Bt0088 cryptography and network security1

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Cryptography and Network Security
BT0088 Part-1
By Milan K Antony

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1. Explain the need for network security.
Computer security is required because many organizations will be damaged by hostile
software or intruders. There may be several forms of damage which are obviously interrelated.
These include:
 Damage or destruction of computer systems.
 Damage or destruction of internal data.
 Loss of sensitive information to hostile parties.
 Use of sensitive information to steal elements of monitary value.
 Use of sensitive information against the customers which may result in legal action by customers
against the organization and loss of customers.
 Damage to the reputation of an organization.
 Monitory damage, due to loss of sensitive information, destruction of data, hostile use of
sensitive data, or damage to the reputation of the organization. The methods used to
accomplish these unscrupulous objectives are many and varied depending on the
circumstances.
2. What is security attack? Explain with examples.
Any action that compromises the security of information owned by an organization is called
security attack. Those who execute such actions, or cause them to be executed, are called
attackers or opponents. Computer-based system has three interrelated and valuable components
namely, hardware, software, and data. Each of these assets offers value to
different members of the community affected by the system. To analyze security, we can

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brainstorm about the ways in which the system or its information can experience some kind of
loss or harm
Vulnerability is a weakness in the security system, For example, you can
see certain system does not verify a user's identity or password before allowing them to access the
data.
A threat to a computing system is a set of circumstances that has the
potential to cause loss or harm. The threats to computing system would be either human-initiated
or computer-initiated. A human who exploits the vulnerability perpetrates an attack on the
system. An attack can also be launched by another system, as when one system sends an
overwhelming set of messages to another, virtually shutting down the second system's ability to
function. we have seen this type of attack frequently, as denial-of-service attacks flood
servers with more messages than they can handle.
3. What is the difference between substitution and transposition techniques?
Substitutions are the simple form of encryption in which one letter is exchanged for
another. A substitution is an acceptable way of encrypting text. Here is few examples.The
Caesar cipher has an important place in history. Julius Caesar is said to have been the first to
use this scheme, in which each letter is translated to a letter a fixed number of places after it in
the alphabet. Caesar used a shift of 3, so that plaintext letter pi was enciphered as ciphertext
letter ci by the rule .A one-time pad is sometimes considered the perfect cipher. The name
comes from an encryption method in which a large, nonrepeating set of keys is written on
sheets of paper, glued together into a pad. The Vernam Cipher is a type of one-time pad
devised by Gilbert Vernam for AT&T. The Vernam cipher is immune to most cryptanalytic

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attacks. The basic encryption involves an arbitrarily long nonrepeating sequence of
numbers that are combined with the plaintext. Vernam's invention used an arbitrarily long punched
paper tape that fed into a teletype machine. The tape contained random numbers that were
combined with characters typed into the teletype. The sequence of random numbers had no
repeats, and each tape was used only once. As long as the key tape does not repeat or is not
reused, this type of cipher is immune to cryptanalytic attack because the available ciphertext
does not display the pattern of the key
.Example:
transposition techniques
The goal of substitution is confusion; the encryption method is an attempt to
make it difficult for a cryptanalyst or intruder to determine how a message and key were
transformed into cipher-text. A transposition is an encryption in which the letters of the
message are rearranged. With transposition, the cryptography aims for diffusion, widely
spreading the information from the message or the key across the ciphertext. Transpositions try
to break established patterns. Because a transposition is a rearrangement of the symbols of a

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message, it is also known as a permutation.
A simplest transposition technique is Columnar Transpositions. The columnar transposition is a
rearrangement of the characters of the plaintext into columns.
The following set of characters is a five-column transposition. The plaintext characters are written
in rows of five and arranged one row after another, as shown here.
c1 c2 C3 c4 c5
c6 c7 C8 c9 c10
c11 c12 etc.
You form the resulting ciphertext by reading down the columns
Encipherment / Decipherment Complexity

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This type is again arranging the letters and reading them off again. Therefore, the
algorithm requires a constant amount of work per character, and the time needed to apply the
algorithm is proportional to the length of the message.
Digrams, Trigrams, and Other Patterns
Just as there are characteristic letter frequencies, there are also characteristic patterns of pairs of
adjacent letters, called digrams. Letter pairs such as -re-, -th-, -en-, and -ed- appear very
frequently
4. Using Caesar Cipher encrypt the plaintext “Sikkim Manipal University” using key value
Plaintext ABCDEFGHIJKLMNOPQRSTUVWXYZ
Ciphertext d e f g h i j k l m n o p q r s t u v w x y z a b c
SIKKIM MANIPAL UNIVERSITY
would be encoded as
SIKKIM MANIPAL UNIVERSITY
vl nnl p pd qls do x q ly huv lwb

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5. Explain different characteristics that identify a good encryption technique.
1. The implementation of the process should be as simple as possible. Principle 3 was
formulated with hand implementation in mind: A complicated algorithm is prone to error or
likely to be forgotten. With the development and popularity of digital computers, algorithms
far too complex for hand implementation became feasible. Still, the issue of complexity is
important. People will avoid an encryption algorithm whose implementation process severely
hinders message transmission, thereby undermining security. And a complex algorithm is
more likely to be programmed incorrectly
2. The enciphering algorithm and set of keys used should be less complex. This principle implies
that we should restrict neither the choice of keys nor the types of plaintext on which the
algorithm can work. For instance, an algorithm that works only on plaintext having an equal
number of As and Es is useless. Similarly, it would be difficult to select keys such that
the sum of the values of the letters of the key is a prime number. Restrictions such as these
make the use of the encipherment prohibitively complex. If the process is too complex, it will
not be used. Furthermore, the key must be transmitted, stored, and remembered, so
it must be short.
3. The amount of secrecy needed should determine the amount of labor appropriate for the
encryption and decryption. Principle 1 is a reiteration of the principle of timeliness and of the

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earlier observation that even a simple cipher may be strong enough to deter the casual
interceptor or to hold off any interceptor for a short time.
4. Errors in ciphering should not propagate and cause corruption of further information in the
message. Principle 4 acknowledges that humans make errors in their use of enciphering
algorithms. One error early in the process should not throw off the entire remaining
ciphertext.
5. The size of the original message and that of enciphered text should be at most same.
6. Compare Symmetric and Asymmetric Encryption Systems.
The two basic kinds of encryption systems are key based and block based.
Key based encryption is based on either single key or multiple keys. Block based encryption is
based on either stream or block of characters We have two types of encryptions based on keys
they are symmetric (also called "secret key") and asymmetric (also called "public key").
Symmetric algorithms use one key, which works for both encryption and decryption.
Usually, the decryption algorithm is closely related to the encryption one The symmetric system
means both encryption and the decryption are performed using the same key. They provide a
two-way channel to their users: A and B share a secret key, and they can both encrypt
information to send to the other as well as decrypt information from the other. As long as
the key remains secret, the system also provides authentication, proof that a message received was
not fabricated by someone other than the declared sender. Authenticity is ensured because only
the legitimate sender can produce a message that will decrypt properly with the shared key.
Public key systems, on the other hand, excel at key management. By the

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nature of the public key approach, you can send a public key in an e-mai message or post it in a
public directory. Only the corresponding private key, which presumably is kept private, can
decrypt what has been encrypted with the public key.
For both kinds of encryption, a key must be kept well secured. Once the
symmetric or private key is known by an outsider, all messages written previously or in the
future can be decrypted (and hence read or modified) by the outsider. So, for all encryption
algorithms, key management is a major issue. It involves storing, safeguarding, and activating
keys
7. Give the Overview of DES Algorithm.
The most widely used encryption scheme is based on Data Encryption
standard. Data Encryption Standard (DES), a system developed for the U.S. government, was
intended for use by the general public The Data Encryption Standard (DES) specifies an
algorithm to be implemented in electronic hardware devices and used for the cryptographic
protection of computer data ... Encrypting data converts it to an unintelligible form called
cipher. Decrypting cipher converts the data back to its original form. The algorithm... specifies
both enciphering and deciphering operations which are based on a binary number called a key
… Data can be recovered from cipher only by using exactly the same key used
to encipher it.

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The Data Encryption algorithm is a combination of both substitution as well
as transposition technique. The strength of DES technique is improved when it uses both the
techniques together. It uses both the technique repeatedly i.e., one on the top of other for a total
of 16 cycles. The sheer complexity of tracing a single bit through 16 iterations of substitutions
and transpositions has so far stopped researchers in the public from identifying
more than a handful of general properties of the algorithm. The algorithm begins by encrypting
the plaintext as blocks of 64 bits. The key is 64 bits long, but in fact it can be any 56-bit
number. (The extra 8 bits are often used as check digits and do not affect encryption in normal
implementations.) The user can change the key at will any time there is uncertainty about the
security of the old key.

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The iterative substitutions and permutations are performed as outlined in
Figure
DES uses only standard arithmetic and logical operations on numbers up to 64 bits long, so it is
suitable for implementation in software on most current computers. Although complex, the
algorithm is repetitive, making it suitable for implementation on a single-purpose chip.
8. Explain RSA technique with an example.
RSA stands for Rivest, Shamir, and Adleman, the people who invented the algorithm at MIT in
1978 which uses two keys: a private key and a public key. Typically, private key algorithms
such as DES can't protect against fraud by the sender or the receiver of a message

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RSA is an exponentiation cipher. You have to follow the following two steps.
1. Choose two large prime numbers p and q, and let n = pq. The totient Ø(n) of n is the number of
numbers less than n with no factors in common with n.
Example: Let n = 10. The numbers that are less than 10 and are relatively prime to (have no factors
in common with) n are 1, 3, 7, and 9. Hence, Ø (10) = 4. Similarly, if n = 21, the numbers that
are relatively prime to n are 1, 2, 4, 5, 8, 10, 11, 13, 16, 17, 19, and 20. So Ø(21) = 12.
2. Choose an integer e < n that is relatively prime to Ø(n). Find a second integer d such that ed mod
Ø(n) = 1. The public key is (e, n), and the private key is d.
Let m be a message. Then:
c = me mod n
and
m = cd mod n.
Example: Let p = 7 and q = 11. Then n = 77 and Ø(n) = 60. Alice chooses e = 17, so her private
key is d = 53. In this cryptosystem, each plaintext character is represented by a number
between 00 (A) and 25 (Z); 26 represents a blank. Bob wants to send Alice the message
"HELLO WORLD." Using the representation above, the plaintext is 07 04 11 11 14
26 22 14 17 11 03. Using Alice's public key, the ciphertext is
0717 mod 77 = 28
0417 mod 77 = 16
1117 mod 77 = 44

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...
0317 mod 77 = 75
or 28 16 44 44 42 38 22 42 19 44 75.
RSA can provide data and origin authentication. If Alice enciphers her message
using her private key, anyone can read it, but if anyone alters it, the (altered) ciphertext cannot
be deciphered correctly.
Example: Suppose Alice wishes to send Bob the message "HELLO WORLD" in such a way
that Bob will be sure that Alice sent it. She enciphers the message with her private key and
sends it to Bob. As indicated above, the plaintext is represented as 07 04 11 11 14 26 22 14 17
11 03.
Using Alice's private key, the ciphertext is
0753 mod 77 = 35
0453 mod 77 = 09
1153 mod 77 = 44
...
0353 mod 77 = 05

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or 35 09 44 44 93 12 24 94 04 05. In addition to origin authenticity, Bob can be sure that no letters
were altered.
9. Explain different approaches used in judging the quality of security.
"secure”, it means that security implies some degree of trust
that the program enforces expected confidentiality, integrity, and availability
Fixing Faults
One approach to judge quality in security is fixing faults. You might argue that a module in which
100 faults were discovered and fixed is better than another in which only 20 faults were
discovered and fixed, suggesting that more rigorous analysis and testing had led to the finding
of the larger number of faults. Early work in computer security was based on the paradigm
of "penetrate and patch," in which analysts searched for and repaired faults. Often, a top-
quality "tiger team" would be convened to test a system's security by attempting to cause it to
fail. The test was considered to be a "proof" of security; if the system withstood the attacks, it
was considered secure. Unfortunately, far too often the proof became a counterexample, in
which not just one but several serious security problems were uncovered. The problem
discovery in turn led to a rapid effort to "patch" the system to repair or restore the security.
However, the patch efforts were largely useless, making the system less secure rather than
more secure because they frequently introduced new faults. There are three reasons why.
 The fault often had non-obvious side effects in places other than the immediate area of the
fault.
 The system functionality or performance would be affected if faults needs to be detected

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properly.
 The pressure to repair a specific problem encouraged a narrow focus on the fault itself and not
on its context. In particular, the analysts paid attention to the immediate cause of the failure
and not to the underlying design or requirements faults.
Unexpected Behavior
The inadequacies of penetrate-and-patch led researchers to seek a better way to be confident that
code meets its security requirements. One way to do that is to compare the requirements with
the behavior. That is, to understand program security, we can examine programs to see whether
they behave as their designers intended or users expected. We call such unexpected behavior a
program security flaw; it is inappropriate program behavior caused by a program vulnerability.
There is no direct mapping of the terms "vulnerability" and "flaw" into the characterization of
faults and failures. A flaw can be either a fault or failure, and a vulnerability usually describes a
class of flaws, such as a buffer overflow. In spite of the inconsistency, it is important for us to
remember that we must view vulnerabilities and flaws from two perspectives, cause and effect,
so that we see what fault caused the problem and what failure (if any) is visible to the user
10. What are the different ways in which an operating system can assist or offer protection?
Separating one user’s object from the other is the basic way of protection. Separation in an
operating system can occur in several ways.
 Logical separation:
In which users operate under the illusion that no other processes exist, as when an operating
system constrains a program's accesses so that the program cannot access objects outside its
permitted domain

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 Physical separation:
Each and every process has its own physical objects, such as separate printers for output
requiring different levels of security.
 Cryptographic separation:
Each process will protect their data and computations in such a way that they are unintelligible to
outside processes
 Temporal separation:
In which processes having different security requirements are executed at different times
There are several ways an operating system can assist, offering protection at any
of several levels.
 Do not protect. Operating systems with no protection are appropriate when sensitive procedures
are being run at separate times.
 Isolate. An operating system providing Isolation feature allow different processes to run
concurrently and are unaware of the presence of the each other. Each process has its own
address space, files, and other objects. The operating system must confine each process
somehow, so that the objects of the other processes are completely concealed.
 Share all or nothing. Each user declare its objects either to be public or private. There by it will
be either available to all users or only to the owners respectively.
 Share via access limitation. With protection by access limitation, the operating system checks the
allowability of each user's potential access to an object. That is, access control is implemented
for a specific user and a specific object. Lists of acceptable actions guide the operating
system in determining whether a particular user should have access to a

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particular object. In some sense, the operating system acts as a guard between users and objects,
ensuring that only authorized accesses occur.
 Share by capabilities. An extension of limited access sharing, this form of protection allows
dynamic creation of sharing rights for objects. The degree of sharing can depend on the owner
or the subject, on the context of the computation, or on the object itself.
 Limit use of an object. This form of protection limits not just the access to an object but the use
made of that object after it has been accessed. For example, a user may be allowed to view a
sensitive document, but not to print a copy of it. More powerfully, a user may be allowed
access to data in a database to derive statistical summaries (such as average salary at
a particular grade level), but not to determine specific data values (salaries of individuals).

Bt0088 cryptography and network security1

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