des Multiple Input Multiple Output (MIMO) techniques, invented by Foschini et al. at Bell Labs in 1996, have become an important means of improving the capacity and robustness of wireless systems. They are now being incorporated in several standards (for example, IEEE 802.11n and 802.16e-2005). MIMO systems use multiple antennas at the transmitter or receiver or both, and exploit the fact that the transmitted signals traverse a "multipath" environment (with scattering, reflection, refraction, and SO on). Before 1996 it was known that using multiple receive antennas and either selecting the best received signal or opti- mally combining received signals could improve transmission reliability. This is known as diversity reception. Where multiple antennas are used instead at the transmitter, Space-Time Block Codes (STBCs) can be used to encode the datastream across the multiple antennas. At the receiver, which can use one or more antennas, some of the received copies of the data will be "better" than others, and this redundancy can be exploited to estimate the transmit- ted digits with minimum error. STBCs offer only diversity gain (compared to single-antenna schemes) and not coding gain. One particular STBC is the code invented by Alamouti in 1998 for a two-transmit-antenna system. It is the simplest example of a class called orthogonal STBCs, and these are particularly attractive because optimal (maximum-likelihood) decod- ing can be achieved at the receiver with only linear processing. MIMO techniques can achieve not only diversity gain but also an increase in capacity. Space-time trellis codes (STTCs) provide both diversity and coding gain by spreading a conventional trellis code over space (multiple antennas) and time (Tarokh et al., 1998). Encryption It is sometimes the case that a sender wishes to send a recipient a message that he wants to keep secret from an eavesdropper. This message, which in its usable form is called plaintext, Encryption must be transmitted over a channel to which the eavesdropper is pre