The strength of a cryptographic method is often measured in terms of work factor, which is equal to the amount of force that can be brought to bear in the analysis of a problem. Coding is the method by which words or numbers are replaced by code equivalents drawn from a code book. A good system will use multiple equivalents for the same word. Code management techniques -- time-of-day codes, one-time codes, storage, distribution, and code lock-up -- can all increase the system's effectiveness. Scrambling is a technique whereby information is hidden through frequency transposition or signal inversion, whereas enciphering is usually done at the level of individual characters or electrical bits of information. As a result, enciphering completely alters the information being transmitted. Modern encryption is based on the use of algorithms, key variables, binary, and other modular mathematics. Selection of an appropriate encryption system is based on the good-enough principle, which relates its cost and strengths to the value of the information being protected. Other factors in choosing a system should be user- friendliness, quality of voice reproduction, connectivity over low-quality transmission media, flexibility, utility, size, and data rates.