Ciphertext/plaintext and ciphertext/key dependence vs number of rounds for the data encryption standard

One property of the Data Encryption Standard (DES) is that each bit of ciphertext is a complicated function of all plaintext bits and all key bits. A method is developed which evaluates how fast this dependence (defined as intersymbol dependence) builds up as a function of repeated mathematical operations called "rounds." It is shown that the minimum number of rounds to achieve intersymbol dependence for plairitext as well as key is five. Cryptography deals with the methods involved in preparing cryptograms-messages or writings intended to be incomprehensible to all except those who legitimately possess the means to recover the original information. The designer of a cryptographic system, or cryptosystem, is a cryptographer. The opponent or antagonist of a cryptosystem is a cryptanalyst. Cryptanalysis is concerned with techniques used to penetrate communications and recover the original information by means other than those available to the legitimate recipient. Cryptology is the science of disguised or secret communications. It embraces both cryptography and cryptanalysis. The basic challenge in cryptography is to devise a method that transforms messages (known as plaintext) into cryptograms (known as ciphertext) in a cryptographically secure way-that is, the method must withstand intense efforts of cryptanalysis. Plaintext can be protected by either of two techniques: it can be encoded using a code system, or it can be enciphered (encrypted) using a cipher System. Code systems require a code book or dictionary that relates the words, phrases, and sentences of the vocabulary (the plaintext) to its equivalent code group (the ciphertext), and vice versa. The number of plaintext messages that can be encoded depends on the number of combinations of phrases that can be obtained from the code book. Although that number may be large, not every combination or pattern of bits can be encoded. Hence the versatility and usefulness of code systems is limited, especially in computer applications, which would have to be programmed to handle all such exceptions.