting is the same process as encrypting,
39    // the key just needs to shift the letters back instead of forward
40var decryptedResult = "";
41    
42    for(var i = 0; i < encryptedMessage.length; i++){
43        // Get the character in the original message
44        var originalCharacter = encryptedMessage.charAt(i);
45        
46        // If it's an alphabetical character, we'll compute the new
47        // shifted character and add it to the decrypted result
48        var alphabeticIndex = ALPHABET.indexOf(originalCharacter);
49        if(alphabeticIndex >= 0){
50            // Compute new index
51            var newIndex = alphabeticIndex + key + ALPHABET.length;
52            newIndex = newIndex % ALPHABET.length;
53            
54            // Get the new character
55            var newCharacter = ALPHABET.charAt(newIndex);
56            
57            // Add the new shifted character to the encrypted result
58            decryptedResult += newCharacter
59        }
60        
61        // Otherwise we'll keep the original character
62        else{
63            decryptedResult += originalCharacter;
64        }
65    }
66    
67    return decryptedResult;
68}
69
70function caesarEncrypt(message, key){
71    var encryptedResult = "";
72    
73    for(var i = 0; i < message.length; i++){
74        // Get the character in the original message
75        var originalCharacter = message.charAt(i);
76        
77        // If it's an alphabetical character, we'll compute the new
78        // shifted character and add it to the encrypted result
79        var alphabeticIndex = ALPHABET.indexOf(originalCharacter);
80        if(alphabeticIndex >= 0){
81            // Compute new index
82            var newIndex = alphabeticIndex + key + ALPHABET.length;
83            newIndex = newIndex % ALPHABET.length;
84            
85            // Get the new character
86            var newCharacter = ALPHABET.charAt(newIndex);
87            
88            // Add the new shifted character to the encrypted result
89            encryptedResult += newCharacter
90        }
91        
92        // Otherwise we'll keep the original character
93        else{
94            encryptedResult += originalCharacter;
95        }
96    }
97    
98    return encryptedResult;
99}