. Any use is subject to the Terms of Use as given at the website. SECURITY COPROCESSORS SECURITY COPROCESSORS 369 classical cryptographic technique called Feistel network. This network is at the heart of many mod- ern ciphers, such as Kasumi. One of the main characteristics of the Feistel structure is that working the properly scheduled subkeys at the receiver and rearranging the left and right sides ensures that decryption is a straightforward process. This property makes the same algorithm capable of encrypt- ing and decrypting. The encryption engine produces the ciphertext output that corresponds to the plaintext block it had previously read in only after all the specified rounds are completed. In the case of DES, it would mean that 64 bits of ciphertext are produced corresponding to the 64 bits (equal to one block length) of plain- text that it just finished processing. If more plaintext is available at the input, the process continues with a next block of input bits. If not, there is usually a specification as to what happens at the last block of plaintext-for example, padding with zeros to generate a block of input bits with some con- trol characters to notify the receiver that no more input is available. In some cases, higher-level protocols on the stack communicate the length of the transmitted bit sequence up front SO by the time encryption occurs, the receiver is already set up with session coun- ters to accept only the amount of bits it was told it was going to receive. Therefore, no cryptographic prowess is necessary to address the issue. At decryption, the process is simply inversed. Based on the decryption key, the receiver starts by performing the same key generation and scheduling process. The list of subkeys is usually engaged with the reverse order, starting from the last one and moving on to the first one that was used at encryption. Stream Ciphers In this case, which is mostly (but not exclusively) favored by the military and intelligence communi- ties, the plaintext is processed bit per bit w