This document proposes two new techniques for authenticating short encrypted messages in mobile and pervasive applications that are more efficient than existing approaches. The first technique utilizes the fact that messages are encrypted to append a short random string for authentication without needing to manage one-time keys. The second technique further improves efficiency by leveraging properties of block cipher encryption algorithms. The proposed techniques allow authentication to benefit from simplicity of unconditional security while avoiding inefficiencies of standalone authentication primitives or general-purpose MAC algorithms.