a recirculating 64-bit block whose security is based on a secret key. DES keys are 64-bit binary vectors consisting of 56 independent information bits and eight parity bits. The 56 in- formation bits are used by the enciphering and deciphering opera- tions and are referred to as the key. Active keys are selected at random from all possible keys by each group of authorized users. Each user should understand that the key must be protected and that any compromise of the key will compromise all data protected by that key. In the encryption computation the 64-bit input is divided into two halves each consisting of 32 bits. One half is used as input to a com- Location "A" Location "B" Clear Cipher Clear Encryption Decryption Algorithm Algorithm Terminal Fig. 5-7. Conventional cryptographic system. NETWORK MANAGEMENT 81 plex nonlinear function, and the result is exclusive OR'ed to the other half. After one round, the two halves of data are swapped and the operation is performed again. The DES algorithm uses 16 rounds to produce a recirculating block cipher. Encryption, however complex the process may seem, is the only realistic way of protecting data in transit. Few security managers would argue the need for system protection, but convincing man- agement acceptance due to costs and system resource implications is the real problem to the solved, prior to installation of any crypto- graphic products. Management must be prepared to weigh the costs and benefits of protecting the organization's information system to decide whether encryption is worth the expense involved. Since some cryptographic systems use cpu cycles, they affect the overall com- munication system performance and response time. The degree to which performance is affected depends upon the specific hardware and software used. Consequently, the project manager should de- mand that vendors demonstrate exactly what effect their product will have on the operating system-prior to purchase and installation. NETWORK STRUCTURES 6.1 TOPOLOGY TRADEOFFS If we look at a