Fix a bug where the password encode function added trailing null bytes

Funny story. Instead of	obtaining the actual bytes from the ByteBuffer in the
password encode function, we obtained the entire buffer. This caused some
trailing null bytes to be added to the encoded password. Luckily (and
strangely), PBKDF2 produces collisions for inputs with trailing null bytes and
thus scrypt does this as well. As such, this bug doesn't affect us right now,
but it would if we were to use that encode function for other purposes in the
future.

This also adds a test that checks for the expected collision behavior of scrypt.

app/src/test/java/com/beemdevelopment/aegis/SCryptTest.java

@@ -0,0 +1,40 @@
+package com.beemdevelopment.aegis;
+
+import com.beemdevelopment.aegis.crypto.CryptoUtils;
+import com.beemdevelopment.aegis.crypto.SCryptParameters;
+import com.beemdevelopment.aegis.encoding.Hex;
+import com.beemdevelopment.aegis.encoding.HexException;
+
+import org.junit.Test;
+
+import javax.crypto.SecretKey;
+
+import static org.junit.Assert.*;
+
+public class SCryptTest {
+    @Test
+    public void testTrailingNullCollision() throws HexException {
+        byte[] salt = new byte[0];
+        SCryptParameters params = new SCryptParameters(
+                CryptoUtils.CRYPTO_SCRYPT_N,
+                CryptoUtils.CRYPTO_SCRYPT_p,
+                CryptoUtils.CRYPTO_SCRYPT_r,
+                salt
+        );
+
+        byte[] expectedKey = Hex.decode("41cd8110d0c66ede16f97ce84fd8e2bd2269c9318532a01437789dfbadd1392e");
+        byte[][] inputs = new byte[][]{
+                new byte[]{'t', 'e', 's', 't'},
+                new byte[]{'t', 'e', 's', 't', '\0'},
+                new byte[]{'t', 'e', 's', 't', '\0', '\0'},
+                new byte[]{'t', 'e', 's', 't', '\0', '\0', '\0'},
+                new byte[]{'t', 'e', 's', 't', '\0', '\0', '\0', '\0'},
+                new byte[]{'t', 'e', 's', 't', '\0', '\0', '\0', '\0', '\0'},
+        };
+
+        for (byte[] input : inputs) {
+            SecretKey key = CryptoUtils.deriveKey(input, params);
+            assertArrayEquals(expectedKey, key.getEncoded());
+        }
+    }
+}