The base interface for implementations of message authentication codes (MACs).
This is from some very old version of bouncycastle, part of package org.bouncycastle.crypto. Android bundled something similar in pre-Gingerbread, but upgraded to a later, incompatible version in Gingerbread. As of Java 1.4 these are in javax.crypto - more or less. To avoid having to make two different versions of our Android app, we rename to org.bouncycastle.oldcrypto.