Use a constant for the tag size.

api/net/sf/briar/api/transport/PacketReader.java

components/net/sf/briar/crypto/SharedSecret.java

...ents/net/sf/briar/transport/ConnectionRecogniserImpl.java

components/net/sf/briar/transport/Constants.java

components/net/sf/briar/transport/PacketEncrypter.java

components/net/sf/briar/transport/PacketEncrypterImpl.java

components/net/sf/briar/transport/TagEncoder.java

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

.../net/sf/briar/transport/ConnectionRecogniserImplTest.java

test/net/sf/briar/transport/PacketEncrypterImplTest.java

test/net/sf/briar/transport/PacketWriterImplTest.java