Added a random salt to prevent ID clashes between anonymous messages.

api/net/sf/briar/api/crypto/CryptoComponent.java

@@ -2,6 +2,7 @@ package net.sf.briar.api.crypto;
 
 import java.security.KeyPair;
 import java.security.MessageDigest;
+import java.security.SecureRandom;
 import java.security.Signature;
 
 import javax.crypto.Cipher;
@@ -36,5 +37,7 @@ public interface CryptoComponent {
 
 	MessageDigest getMessageDigest();
 
+	SecureRandom getSecureRandom();
+
 	Signature getSignature();
 }

api/net/sf/briar/api/protocol/Message.java

@@ -13,6 +13,9 @@ public interface Message {
 	/** The maximum length of a signature in bytes. */
 	static final int MAX_SIGNATURE_LENGTH = 100;
 
+	/** The length of the random salt in bytes. */
+	static final int SALT_LENGTH = 8;
+
 	/** Returns the message's unique identifier. */
 	MessageId getId();