Cryptography and Network Security: Principles and Practice, 6 th Edition,
by William Stallings
CHAPTER 2: CLASSICAL ENCRYPTION TECHNIQUES
TRUE OR FALSE
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F
1. Symmetric encryption remains by far the most widely
used of the
two types of encryption.
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2. Rotor machines are sophisticated precomputer
hardware devices
that use substitution techniques.
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which
3. Symmetric encryption is a form of cryptosystem in
encryption and decryption are performed using
different keys. It is
also known as non- conventional encryption.
Lo correcto: el cifrado simtrico es una forma de criptosistema en el
cual el cifrado y decifrado son REALIZADOS CON LA MISMA CLAVE.
El simtrico es conocido tambin como CIFRADO CONVENCIONAL.
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security
4. With the use of symmetric encryption, the principal
problem is maintaining the secrecy of the key.
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known as
5. The process of converting from plaintext to ciphertext is
deciphering or decryption.
Lo correcto: el proceso de convertir de plaintext a cifrado es conocido
como CIFRADO. El proceso de resturar el plaintext desde el cifrado se
conoce como DECIFRADO.
Pag 28
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on the
6. The algorithm will produce a different output depending
specific secret key being used at the time. The exact
substitutions
and transformations performed by the algorithm
depend on the
key.
7. When using symmetric encryption it is very important to
Cryptography and Network Security: Principles and Practice, 6 th Edition,
by William Stallings
keep the
algorithm secret.
Lo correcto: cuando se usa cifrado simtrico NO ES necesario guardar
el algoritmo, se necesita solo la clave secreta.
Pag 29
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achieve
8. On average, half of all possible keys must be tried to
success with a brute-force attack.
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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.
Lo correcto: Un texto cifrado usando un esquema de cifrado es
computacionalmente seguro si cumple dos cosas: el costo de romper el
cifrado excede el valor de la informacin encriptado, o el tiempo de
romper el cifrado excede el tiempo de vida de la informacin.
T
F
10. Monoalphabetic ciphers are easy to break because they
reflect the
frequency data of the original alphabet.
T
11. As with Playfair, the strength of the Hill cipher is that it
completely hides single letter frequencies.
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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.
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13. The one-time pad has unlimited utility and is useful
primarily for
high-bandwidth channels requiring low security.
Me parece que one-time pad (cojin de una vez) no vimos.
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Standard.
14. The most widely used cipher is the Data Encryption
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15. Steganography renders the message unintelligible to
outsiders by
Cryptography and Network Security: Principles and Practice, 6 th Edition,
by William Stallings
various transformations of the text.
Los mtodos de criptografa son los que hacen que el mensaje sea
inteligible para los forasteros por diversas transformaciones de
texto.
Los mtodos de estenografa ocultan la existencia de un mensaje.
Pag 52.
MULTIPLE CHOICE
1. __________ techniques map plaintext elements (characters, bits)
into ciphertext elements.
A) Transposition
B) Substitution
C) Traditional
D) Symmetric
Tecnicas de sustitucion mapean elementos de texto dentro de
elementos cifrados.
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
C) polycipher
Creo que no vimos este tema
B) one-time pad
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
C) deciphering, enciphering
B) plaintext, ciphertext
D) cipher, plaintext
4. Restoring the plaintext from the ciphertext is __________ .
Cryptography and Network Security: Principles and Practice, 6 th Edition,
by William Stallings
A) deciphering
C) steganography
B) transposition
D) encryption
5. A __________ attack involves trying every possible key until an
intelligible translation of the ciphertext is obtained.
A) brute-force
C) ciphertext only
B) Caesar attack
D) chosen plaintext
6. Techniques used for deciphering a message without any
knowledge of the enciphering details is ___________ .
A) blind deciphering
C) cryptanalysis
B) steganography
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
C) asymmetric
(tambien llamada simetrica)
B) two-key
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
Cryptography and Network Security: Principles and Practice, 6 th Edition,
by William Stallings
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
C) Hill cipher
B) Playfair 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) Streaming
C) Digrams
cipher
B) Transposition
D) Polyalphabetic
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
cipher
D) polyanalysis
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
Cryptography and Network Security: Principles and Practice, 6 th Edition,
by William Stallings
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. Symtmetric 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 2.
Steganography
3. An encryption scheme is said to be 3. computationally secure 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 4.
brute-force 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 5.
The
number of keys used.
6. All encryption algorithms are based on two general principles:
substitution and
Permutation .
7. One of the simplest and best known polyalphabetic ciphers is 7.
Vigenre 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 Block cipher processes the input one block of elements at a time
Cryptography and Network Security: Principles and Practice, 6 th Edition,
by William Stallings
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 unconditionally 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 caesar 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 Playfair
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 one-time pad scheme.
13. The simplest transposition cipher is the rail fence 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 DES .
15. The rotor machines 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.
