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1import java.net.*;
2import java.io.*;
3import javax.crypto.*;
4import javax.crypto.spec.IvParameterSpec;
5import javax.crypto.spec.SecretKeySpec;
6import java.security.*;
7
8public class Client {
9    public static void main(String[] args) {
10
11        Socket s = null;
12        int serversocket = 4567;
13        try {
14            s = new Socket("127.0.0.1", serversocket);
15
16            DataInputStream in = new DataInputStream(s.getInputStream());
17            DataOutputStream out = new DataOutputStream(s.getOutputStream());
18
19            while (true){
20                SecureRandom sr = new SecureRandom();
21
22                //shared private key
23                byte [] key = createPrivateKey(sr, out, in);
24
25                //hash function and new keys
26                byte [] byte1 = {1};
27                SecretKey k1 = createHashKeys(byte1, key);
28                byte [] byte2 = {2};
29                SecretKey k2 = createHashKeys(byte2, key);
30
31                //message authentication code
32                Mac mac = Mac.getInstance("HmacSHA256");
33                mac.init(k2);
34                int nSeq = 0;
35
36                //nonce - sends the bytes in plaintext so that the server can create the same IV
37                byte[] bytesForIV=new byte[16];
38                sr.nextBytes(bytesForIV);
39                IvParameterSpec iv = new IvParameterSpec(bytesForIV);
40                out.write(bytesForIV);
41
42                //E(m, k1) = c
43                Cipher c = Cipher.getInstance("AES/CTR/PKCS5Padding");
44                c.init(Cipher.ENCRYPT_MODE, k1, iv);
45
46                String text;
47                InputStreamReader input = new InputStreamReader(System.in);
48                BufferedReader reader = new BufferedReader(input);
49
50                while (true) {
51                    try {
52                        //encrypt text sent by client
53                        text = reader.readLine();
54                        byte [] ciphertext = c.doFinal(text.getBytes());
55
56                        //calculates the authentication tag
57                        byte [] nSeqByte = intToByteArray(nSeq);
58                        byte [] authTagAux = concat(ciphertext, nSeqByte);
59                        byte [] authTag = mac.doFinal(authTagAux);
60
61                        // info = [size of ciphertext(4 bytes)][mac(32 bytes)][ciphertext]
62                        byte[] info = concat(intToByteArray(ciphertext.length),concat(authTag,ciphertext));
63                        out.write(info);
64                        nSeq++;
65                    }
66                    catch (Exception ignored) { }
67                }
68            }
69        } catch (UnknownHostException e) {
70            System.out.println("Sock:" + e.getMessage());
71        } catch (EOFException e) {
72            System.out.println("EOF:" + e.getMessage());
73        } catch (IOException e) {
74            System.out.println("IO:" + e.getMessage());
75        } catch (NoSuchPaddingException | NoSuchAlgorithmException | InvalidAlgorithmParameterException | InvalidKeyException e) {
76            e.printStackTrace();
77        } finally {
78            if (s != null)
79                try {
80                    s.close();
81                } catch (IOException e) {
82                    System.out.println("close:" + e.getMessage());
83                }
84        }
85    }
86
87    private static byte [] createPrivateKey(SecureRandom sr, DataOutputStream out, DataInputStream in) throws IOException {
88        //diffie-hellman
89        //create the variables p, q and g and send them to the server
90        byte [] p = new byte[16];
91        sr.nextBytes(p);
92        out.write(p);
93        byte [] q = new byte[16];
94        sr.nextBytes(q);
95        out.write(q);
96        byte [] g = new byte[16];
97        sr.nextBytes(g);
98        out.write(g);
99
100        //the private key of the client should range from 0 to q-1
101        int privateKeyClient = getRandomNumber(byteArrayToInt(q)-1);
102
103        //calculate and send to the server the public key of the client - g^(privateKeyClient) mod p
104        int powClient = (int)Math.pow(byteArrayToInt(g), privateKeyClient);
105        int publicKeyClientAux = powClient%byteArrayToInt(p);
106        byte [] publicKeyClient = intToByteArray(publicKeyClientAux);
107        out.write(publicKeyClient);
108
109        //receive the public key of the server
110        byte [] publicKeyServer = new byte[16];
111        in.read(publicKeyServer);
112
113        //calculate the shared private key - publickKeyServer^(privateKeyClient) mod p
114        int pow = (int)Math.pow(byteArrayToInt(publicKeyServer), privateKeyClient);
115        int privateKeyAux = pow%byteArrayToInt(p);
116
117        return intToByteArray(privateKeyAux);
118    }
119
120    private static SecretKey createHashKeys(byte [] b, byte [] key){
121        SecretKey k = null;
122        try {
123            MessageDigest md = MessageDigest.getInstance("SHA-256");
124
125            byte[] aux = concat(key, b);
126            byte[] hash = md.digest(aux);
127            k = new SecretKeySpec(hash, 0, hash.length, "AES");
128        } catch (NoSuchAlgorithmException | IOException e) {
129            e.printStackTrace();
130        }
131        return k;
132    }
133
134    private static byte [] concat(byte[] first, byte[] second) throws IOException {
135        ByteArrayOutputStream baos = new ByteArrayOutputStream();
136        baos.write(first);
137        baos.write(second);
138        byte [] result = baos.toByteArray();
139        baos.reset();
140        return result;
141    }
142
143    //https://mkyong.com/java/java-generate-random-integers-in-a-range/
144    private static int getRandomNumber(int max) {
145        return (int)(Math.random() * ((max) + 1));
146    }
147
148    //https://stackoverflow.com/questions/2183240/java-integer-to-byte-array
149    //the function needs to be final because if it's not the MAC is invalid
150    private static final byte[] intToByteArray(int value) {
151        return new byte[]{
152                (byte)(value >>> 24),
153                (byte)(value >>> 16),
154                (byte)(value >>> 8),
155                (byte)value};
156    }
157
158    //https://stackoverflow.com/questions/4950598/convert-byte-to-int
159    private static int byteArrayToInt(byte[] b) {
160        int value= 0;
161        for (byte item : b) value = (value << 8) | item;
162        return value;
163    }
164}