Encryption Key Plaintext) Encryption Decryption Active Adversary FIGURE 97.3 Basic cryptosystem functions 64-bit plaintext bloc ck 56-bit key(8 parity checks discarded) split into two 32-bit groups split into two 28-bit groups 匚 expansion to48bts 48-bit key extract combine/extract/transpose □8 S-boxes:6m4ou permutation nterchange (total of 16 sequentially produ and similar key extractions total of 16 sequentially produced and similar rounds") 匚64 bit ciphertext block FIGURE 97.4 Basic function of the DES algorithm. which is used to encrypt information by the sender and to decrypt information by the recipient. a prior secure process is necessary so that both sender and recipient know(and no adversary knows)the key. The most well-known and most widely used single-key cryptosystem in history is the Data Encryption Standard(DES), published by the U.S. National Bureau of Standards [1977](now the National Institute of Standards and Technology, NIST), with National Security Agency(NSA) consultation. DES utilizes a 56-bit key(some weak and semi-weak keys are excluded) to encipher information in blocks of 64 bits. It involves substitution and permutation, linear and nonlinear transformations, and 16 successive rounds" of key-depen- dent processing(general indication of logic shown in Fig. 97. 4). The DES cryptosystem is identical for encryp- on and decryption, except that the order of application of the 16 key extractions is reversed. Like most cryptosystems of this type, DES is usually used with some form of chaining(mixing ciphertext or information that produces ciphertext from one block with plaintext or information that produces ciphertext in the quent block at the transmitter, and then inverting the process at the receiver). Three chaining technique specified for DES (and usable in most other cryptosystems)are indicated in Fig. 97.5, along with the basic electronic codebook block form. The k bits shown are typically eight bits, and these are shifted into the first k positions of a shift-register/buffer after each encryption. Coordinated time stamps or initial values(Ivs)ar used to prevent identical transformation for each system start. c 2000 by CRC Press LLC© 2000 by CRC Press LLC which is used to encrypt information by the sender and to decrypt information by the recipient. A prior secure process is necessary so that both sender and recipient know (and no adversary knows) the key. The most well-known and most widely used single-key cryptosystem in history is the Data Encryption Standard (DES), published by the U.S. National Bureau of Standards [1977] (now the National Institute of Standards and Technology, NIST), with National Security Agency (NSA) consultation. DES utilizes a 56-bit key (some weak and semi-weak keys are excluded) to encipher information in blocks of 64 bits. It involves substitution and permutation, linear and nonlinear transformations, and 16 successive “rounds” of key-depen￾dent processing (general indication of logic shown in Fig. 97.4). The DES cryptosystem is identical for encryp￾tion and decryption, except that the order of application of the 16 key extractions is reversed. Like most cryptosystems of this type, DES is usually used with some form of chaining (mixing ciphertext or information that produces ciphertext from one block with plaintext or information that produces ciphertext in the subse￾quent block at the transmitter, and then inverting the process at the receiver). Three chaining techniques specified for DES (and usable in most other cryptosystems) are indicated in Fig. 97.5, along with the basic electronic codebook block form. The k bits shown are typically eight bits, and these are shifted into the first k positions of a shift-register/buffer after each encryption. Coordinated time stamps or initial values (IVs) are used to prevent identical transformation for each system start. FIGURE 97.3 Basic cryptosystem functions. FIGURE 97.4 Basic function of the DES algorithm