Galois Message Authentication Code (GMAC) is an specialization of the GCM(Galois/Counter mode) and used for authentication, as defined in NIST800_38D. In this version, Advanced Encryption Standard (AES) based processing ability is provided. The cipherkey length for AES should be 128/192/256 bits. The implementation takes a fix-sized (128 bits per block) data stream, but the text in real world has a variety of lengths. Thus, you need to provide the data length in bits accompany with the data.