Add a client nonce in the transactions to prevent replay attacks.

Change-Id: Icf98c918223fc4b738c569f764726032329cffee

5 files changed, 44 insertions, 8 deletions

diff --git a/security/AsymmetricFingerprintDialog/Application/src/main/java/com/example/android/asymmetricfingerprintdialog/FingerprintAuthenticationDialogFragment.java b/security/AsymmetricFingerprintDialog/Application/src/main/java/com/example/android/asymmetricfingerprintdialog/FingerprintAuthenticationDialogFragment.java
index d40661c6..a56556f1 100644
--- a/security/AsymmetricFingerprintDialog/Application/src/main/java/com/example/android/asymmetricfingerprintdialog/FingerprintAuthenticationDialogFragment.java
+++ b/security/AsymmetricFingerprintDialog/Application/src/main/java/com/example/android/asymmetricfingerprintdialog/FingerprintAuthenticationDialogFragment.java
@@ -42,6 +42,7 @@ import java.security.KeyStore;
 import java.security.KeyStoreException;
 import java.security.NoSuchAlgorithmException;
 import java.security.PublicKey;
+import java.security.SecureRandom;
 import java.security.Signature;
 import java.security.SignatureException;
 import java.security.cert.CertificateException;
@@ -217,7 +218,7 @@ public class FingerprintAuthenticationDialogFragment extends DialogFragment
      * the activity know about the result.
      */
     private void verifyPassword() {
-        Transaction transaction = new Transaction("user", 1);
+        Transaction transaction = new Transaction("user", 1, new SecureRandom().nextLong());
         if (!mStoreBackend.verify(transaction, mPassword.getText().toString())) {
             return;
         }
@@ -284,7 +285,10 @@ public class FingerprintAuthenticationDialogFragment extends DialogFragment
         // successful.
         mPassword.setText("");
         Signature signature = mCryptoObject.getSignature();
-        Transaction transaction = new Transaction("user", 1);
+        // Include a client nonce in the transaction so that the nonce is also signed by the private
+        // key and the backend can verify that the same nonce can't be used to prevent replay
+        // attacks.
+        Transaction transaction = new Transaction("user", 1, new SecureRandom().nextLong());
         try {
             signature.update(transaction.toByteArray());
             byte[] sigBytes = signature.sign();
diff --git a/security/AsymmetricFingerprintDialog/Application/src/main/java/com/example/android/asymmetricfingerprintdialog/MainActivity.java b/security/AsymmetricFingerprintDialog/Application/src/main/java/com/example/android/asymmetricfingerprintdialog/MainActivity.java
index e6031c52..5086a173 100644
--- a/security/AsymmetricFingerprintDialog/Application/src/main/java/com/example/android/asymmetricfingerprintdialog/MainActivity.java
+++ b/security/AsymmetricFingerprintDialog/Application/src/main/java/com/example/android/asymmetricfingerprintdialog/MainActivity.java
@@ -55,10 +55,7 @@ import javax.inject.Inject;
  */
 public class MainActivity extends Activity {
 
-    private static final String TAG = MainActivity.class.getSimpleName();
-
     private static final String DIALOG_FRAGMENT_TAG = "myFragment";
-    private static final String SECRET_MESSAGE = "Very secret message";
     /** Alias for our key in the Android Key Store */
     public static final String KEY_NAME = "my_key";
 
diff --git a/security/AsymmetricFingerprintDialog/Application/src/main/java/com/example/android/asymmetricfingerprintdialog/server/StoreBackend.java b/security/AsymmetricFingerprintDialog/Application/src/main/java/com/example/android/asymmetricfingerprintdialog/server/StoreBackend.java
index dde81519..87921ae5 100644
--- a/security/AsymmetricFingerprintDialog/Application/src/main/java/com/example/android/asymmetricfingerprintdialog/server/StoreBackend.java
+++ b/security/AsymmetricFingerprintDialog/Application/src/main/java/com/example/android/asymmetricfingerprintdialog/server/StoreBackend.java
@@ -57,5 +57,4 @@ public interface StoreBackend {
      * @return true if the enrollment was successful, false otherwise
      */
     boolean enroll(String userId, String password, PublicKey publicKey);
-
 }
diff --git a/security/AsymmetricFingerprintDialog/Application/src/main/java/com/example/android/asymmetricfingerprintdialog/server/StoreBackendImpl.java b/security/AsymmetricFingerprintDialog/Application/src/main/java/com/example/android/asymmetricfingerprintdialog/server/StoreBackendImpl.java
index 8bf48d89..b28dce49 100644
--- a/security/AsymmetricFingerprintDialog/Application/src/main/java/com/example/android/asymmetricfingerprintdialog/server/StoreBackendImpl.java
+++ b/security/AsymmetricFingerprintDialog/Application/src/main/java/com/example/android/asymmetricfingerprintdialog/server/StoreBackendImpl.java
@@ -23,7 +23,9 @@ import java.security.PublicKey;
 import java.security.Signature;
 import java.security.SignatureException;
 import java.util.HashMap;
+import java.util.HashSet;
 import java.util.Map;
+import java.util.Set;
 
 /**
  * A fake backend implementation of {@link StoreBackend}.
@@ -31,10 +33,17 @@ import java.util.Map;
 public class StoreBackendImpl implements StoreBackend {
 
     private final Map<String, PublicKey> mPublicKeys = new HashMap<>();
+    private final Set<Transaction> mReceivedTransactions = new HashSet<>();
 
     @Override
     public boolean verify(Transaction transaction, byte[] transactionSignature) {
         try {
+            if (mReceivedTransactions.contains(transaction)) {
+                // It verifies the equality of the transaction including the client nonce
+                // So attackers can't do replay attacks.
+                return false;
+            }
+            mReceivedTransactions.add(transaction);
             PublicKey publicKey = mPublicKeys.get(transaction.getUserId());
             Signature verificationFunction = Signature.getInstance("SHA256withECDSA");
             verificationFunction.initVerify(publicKey);
@@ -65,5 +74,4 @@ public class StoreBackendImpl implements StoreBackend {
         // backend.
         return true;
     }
-
 }
diff --git a/security/AsymmetricFingerprintDialog/Application/src/main/java/com/example/android/asymmetricfingerprintdialog/server/Transaction.java b/security/AsymmetricFingerprintDialog/Application/src/main/java/com/example/android/asymmetricfingerprintdialog/server/Transaction.java
index 33b65ef8..789cc0e7 100644
--- a/security/AsymmetricFingerprintDialog/Application/src/main/java/com/example/android/asymmetricfingerprintdialog/server/Transaction.java
+++ b/security/AsymmetricFingerprintDialog/Application/src/main/java/com/example/android/asymmetricfingerprintdialog/server/Transaction.java
@@ -19,6 +19,7 @@ package com.example.android.asymmetricfingerprintdialog.server;
 import java.io.ByteArrayOutputStream;
 import java.io.DataOutputStream;
 import java.io.IOException;
+import java.util.Objects;
 
 /**
  * An entity that represents a single transaction (purchase) of an item.
@@ -31,9 +32,16 @@ public class Transaction {
     /** The unique user ID who made the transaction */
     private final String mUserId;
 
-    public Transaction(String userId, long itemId) {
+    /**
+     * The random long value that will be also signed by the private key and verified in the server
+     * that the same nonce can't be reused to prevent replay attacks.
+     */
+    private final Long mClientNonce;
+
+    public Transaction(String userId, long itemId, long clientNonce) {
         mItemId = itemId;
         mUserId = userId;
+        mClientNonce = clientNonce;
     }
 
     public String getUserId() {
@@ -47,6 +55,7 @@ public class Transaction {
             dataOutputStream = new DataOutputStream(byteArrayOutputStream);
             dataOutputStream.writeLong(mItemId);
             dataOutputStream.writeUTF(mUserId);
+            dataOutputStream.writeLong(mClientNonce);
             return byteArrayOutputStream.toByteArray();
         } catch (IOException e) {
             throw new RuntimeException(e);
@@ -63,4 +72,23 @@ public class Transaction {
             }
         }
     }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) {
+            return true;
+        }
+        if (o == null || getClass() != o.getClass()) {
+            return false;
+        }
+
+        Transaction that = (Transaction) o;
+        return Objects.equals(mItemId, that.mItemId) && Objects.equals(mUserId, that.mUserId) &&
+                Objects.equals(mClientNonce, that.mClientNonce);
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(mItemId, mUserId, mClientNonce);
+    }
 }