Javadoc and unit test for SharedSecret.

test/net/sf/briar/crypto/SharedSecretTest.java

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+package net.sf.briar.crypto;
+
+import java.util.Arrays;
+import java.util.Random;
+
+import junit.framework.TestCase;
+
+import org.junit.Test;
+
+public class SharedSecretTest extends TestCase {
+
+	@Test
+	public void testDecodeAndEncode() {
+		Random random = new Random();
+		byte[] secret = new byte[40];
+		random.nextBytes(secret);
+		secret[16] = (byte) 0;
+		SharedSecret s = new SharedSecret(secret);
+		assertTrue(Arrays.equals(secret, s.getBytes()));
+		secret[16] = (byte) 1;
+		s = new SharedSecret(secret);
+		assertTrue(Arrays.equals(secret, s.getBytes()));
+		// The Alice flag must be either 0 or 1
+		secret[16] = (byte) 2;
+		try {
+			s = new SharedSecret(secret);
+			fail();
+		} catch(IllegalArgumentException expected) {}
+		// The secret must be at least 18 bytes long
+		secret = new byte[17];
+		random.nextBytes(secret);
+		secret[16] = (byte) 0;
+		try {
+			s = new SharedSecret(secret);
+			fail();
+		} catch(IllegalArgumentException expected) {}
+	}
+}