Exam 3: Block Ciphers and the Data Encryption Standard

The vast majority of network based symmetric cryptographic applications make use of stream ciphers.

The ________ cipher structure,which dates back over a quarter century and which,in turn,is based on Shannon's proposal of 1945,is the structure used by many significant symmetric block ciphers currently in use.

Feistel proposed that we can approximate the ideal block cipher by utilizing the concept of a __________ cipher,which is the execution of two or more simple ciphers in sequence in such a way that the final result or product is cryptographically stronger than any of the component ciphers.

Two alternatives to DES are AES and _________ DES.

A prime concern with DES has been its vulnerability to brute-force attack because of its relatively short key length.

A __________ cipher is one that encrypts a digital data stream one bit or one byte at a time.

The heart of a Feistel block cipher is the function F,which relies on the use of S-boxes.

__________ is when each plaintext element or group of elements is uniquely replaced by a corresponding ciphertext element or group of elements.

The Feistel cipher structure,based on Shannon's proposal of 1945, dates back over a quarter of a century and is the structure used by many significant symmetric block ciphers currently in use.

Key sizes of __________ or less are now considered to be inadequate.

A sequence of plaintext elements is replaced by a __________ of that sequence which means that no elements are added,deleted or replaced in the sequence,but rather the order in which the elements appear in the sequence is changed.

A __________ cipher is one in which a block of plaintext is treated as a whole and used to produce a ciphertext block of equal length.

Confusion seeks to make the statistical relationship between the plaintext and ciphertext as complex as possible in order to thwart attempts to deduce the key.

In ___________ the statistical structure of the plaintext is dissipated into long-range statistics of the ciphertext.This is achieved by having each plaintext digit affect the value of many ciphertext digits.

All other things being equal,smaller block sizes mean greater security.

A problem with the ideal block cipher using a small block size is that it is vulnerable to a statistical analysis of the plaintext.

Greater complexity in the subkey generation algorithm should lead to greater difficulty of cryptanalysis.

The vast majority of network-based symmetric cryptographic applications make use of ________ ciphers.

DES exhibits the classic __________ block cipher structure,which consists of a number of identical rounds of processing.

If the bit-stream generator is a key-controlled algorithm the two users only need to share the generating key and then each can produce the keystream.