Chapter 3 1 English Language Streamings Transpositions Digrams Polyalphabetic Ciphers

CHAPTER 3: CLASSICAL ENCRYPTION TECHNIQUES

TRUE OR FALSE

T F 1. Symmetric encryption remains by far the most widely used of the

two types of encryption.

T F 2. Rotor machines are sophisticated precomputer hardware devices

that use substitution techniques.

T F 3. Symmetric encryption is a form of cryptosystem in which

encryption and decryption are performed using different keys. It is

also known as non-conventional encryption.

T F 4. With the use of symmetric encryption, the principal security

problem is maintaining the secrecy of the key.

T F 5. The process of converting from plaintext to ciphertext is known as

deciphering or decryption.

T F 6. The algorithm will produce a different output depending on the

specific secret key being used at the time. The exact substitutions

and transformations performed by the algorithm depend on the

key.

T F 7. When using symmetric encryption it is very important to keep the

algorithm secret.

T F 8. On average, half of all possible keys must be tried to achieve

success with a brute-force attack.

T F 9. Ciphertext generated using a computationally secure encryption

scheme is impossible for an opponent to decrypt simply because

the required information is not there.

T F 10. Monoalphabetic ciphers are easy to break because they reflect the

frequency data of the original alphabet.

T F 11. As with Playfair, the strength of the Hill cipher is that it

completely hides single letter frequencies.

T F 12. A scheme known as a one-time pad is unbreakable because it

produces random output that bears no statistical relationship to

the plaintext.

T F 13. The one-time pad has unlimited utility and is useful primarily for

high-bandwidth channels requiring low security.

T F 14. The most widely used cipher is the Data Encryption Standard.

T F 15. Steganography renders the message unintelligible to outsiders by

various transformations of the text.

MULTIPLE CHOICE

1. __________ techniques map plaintext elements (characters, bits) into ciphertext

elements.

A) Transposition B) Substitution

C) Traditional D) Symmetric

2. Joseph Mauborgne proposed an improvement to the Vernam cipher that uses

a random key that is as long as the message so that the key does not need to

be repeated. The key is used to encrypt and decrypt a single message and

then is discarded. Each new message requires a new key of the same

length as the new message. This scheme is known as a(n) __________ .

A) pascaline B) one-time pad

C) polycipher D) enigma

3. An original intelligible message fed into the algorithm as input is known as

_________ , while the coded message produced as output is called the __________ .

A) decryption, encryption B) plaintext, ciphertext

C) ciphertext, plaintext D) encryption, decryption

4. Restoring the plaintext from the ciphertext is __________ .

A) deciphering B) transposition

C) steganography D) encryption

5. A __________ attack involves trying every possible key until an intelligible

translation of the ciphertext is obtained.

A) brute-force B) Caesar attack

C) ciphertext only D) chosen plaintext

6. Techniques used for deciphering a message without any knowledge of the

enciphering details is ___________ .

A) blind deciphering B) steganography

C) cryptanalysis D) transposition

7. The ___________ takes the ciphertext and the secret key and produces the

original plaintext. It is essentially the encryption algorithm run in reverse.

A) Voronoi algorithm B) decryption algorithm

C) cryptanalysis D) diagram algorithm

8. If both sender and receiver use the same key, the system is referred to as:

A) public-key encryption B) two-key

C) asymmetric D) conventional encryption

9. __________ attacks exploit the characteristics of the algorithm to attempt to

deduce a specific plaintext or to deduce the key being used.

A) Brute-force B) Cryptanalytic

C) Block cipher D) Transposition

10. The __________ was used as the standard field system by the British Army in

World War I and was used by the U.S. Army and other Allied forces during

World War II.

A) Caesar cipher B) Playfair cipher

C) Hill cipher D) Rail Fence cipher

11. The __________ attack is the easiest to defend against because the opponent has

the least amount of information to work with.

A) ciphertext-only B) chosen ciphertext

C) known plaintext D) chosen plaintext

12. _________ refer to common two-letter combinations in the English language.

A) Streamings B) Transpositions

C) Digrams D) Polyalphabetic ciphers

13. A way to improve on the simple monoalphabetic technique is to use different

monoalphabetic substitutions as one proceeds through the plaintext

message. The general name for this approach is ___________ .

A) rail fence cipher B) cryptanalysis

C) polyalphabetic substitution cipher D) polyanalysis cipher

14. A technique referred to as a __________ is a mapping achieved by performing

some sort of permutation on the plaintext letters.

A) transposition cipher B) polyalphabetic cipher

C) Caesar cipher D) monoalphabetic cipher

15. The methods of __________ conceal the existence of the message in a graphic

image.

A) steganography B) decryptology

C) cryptology D) cryptography

SHORT ANSWER

1. __________ encryption is a form of cryptosystem in which encryption and

decryption are performed using the same key.

2. A technique for hiding a secret message within a larger document or picture in

such a way that others cannot discern the presence or contents of the hidden

message is __________ .

3. An encryption scheme is said to be __________ if the cost of breaking the cipher

exceeds the value of the encrypted information and the time required to break the

cipher exceeds the useful lifetime of the information.

4. The two types of attack on an encryption algorithm are cryptanalysis, based on

properties of the encryption algorithm, and _________ which involves trying all

possible keys.

5. Cryptographic systems are characterized along three independent dimensions:

the type of operations used for transforming plaintext to ciphertext; the way in

which the plaintext is processed; and __________ .

6. All encryption algorithms are based on two general principles: substitution and

____________ .

7. One of the simplest and best known polyalphabetic ciphers is _________ cipher. In

this scheme, the set of related monoalphabetic substitution rules consists of the 26

Caesar ciphers with shifts of 0 through 25. Each cipher is denoted by a key letter

which is the ciphertext letter that substitutes for the plaintext letter a.

8. A __________ cipher processes the input one block of elements at a time, producing

an output block for each input block, whereas a stream cipher processes the input

elements continuously, producing output one element at a time.

9. An encryption scheme is __________ secure if the ciphertext generated by the

scheme does not contain enough information to determine uniquely the

corresponding plaintext, no matter how much ciphertext is available.

10. The earliest known, and simplest use, of a substitution cipher was called the

__________ cipher and involved replacing each letter of the alphabet with the letter

standing three places further down the alphabet.

11. The best known multiple letter encryption cipher is the __________ , which treats

digrams in the plaintext as single units and translates these units into ciphertext

digrams.

12. The task of making large quantities of random keys on a regular basis and

distributing a key of equal length to both sender and receiver for every message

sent are difficulties of the __________ scheme.

13. The simplest transposition cipher is the _________ technique, in which the

plaintext is written down as a sequence of diagonals and then read off as a sequence

of rows.

14. The most widely used cipher ever is the __________ .

15. The __________ consist of a set of independently rotating cylinders through which

electrical pulses can flow. Each cylinder has 26 input pins and 26 output pins with

internal wiring that connects each input pin to a unique output pin.

CHAPTER 3: CLASSICAL ENCRYPTION TECHNIQUES

TRUE OR FALSE

MULTIPLE CHOICE

SHORT ANSWER