From 67115b9693d4c3028337bdef98bf8a354ba5cd38 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?C=C3=A9dric=20Seron?= <Cedric.Seron@student.uliege.be>
Date: Wed, 12 Nov 2025 13:41:11 +0100
Subject: [PATCH] first pass step3

---
 src/main/java/step3/Main.java | 97 +++++++++++++++++++++++++++++++++++
 1 file changed, 97 insertions(+)
 create mode 100644 src/main/java/step3/Main.java

diff --git a/src/main/java/step3/Main.java b/src/main/java/step3/Main.java
new file mode 100644
index 0000000..691a952
--- /dev/null
+++ b/src/main/java/step3/Main.java
@@ -0,0 +1,97 @@
+package step3;
+
+import java.io.BufferedReader;
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import java.io.InputStreamReader;
+import java.math.BigInteger;
+import java.net.ServerSocket;
+import java.net.Socket;
+import java.security.GeneralSecurityException;
+import java.util.Base64;
+
+import static common.Logger.displayInfo;
+import static common.Logger.displayReceived;
+import static common.Logger.displaySent;
+import static common.SocketManager.readResponse;
+import static common.SocketManager.send;
+
+public class Main {
+
+    private static final BigInteger P = new BigInteger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
+    private static final BigInteger G = new BigInteger("2", 16);
+
+    public static void main(String[] args){
+        int port = 8888;
+        if (args.length >= 2 && args[0].equals("--port")) {
+            port = Integer.parseInt(args[1]);
+        }
+
+        try (ServerSocket socket = new ServerSocket(port)) {
+            displayInfo("Server is running on port " + port + "\n waiting for the client to connect...");
+            while (true) {
+                try {
+                    handleClient(socket.accept());
+                    return;
+                } catch (IOException ignored) {}
+            }
+        } catch (Exception e) {
+            e.printStackTrace();
+        }
+    }
+
+    private static void handleClient(Socket socket) throws IOException, GeneralSecurityException {
+        socket.setSoTimeout(30000);
+        displayInfo("Client connected");
+
+        BufferedReader reader = new BufferedReader(
+                new InputStreamReader(socket.getInputStream())
+        );
+
+        String message = readResponse(reader);
+        displayReceived("Received A : " + message);
+        byte[] A_bytes = Base64.getDecoder().decode(message.substring(2));
+        BigInteger A = new BigInteger(1, A_bytes);
+
+        // Compute b and B
+        BigInteger b = new BigInteger(256, new java.security.SecureRandom());
+        BigInteger B = G.modPow(b, P);
+        byte[] B_bytes = toFixedLen(B, 256);
+        
+        // Send B
+        message = "B:" + Base64.getEncoder().encodeToString(B_bytes);
+        send(socket, message);
+        displaySent("B sent : " + message);
+
+        // Compute shared Z
+
+        BigInteger Z = A.modPow(b, P);
+        byte[] Z_bytes = toFixedLen(Z, 256);
+        
+    }
+
+    private static byte[] toFixedLen(BigInteger x, int len) {
+        byte[] t = x.toByteArray();
+        if (t.length == len) return t;
+        if (t[0] == 0 && t.length == len + 1) { // strip leading zero
+            byte[] r = new byte[len];
+            System.arraycopy(t, 1, r, 0, len);
+            return r;
+        }
+        byte[] r = new byte[len];
+        System.arraycopy(t, Math.max(0, t.length - len), r, len - Math.min(len, t.length),
+        Math.min(len, t.length));
+        return r;
+    }
+
+    private static byte[] PBKDF_HMAC_key(BigInteger A, BigInteger B, BigInteger p, BigInteger g){
+
+        ByteArrayOutputStream outputStream = new ByteArrayOutputStream( );
+        outputStream.write( toFixedLen(g, 8) );
+        outputStream.write( toFixedLen(p, 256) );
+        outputStream.write( toFixedLen(A, 256) );
+        outputStream.write( toFixedLen(B, 256) );
+
+    }
+
+}
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