The relevant concepts of the most efficient uniquely decipherable and maximal uniquely decipherable encoding schemes similar to highly efficient instantaneous encoding schemes are introduced. We show that the Huffman encoding schemes are the most efficient possible among both all uniquely decipherable encoding schemes and all maximal uniquely decipherable encoding schemes, respectively. Generally, decoding messages encrypted by a uniquely decipherable encoding scheme is more difficult than decoding messages encrypted by an instantaneous encoding scheme. However, from the security point of view, it is much more secure to apply a uniquely decipherable encoding scheme than to apply an instantaneous encoding scheme to an information system.