added standard curve25519 decryption

tests/Curve25519Test.cs

@@ -13,6 +13,32 @@ namespace Curve25519Tests
         {
         }
 
+        [Test]
+        public void TestCurve25519Decrypt()
+        {
+            var alice = PublicKeyBox.GenerateKeyPair();
+            var bob = PublicKeyBox.GenerateKeyPair();
+            string message = "Hello World";
+            byte[] message_bytes = UTF8Encoding.UTF8.GetBytes(message);
+            byte[] nonce = Sodium.PublicKeyBox.GenerateNonce();
+
+            byte[] encrypted =
+                    Sodium.PublicKeyBox.Create(
+                        message,
+                        nonce,
+                        alice.PrivateKey,
+                        bob.PublicKey
+                    );
+            byte[] both = new byte[nonce.Length + encrypted.Length];
+            Buffer.BlockCopy(nonce, 0, both, 0, nonce.Length);
+            Buffer.BlockCopy(encrypted, 0, both, nonce.Length, encrypted.Length);
+            byte[] decrypted = chestcrypto.Curve25519.decrypt(bob.PrivateKey, alice.PublicKey, both);
+            string decrypted_string = Encoding.UTF8.GetString(decrypted, 0, decrypted.Length);
+            if (! decrypted_string.Equals(message)){
+                Assert.Fail();
+            }
+        }
+
         [Test]
         public void TestCurve25519Encrypt()
         {
@@ -26,15 +52,13 @@ namespace Curve25519Tests
             byte[] encrypted_without_nonce = new byte[encrypted_with_nonce.Length - 24];
 
             int counter = 0;
-            Console.WriteLine(encrypted_without_nonce.Length);
+
             for (int i = 24; i < encrypted_with_nonce.Length; i++){
-                //Console.WriteLine(counter);
                 encrypted_without_nonce[counter] = encrypted_with_nonce[i];
                 counter += 1;
             }
 
             for (int i = 0; i < chestcrypto.Curve25519.NONCE_BYTE_AMOUNT; i++){
-                //Console.WriteLine(i);
                 used_nonce[i] = encrypted_with_nonce[i];
             }
             for (int i = 0; i < chestcrypto.Curve25519.NONCE_BYTE_AMOUNT; i++){
@@ -44,7 +68,6 @@ namespace Curve25519Tests
             }
             byte[] decrypted = PublicKeyBox.Open(encrypted_without_nonce, used_nonce, bob.PrivateKey, alice.PublicKey);
             if (!Encoding.UTF8.GetString(decrypted, 0, decrypted.Length).Equals(message)){
-                Console.WriteLine(Encoding.UTF8.GetString(decrypted, 0, decrypted.Length));
                 Assert.Fail();
             }
         }

treasurechest/chestcrypto/curve25519.cs

@@ -5,6 +5,7 @@ namespace chestcrypto {
 
         public static int NONCE_BYTE_AMOUNT = 24;
         public static byte[] encrypt(byte[] privkey, byte[] pubkey, byte[] message){
+            // Take a byte message and priv/pubkey for authenticated encryption and return encrypted data with prepended nonce
             byte[] nonce = Sodium.PublicKeyBox.GenerateNonce();
             return ByteCombiner.Combine
                     (nonce,
@@ -15,5 +16,21 @@ namespace chestcrypto {
                         pubkey
                     ));
         }
+
+        public static byte[] decrypt(byte[] privkey, byte[] pubkey, byte[] message){
+            byte[] nonce = new byte[NONCE_BYTE_AMOUNT];
+            byte[] encrypted = new byte[message.Length - NONCE_BYTE_AMOUNT];
+            int counter = 0;
+            for (int i = 0; i < message.Length; i++){
+                if (i < NONCE_BYTE_AMOUNT){
+                    nonce[i] = message[i];
+                    continue;
+                }
+                encrypted[counter] = message[i];
+                counter += 1;
+            }
+
+            return Sodium.PublicKeyBox.Open(encrypted, nonce, privkey, pubkey);
+        }
     }
 }