2wJq05yF

1import java.nio.ByteBuffer;
2import java.nio.charset.Charset;
3import java.security.Key;
4import java.security.MessageDigest;
5import java.security.SecureRandom;
6import java.security.spec.KeySpec;
7import java.util.Arrays;
8
9import javax.crypto.Cipher;
10import javax.crypto.SecretKeyFactory;
11import javax.crypto.spec.IvParameterSpec;
12import javax.crypto.spec.PBEKeySpec;
13import javax.crypto.spec.PBEParameterSpec;
14import javax.xml.bind.DatatypeConverter;
15
16public class Solution {
17
18    private static final String ALGORITHM = "PBEWITHHMACSHA512ANDAES_128";
19    private static final Charset charset = Charset.forName("utf-8");
20    private static final int ITERATION_COUNT = 100;
21
22    private static byte[] getIvAndMac(String message, String key, byte[] salt) {
23        try {
24            KeySpec spec = new PBEKeySpec(key.toCharArray(), salt, ITERATION_COUNT);
25            // salt is also used as IV
26            IvParameterSpec ivSpec = new IvParameterSpec(salt);
27            PBEParameterSpec pbSpec = new PBEParameterSpec(salt, ITERATION_COUNT, ivSpec);
28            Key secretKey = SecretKeyFactory.getInstance(ALGORITHM).generateSecret(spec);
29            Cipher eCipher = Cipher.getInstance(ALGORITHM);
30            eCipher.init(Cipher.ENCRYPT_MODE, secretKey, pbSpec);
31
32            byte[] mac = eCipher.doFinal(message.getBytes(charset));
33            // then hash the MAC so that it will always be of fixed length
34            MessageDigest md = MessageDigest.getInstance("SHA");
35            byte[] hash = md.digest(mac);
36            ByteBuffer bb = ByteBuffer.allocate(salt.length + hash.length);
37            bb.put(salt);
38            bb.put(hash);
39            
40            return bb.array();
41        }
42        catch(Exception e) {
43            throw new RuntimeException(e);
44        }
45    }
46
47    // Add a message autentication code to the message using a specific key.
48    public static String addMac(String message, String key, int messagelength) {
49        // here the messagelength property is not used at all, not sure why it is even passed when the whole message needs to be MACed
50
51        // salt is to keep the generated MAC unique even when the same message is passed as input
52        // instead of using a counter which requires storing the messages for future reference to detect duplicates
53        byte[] salt = new byte[16];
54        SecureRandom random = new SecureRandom();
55        random.nextBytes(salt);
56        
57        byte[] ivAndMac = getIvAndMac(message, key, salt);
58
59        return message + DatatypeConverter.printHexBinary(ivAndMac);
60    }
61
62    // Verify a message authentication code to the message given a specific key.
63    public static boolean checkMac(String message, String key, int messagelength) {
64        if(message == null) {
65            return false;
66        }
67
68        if(messagelength > message.length()) {
69            return false;
70        }
71
72        String plainText = message.substring(0, messagelength);
73        String macHex = message.substring(messagelength);
74        byte[] ivAndMac = DatatypeConverter.parseHexBinary(macHex);
75        if(ivAndMac.length < 36) { // salt itself is 16 bytes + hash is 20 bytes
76            return false;
77        }
78
79        byte[] salt = new byte[16];
80        System.arraycopy(ivAndMac, 0, salt, 0, 16);
81
82        byte[] reComputedIvAndMac = getIvAndMac(plainText, key, salt);
83        
84        return Arrays.equals(ivAndMac, reComputedIvAndMac);
85    }
86    
87    public static void main(String[] args) {
88        String message = "this is a secret message";
89        String key = "and this is my 32 character secret key";
90
91        String authenticatedMessage = addMac(message, key, message.length());
92        //System.out.println(authenticatedMessage);
93        boolean verified = checkMac(authenticatedMessage, key, message.length());
94        //System.out.println(verified);
95        if(!verified) {
96            System.out.println("corrupted message, MAC check failed");
97        }
98        
99        String anotherAuthMessageWithSameMessage = addMac(message, key, message.length());
100        if(authenticatedMessage.equals(anotherAuthMessageWithSameMessage)) {
101            System.out.println("program failed to generate unique MAC for the same input");
102        }
103        
104        // check MAC by altering the message
105        verified = checkMac("abc" + anotherAuthMessageWithSameMessage, key, anotherAuthMessageWithSameMessage.length() + 3);
106        if(verified) {
107            System.out.println("MAC check should fail");
108        }
109    }
110}