2 Marks Questions (Any 4)
Q1. What is the purpose of security.
Information security is needed because some organizations can be damaged by hostile
application or intruders.
There can be multiple forms of damage which are interrelated. These includes −
▪ It can be damage or destruction of computer systems.
▪ It can be damage or destruction of internal data.
▪ It can be used to loss of sensitive information to hostile parties.
▪ It is the use of sensitive information to steal items of monetary value.
▪ It is the use of sensitive information against the organization’s customers which may
result in legal action by customers against the organization and loss of customers.
▪ It is used to damage to the reputation of an organization.
▪ It can be used to monetary damage due to loss of sensitive information, destruction
of data, hostile use of sensitive data, or damage to the organization’s reputation.
Q2. What is the difference between plain text and cipher text.
Plaintext can refer to anything which humans can understand and/or relate to. This may be
as simple as English sentences, a script, or Java code. If you can make sense of what is
written, then it is in plaintext.
Ciphertext, or encrypted text, is a series of randomized letters and numbers which humans
cannot make any sense of. An encryption algorithm takes in a plaintext message, runs the
algorithm on the plaintext, and produces a ciphertext. The ciphertext can be reversed
through the process of decryption, to produce the original plaintext.
Example: We will encrypt a sentence using Caesar Cipher. The key is 7, which means the
letter a becomes h.
Plaintext: This is a plaintext.
Ciphertext: Aopz pz h wshpualea.
Q3. What is attack? How it can be prevented.
An attack is an information security threat that involves an attempt to obtain, alter, destroy,
remove, implant, or reveal information without authorized access or permission. It happens
to both individuals and organizations. There are many kinds of attacks, including but not
limited to passive, active, targeted, clickjacking, brandjacking, botnet, phishing, spamming,
inside and outside.
Q4. What is difference between Confidentiality and Authorization?
Confidentiality: The degree of confidentiality determines the secrecy of the information. The
principle specifies that only the sender and receiver will be able to access the information
shared between them. Confidentiality compromises if an unauthorized person is able to
access a message.
For example, let us consider sender A wants to share some confidential information with
receiver B and the information gets intercepted by the attacker C. Now the confidential
information is in the hands of an intruder C.
Authentication: Authentication is the mechanism to identify the user or system or the entity.
It ensures the identity of the person trying to access the information. The authentication is
mostly secured by using username and password. The authorized person whose identity is
preregistered can prove his/her identity and can access the sensitive information.
Q5. Give two differences between Block Cipher and Stream cipher.
Block Cipher
Block Cipher Converts the plain text into
cipher text by taking plain text’s block at a
time.
Block cipher uses either 64 bits or more
than 64 bits.
The complexity of block cipher is simple.
Block cipher Uses confusion as well as
diffusion.
In block cipher, reverse encrypted text is
hard.
The algorithm modes which are used in
block cipher are ECB (Electronic Code Book)
and CBC (Cipher Block Chaining).
Block cipher works on transposition
techniques like rail-fence technique,
columnar transposition technique, etc.
Block cipher is slow as compared to a
stream cipher.
Stream Cipher
Stream Cipher Converts the plain text
into cipher text by taking 1 byte of plain
text at a time.
While stream cipher uses 8 bits.
While stream cipher is more complex.
While stream cipher uses only confusion.
While in-stream cipher, reverse
encrypted text is easy.
The algorithm modes which are used in
stream cipher are CFB (Cipher Feedback)
and OFB (Output Feedback).
While stream cipher works on
substitution techniques like Caesar
cipher, polygram substitution cipher, etc.
While stream cipher is fast in comparison
to block cipher.
Q6. What is the purpose of symmetric encryption?
Since symmetric encryption performs smoother and quicker than asymmetric encryption, it
is commonly used for bulk encryption / encrypting massive volumes of data, such as
database encryption. In a database, the secret key can be used only by the database to
encrypt or decrypt data.
Here are a few instances of where symmetric cryptography is used:
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Payment applications, such as bank purchases, where personally identifiable information
(PII) must be secured to avoid identity theft or fraudulent charges
Validations are performed to ensure that the sender of a message is who he appears to
be.
Hashing or random number creation
Q7. Explain the concept of Feistel Block Cipher.
The Feistel cipher is a design model or structure used to build various symmetric block
ciphers, such as DES. This design model can have invertible, non-invertible, and selfinvertible components. Additionally, the Feistel block cipher uses the same encryption and
decryption algorithms.
Q8. Illustrate the concept of Blowfish algorithm.
Blowfish is a variable-length, symmetric, 64-bit block cipher. It was intended to provide a
fast, free, drop-in alternative to the aging Data Encryption Standard (DES) and International
Data Encryption Algorithm (IDEA) encryption algorithms.
Q9. Explain the concept of IDEA cryptography?
The International Data Encryption Algorithm (IDEA) is a symmetric key block cipher
encryption algorithm designed to encrypt text to an unreadable format for transmission via
the internet. It uses a typical block size of 128 bits and takes 64 bits as an input, i.e., 64-bit
data.
6 Marks Questions (Any 2)
Q1. Define key size and key range in cryptography
Key Range is total number of keys from smallest to largest available key.
Key Size is measured in bits and is represented using binary number system. Thus, if the key
ranges from 0 to 8, then the key size is 3 bits or in other words we can say if the size is bits,
then the key range is 0 to 256.
A key range may contain individual single arbitrary quantity whereas key size defines the
total or maximum capacity of all the keys.
Q2. Explain cryptanalytic attacks.
The attacks rely on nature of the algorithm and knowledge of the general characteristics of
the plaintext, i.e., plaintext can be a regular document written in English or it can be a code
written in Java. Therefore, nature of the plaintext should be known before trying to use the
attacks. This type of attacks is called Cryptanalytic attacks.
Types of Cryptanalytic attacks :
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Known-Plaintext Analysis (KPA)
Chosen-Plaintext Analysis (CPA) :
Ciphertext-Only Analysis (COA)
Man-In-The-Middle (MITM) attack
Adaptive Chosen-Plaintext Analysis (ACPA)
Q3. Differentiate between Substitution cipher and Transposition
Substitution Cipher Technique
In substitution Cipher Technique, plain
text characters are replaced with other
characters, numbers and symbols.
Substitution Cipher’s forms are: Mono
alphabetic substitution cipher and poly
alphabetic substitution cipher.
In substitution Cipher Technique,
character’s identity is changed while its
position remains unchanged.
In substitution Cipher Technique, the
letter with low frequency can detect
plain text.
The example of substitution Cipher is
Caesar Cipher.
Transposition Cipher Technique
In transposition Cipher Technique, plain text
characters are rearranged with respect to
the position.
Transposition Cipher’s forms are Key-less
transposition cipher and keyed transposition
cipher.
While in transposition Cipher Technique, the
position of the character is changed but
character’s identity is not changed.
While in transposition Cipher Technique,
The Keys which are nearer to correct key
can disclose plain text.
The example of transposition Cipher is Rail
Fence Cipher.
Q4. What do you understand by substitution technique.
In Substitution Cipher Technique plain text characters are replaced with other characters,
numbers, and symbols as well as in substitution Cipher Technique, character’s identity is
changed while its position remains unchanged.
Types of Substitution Technique:
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Caesar Cipher
Monoalphabetic Cipher
Homophonic Cipher
PolyGram Cipher
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Playfair Cipher
Vernam Cipher
Hill Cipher
Q5. Differentiate between symmetric key cryptography and asymmetric key cryptography.
Symmetric Key Encryption
It only requires a single key for both
encryption and decryption.
The size of cipher text is the same or
smaller than the original plain text.
The encryption process is very fast.
It is used when a large amount of data is
required to transfer.
It only provides confidentiality.
The length of key used is 128 or 256 bits
In symmetric key encryption, resource
utilization is low as compared to
asymmetric key encryption.
Asymmetric Key Encryption
It requires two keys, a public key, and a
private key, one to encrypt and the other
one to decrypt.
The size of cipher text is the same or
larger than the original plain text.
The encryption process is slow.
It is used to transfer small amounts of
data.
It provides confidentiality, authenticity,
and non-repudiation.
The length of key used is 2048 or higher
In asymmetric key encryption, resource
utilization is high.
It is efficient as it is used for handling large
amount of data.
Security is less as only one key is used for
both encryption and decryption purpose.
It is comparatively less efficient as it can
handle a small amount of data.
It is more secure as two keys are used
here- one for encryption and the other for
decryption.
Q6. Explain the main concepts of DES.
Data encryption standard (DES) has been found vulnerable to very powerful attacks and
therefore, the popularity of DES has been found slightly on the decline. DES is a block cipher
and encrypts data in blocks of size of 64 bits each, which means 64 bits of plain text go as
the input to DES, which produces 64 bits of ciphertext. The same algorithm and key are used
for encryption and decryption, with minor differences. The key length is 56 bits.
Q7. Explain the concept of confusion and diffusion.
Confusion is a cryptographic technique devised to increase the vagueness of the cipher text.
In simple words, confusion is a technique of ensuring that a cipher text gives no clue about
the original plain text.
Diffusion means that if we change a character of the plaintext, then several characters of
the ciphertext should change, and similarly, if we change a character of the ciphertext, then
several characters of the plaintext should change. Thus, diffusion is a cryptographic
technique invented to increase the redundancy of the plain text
Q8. How does the one-time initialization step work in AES
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As AES requires 10 rounds it while need 10 keys and 1 more key for OTI
In all eleven keys are required
So, the 16-byte key is expanded to get the actual block i.e., the 16-byte key is
expanded into a key containing 4*4 entries
Out of the 11 keys 1 key is used for OTI and the remaining 10 keys are used for 10
rounds
Q10. Illustrate the steps of RSA Algorithm.
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Choose two large prime numbers (p and q)
Calculate n = p*q and z = (p-1)(q-1)
Choose a number e where 1 < e < z.
Calculate d = e-1mod(p-1)(q-1)
You can bundle private key pair as (n,d)
You can bundle public key pair as (n,e)
10 Marks Question (Any 2)
Q1. Explain network security attacks on the basis of security goals.
A network attack is an attempt to gain unauthorized access to an organization’s network,
with the objective of stealing data or perform other malicious activity. There are two main
types of network attacks:
Passive Attack : A passive attack attempts to learn or make use of information from the
system but does not affect system resources.
Active attacks : An active attack attempts to alter system resources or affect their operation.
The security attacks aim to compromise the five major security goals for network security:
Confidentiality, Availability, Authentication, Integrity, and Nonrepudiation.
Q2. Explain Playfair cipher with example. Encrypt HIDE MONEY using playfair.
The Playfair Cipher, also called as Playfair Square, is a cryptographic technique that is used
for manual encryption of data. This scheme was invented by Charles Wheatstone in 1854.
However, eventually the scheme came to be known by the name of Lord Playfair.
It initially creates a key-table of 5*5 matrix. The matrix contains alphabets that act as the
key for encryption of the plaintext. Note that any alphabet should not be repeated. Another
point to note that there are 26 alphabets and we have only 25 blocks to put a letter inside it.
Therefore, one letter is excess so, a letter will be omitted (usually J) from the matrix.
Nevertheless, the plaintext contains J, then J is replaced by I. It means treat I and J as the
same letter, accordingly.
Let us solve “HIDE MONEY” with Playfair Cipher using the key “TUTORIALS”.
Playfair Cipher Key Matrix:
T
U
O
R
I
A
L
S
B
C
D
E
F
G
H
K
M
N
P
Q
V
W
X
Y
Z
Encrypted Message: QCEFNUMFZY
Decrypted Message: HIDEMONEYX
Q3. Explain the basic purpose of IDEA. How IDEA works?
The International Data Encryption Algorithm (IDEA) is a symmetric key block cipher
encryption algorithm designed to encrypt text to an unreadable format for transmission via
the internet. It uses a typical block size of 128 bits and takes 64 bits as an input, i.e., 64-bit
data.
Q4. Describe the difference between double DES and Triple DES.
Double DES: Double DES is an encryption technique which uses two instances of DES on
same plain text. In both instances it uses different keys to encrypt the plain text. Both keys
are required at the time of decryption. The 64-bit plain text goes into first DES instance
which then converted into a 64-bit middle text using the first key and then it goes to second
DES instance which gives 64-bit cipher text by using second key.
Triple DES: Triple DES is an encryption technique which uses three instances of DES on same
plain text. It uses their different types of keys choosing technique in first all used keys are
different and in second two keys are same and one is different and in third all keys are
same.