fYiTxkcf

1package arp;
2
3import java.nio.ByteBuffer;
4import java.nio.charset.Charset;
5import java.security.Key;
6import java.security.SecureRandom;
7import java.security.spec.KeySpec;
8import java.util.Arrays;
9
10import javax.crypto.Cipher;
11import javax.crypto.SecretKeyFactory;
12import javax.crypto.spec.IvParameterSpec;
13import javax.crypto.spec.PBEKeySpec;
14import javax.crypto.spec.PBEParameterSpec;
15import javax.xml.bind.DatatypeConverter;
16
17public class Solution {
18
19    private static final String ALGORITHM = "PBEWITHHMACSHA512ANDAES_128";
20    private static final Charset charset = Charset.forName("utf-8");
21    private static final int ITERATION_COUNT = 100;
22
23    private static byte[] getIvAndMac(String message, String key, byte[] salt) {
24        try {
25            KeySpec spec = new PBEKeySpec(key.toCharArray(), salt, ITERATION_COUNT);
26            // salt is also used as IV
27            IvParameterSpec ivSpec = new IvParameterSpec(salt);
28            PBEParameterSpec pbSpec = new PBEParameterSpec(salt, ITERATION_COUNT, ivSpec);
29            Key secretKey = SecretKeyFactory.getInstance(ALGORITHM).generateSecret(spec);
30            Cipher eCipher = Cipher.getInstance(ALGORITHM);
31            eCipher.init(Cipher.ENCRYPT_MODE, secretKey, pbSpec);
32
33            byte[] mac = eCipher.doFinal(message.getBytes(charset));
34            ByteBuffer bb = ByteBuffer.allocate(salt.length + mac.length);
35            bb.put(salt);
36            bb.put(mac);
37            
38            return bb.array();
39        }
40        catch(Exception e) {
41            throw new RuntimeException(e);
42        }
43    }
44
45    // Add a message autentication code to the message using a specific key.
46    public static String addMac(String message, String key, int messagelength) {
47        // here the messagelength property is not used at all, not sure why it is even passed when the whole message needs to be MACed
48
49        // salt is to keep the generated MAC unique even when the same message is passed as input
50        // instead of using a counter which requires storing the messages for future reference to detect duplicates
51        byte[] salt = new byte[16];
52        SecureRandom random = new SecureRandom();
53        random.nextBytes(salt);
54        
55        byte[] ivAndMac = getIvAndMac(message, key, salt);
56
57        return message + DatatypeConverter.printHexBinary(ivAndMac);
58    }
59
60    // Verify a message authentication code to the message given a specific key.
61    public static boolean checkMac(String message, String key, int messagelength) {
62        if(message == null) {
63            return false;
64        }
65
66        if(messagelength > message.length()) {
67            return false;
68        }
69
70        String plainText = message.substring(0, messagelength);
71        String macHex = message.substring(messagelength);
72        byte[] ivAndMac = DatatypeConverter.parseHexBinary(macHex);
73        if(ivAndMac.length < 20) { // salt itself is 16 bytes so the whole array should be much more than just 16 bytes
74            return false;
75        }
76
77        byte[] salt = new byte[16];
78        System.arraycopy(ivAndMac, 0, salt, 0, 16);
79
80        byte[] reComputedIvAndMac = getIvAndMac(plainText, key, salt);
81        
82        return Arrays.equals(ivAndMac, reComputedIvAndMac);
83    }
84    
85    public static void main(String[] args) {
86        String message = "this is a secret message";
87        String key = "and this is my 32 character secret key";
88
89        String authenticatedMessage = addMac(message, key, message.length());
90        //System.out.println(authenticatedMessage);
91        boolean verified = checkMac(authenticatedMessage, key, message.length());
92        //System.out.println(verified);
93        if(!verified) {
94            System.out.println("corrupted message, MAC check failed");
95        }
96        
97        String anotherAuthMessageWithSameMessage = addMac(message, key, message.length());
98        if(authenticatedMessage.equals(anotherAuthMessageWithSameMessage)) {
99            System.out.println("program failed to generate unique MAC for the same input");
100        }
101        
102        // check MAC by altering the message
103        verified = checkMac("abc" + anotherAuthMessageWithSameMessage, key, anotherAuthMessageWithSameMessage.length() + 3);
104        if(verified) {
105            System.out.println("MAC check should fail");
106        }
107    }
108}