Fall 2013 Assignment
Bachelor of Science in Information Technology (BSc
IT) – Semester 5
BT0088 – Cryptography and Network Security – 4
Credits
(Book ID: B1183)
Assignment Set (60 Marks)
1.
What is the need
for security? Explain types of security attacks.
Ans.- 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.
·
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.
Types of Threats
(Attacks)
Now
you would see the various types of threats which a computing environment would
encounter.
·
Interception:
This
type of threat occurs when an unauthorized party(outsider) has gained access.
The outside party can be a person, a program, or a computing system. Examples
of this type of failure are illicit copying of program or data files, or
wiretapping to obtain data in a network. Although a loss may be discovered
fairly quickly, a silent interceptor may leave no traces by which the interception
can be readily detected.
·
Interruption:
This occurs when an asset of
the system becomes lost, unavailable, or unusable. An example is the malicious
destruction of a hardware device, erasure of a program or data file, or
malfunction of an operating system file manager so that it cannot find a
particular disk file.
Passive
attacks:- Passive
attacks are in the nature of eavesdropping on, or monitoring of transmissions.
The goal of the opponent is to obtain information that is being transmitted.
Two types of passive attacks are release of message contents and traffic
analysis.
The release of message content is easily
understood. A telephone conversation, an electronic mail message, and a
transferred file may contain sensitive or confidential information. We would
like to prevent the opponent from learning the contents of these transmissions.
A second type of passive attack is
traffic analysis. Suppose a sender is masking the content by using encryption(
will be discussed later) an attacker still be able to observe the pattern of
these messages. The attacker (Opponent) could determine the location and
identify the communicating hosts and could observe the frequency and length of
messages being exchanged. This information might be useful in guessing the nature
of the communication that has taken place.
Passive attacks are very difficult to
detect because they do not involve any alteration of the data.
Active Attacks:-
Active
attacks involve some modification of the data stream or the creation of a false
stream and can be subdivided into four categories: masquerade, replay,
modification of messages and denial of service.
A Masquerade takes place when one entity
pretends to be a different entity. A masquerade attack usually includes one of
the other forms of active attack. Replay involves the passive capture of a data
unit and its subsequent retransmission to produce an unauthorized effect.
2.
List
substitution techniques. Explain Ceaser’s cipher.
Ans.- Substitutions are the simple form of
encryption in which one letter is exchanged for another. A substitution is an
acceptable way of encrypting text. There are four types of Substitutions
techniques--
1.
The
Caesar Cipher
2.
One-Time
Pads
3.
The
Vernam Cipher
4.
Book
Cipher
The
Caesar Cipher:- 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
full translation chart of the Caesar cipher is shown here.
Using
this encryption, the message
SIKKIM
MANIPAL UNIVERSITY
would
be encoded as
S
I K K I M M A N I P A L U N I V E R S I T Y
v
l n n l p p d q l s d o x q l y h u v l w b
Cryptanalysis
of the Caesar Cipher
Let us take a closer look at the result
of applying Caesar's encryption technique to "SIKKIM MANIPAL
UNIVERSITY" If we did not know the plaintext and were trying to guess it,
we would have many clues from the ciphertext. For example, the break between
the two words is preserved in the ciphertext, and double letters are preserved:
The SS is translated to vv. We might also notice that when a letter is
repeated, it maps again to the same ciphertext as it did previously. So the
letter K always translate to n. These clues make this cipher easy to break.
Suppose
you are given the following ciphertext message, and you want to try to
determine the original plaintext.
wklv
phvvdjh lv qrw wrr kdug wr euhdn
The message has actually been enciphered
with a 27-symbol alphabet: A through Z plus the "blank" character or
separator between words. As a start, assume that the coder was lazy and has
allowed the blank to be translated to itself. If your assumption is true, it is
an exceptional piece of information; knowing where the spaces are allows us to
see which are the small words. English has relatively few small words, such as
am, is, to, be, he, we, and, are, you, she, and so on. Therefore, one way to
attack this problem and break the encryption is to substitute known short words
at appropriate places in the ciphertext until you have something that seems to
be meaningful. Once the small words fall into place, you can try substituting
for matching characters at other places in the ciphertext.
Look again at the ciphertext you are
decrypting. There is a strong clue in the repeated r of the word wrr. You might
use this text to guess at three-letter words that you know. For instance, two
very common three-letter words having the pattern xyy are see and too; other
less common possibilities are add, odd, and off. (Of course, there are also
obscure possibilities like woo or gee, but it makes more sense to try the
common cases first.) Moreover, the combination wr appears in the ciphertext,
too, so you can determine whether the first two letters of the three-letter
word also form a two-letter word.
3.
Explain in brief
types of encryption systems.
Ans.- 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.
Based
on Key :- 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 nature of the public key approach, you can send
a public key in an e-mail 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.
But 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.
Based
on Block:- Block
based encryption system is classified as stream and block encryption system.
Stream encryption algorithm convert one symbol of plaintext immediately into a
symbol of ciphertext. The transformation depends only on the symbol, the key,
and the control information of the encipherment algorithm. A model of stream
enciphering is shown in below figure.
Some kinds of errors, such as skipping a
character in the key during encryption, affect the encryption of all future
characters. However, such errors can sometimes be recognized during decryption
because the plaintext will be properly recovered up to a point, and then all
following characters will be wrong. If that is the case, the receiver may be
able to recover from the error by dropping a character of the key on the
receiving end. Once the receiver has successfully recalibrated the key with the
ciphertext, there will be no further effects from this error.
To address this problem and make it
harder for a cryptanalyst to break the code, we can use block encryption
algorithm. A block encryption encrypts a group of plaintext symbols as one
block. The columnar transposition and other transpositions are examples of
block ciphers. In the columnar transposition, the entire message is translated
as one block. The block size need not have any particular relationship to the
size of a character. Block ciphers work on blocks of plaintext and produce
blocks of ciphertext, as shown in below figure. In this figure, the central box
represents an encryption machine: The previous plaintext pair is converted to
po, the current one being converted is IH, and the machine is soon to convert
ES.
4.
Explain
authentication header with necessary diagrams.
Ans.- Authentication Header (AH) is one of the
two core security protocols in IPSec protocol suite. AH provides data
integrity, data source authentication, and protection against replay attacks.
It does not provide confidentiality. This makes AH header much simpler than
ESP. It is merely a header and not a header plus trailer. The below figure
shows the AH protected IP packet.
It
provides authentication of either all or part of the contents of a datagram
through the addition of a header that is calculated based on the values in the
datagram. What parts of the datagram are used for the calculation, and the
placement of the header, depends on the mode (tunnel or transport) and the
version of IP. The below figure shows the AH protocol structure.
The
fields comprising the AH header are:
·
Next
Header: The next header field identifies the protocol type of the next packet
header after the AH packet header.
·
Payload
Length: The length field states the length of the AH header information.
·
Reserved
field: It is for future extensions of the AH protocol.
·
SPI
field: shows to which SA the packet belongs.
·
Sequence
number: It is an incrementing value that prevents against replay attacks.
·
The
authentication data: contains the information for authenticating the packet.
The
operation of the AH protocol is simple especially for any protocol that has anything
to do with network security. It can be considered analogous to the algorithms
used to calculate checksums or perform CRC checks for error detection. In those
cases, a standard algorithm is used by the sender to compute a checksum or CRC
code based on the contents of a message. This computed result is transmitted
along with the original data to the destination, which repeats the calculation
and discards the message if any discrepancy is found between its calculation
and the one done by the source.
This
is the same idea behind AH, except that instead of using a simple algorithm
known to everyone, it uses a special hashing algorithm and a specific key known
only to the source and the destination. SA between two devices is set up that
specifies these particulars so that the source and destination know how to
perform the computation, but nobody else can. On the source device, AH performs
the computation and puts the result (called the Integrity Check Value or ICV)
into a special header with other fields for transmission. The destination
device does the same calculation using the key the two devices share, which
enables it to see immediately if any of the fields in the original datagram
were modified either due to error or malice.
It's
important to point here that just as a checksum doesn't change the original
data, neither does the ICV calculation change it. The presence of the AH header
allows us to verify the integrity of the message, but doesn't encrypt it. Thus,
AH provides authentication but not privacy.
5.
Explain the
processing of Encrypted E-Mail.
Ans.- The sender chooses a (random) symmetric
algorithm encryption key. Then, the sender encrypts a copy of the entire
message to be transmitted, including FROM:, TO:, SUBJECT:, and DATE: headers.
Next, the sender prepends plaintext headers. For key management, the sender
encrypts the message key under the recipient's public key, and attaches that to
the message as well. The process of creating an encrypted e-mail message is
shown in Figure A.
Encryption
can potentially yield any string as output. Many e-mail handlers expect that
message traffic will not contain characters other than the normal printable
characters. Network e-mail handlers use unprintable characters as control
signals in the traffic stream. To avoid problems in transmission, encrypted
e-mail converts the entire ciphertext message to printable characters. An
example of an encrypted e-mail message is shown in above Figure A. Notice the
three portions: an external (plaintext) header, a section by which the message
encryption key can be transferred, and the encrypted message itself. (The
encryption is shown with shading.)
The
encrypted e-mail standard works most easily as just described, using both
symmetric and asymmetric encryption. The standard is also defined for symmetric
encryption only: To use symmetric encryption, the sender and receiver must have
previously established a shared secret encryption key. The processing type
("Proc-Type") field tells what privacy enhancement services have been
applied. In the data exchange key field ("DEK-Info"), the kind of key
exchange (symmetric or asymmetric) is shown. The key exchange
("Key-Info") field contains the message encryption key, encrypted
under this shared encryption key. The field also identifies the originator
(sender) so that the receiver can determine which shared symmetric key was
used. If the key exchange technique were to use asymmetric encryption, the key
exchange field would contain the message encryption field, encrypted under the
recipient's public key. Also included could be the sender's certificate (used
for determining authenticity and for generating replies).
To
ensure the authenticity of the sender, the encrypted e-mail messages always
carry a digital signature along with the message. The integrity is also assured
because of a hash function (called a message integrity check, or MIC) in the
digital signature. Optionally, encrypted e-mail messages can be encrypted for
confidentiality.
Notice
in above Figure A. that the header inside the message (in the encrypted
portion) differs from that outside. A sender's identity or the actual subject
of a message can be concealed within the encrypted portion.
The
encrypted e-mail processing can integrate with ordinary e-mail packages, so a
person can send both enhanced and nonenhanced messages, as shown in below
Figure B. If the sender decides to add enhancements, an extra bit of encrypted
e-mail processing is invoked on the sender's end; the receiver must also remove
the enhancements. But without enhancements, messages flow through the mail
handlers as usual.
6.
Explain
characteristics of good security policy.
Ans.- Characteristics
of a good security policy
If
a security policy is written poorly, it cannot guide the developers and users
in providing appropriate security mechanisms to protect important assets.
Certain characteristics make a security policy a good one.
1. Coverage: A security policy must be comprehensive:
It must either apply to or explicitly exclude all possible situations.
Furthermore, a security policy may not be updated as each new situation arises,
so it must be general enough to apply naturally to new cases that occur as the
system is used in unusual or unexpected ways.
2.Durability: A security policy must grow and
adapt well. In large measure, it will survive the system's growth and expansion
without change. If written in a flexible way, the existing policy will be
applicable to new situations. However, there are times when the policy must
change (such as when government regulations mandate new security constraints),
so the policy must be changeable when it needs to be.
An
important key to durability is keeping the policy free from ties to specific
data or protection mechanisms that almost certainly will change. For example,
an initial version of a security policy might require a ten-character password
for anyone needing access to data on the Sun workstation in room 110. But when
that workstation is replaced or moved, the policy's guidance becomes useless.
It is preferable to describe assets needing protection in terms of their
function and characteristics, rather than in terms of specific implementation.
For example, the policy on Sun workstations could be re-worded to mandate
strong authentication for access to sensitive student grades or customers'
proprietary data. Better still, we can separate the elements of the policy,
having one policy statement for student grades and another for customers'
proprietary data. Similarly, we may want to define one policy that applies to
preserving the confidentiality of relationships, and another protecting the use
of system through strong authentication.
3.Realism: The policy must be realistic. That is,
it must be possible to implement the stated security requirements with existing
technology. Moreover, the implementation must be beneficial in terms of time,
cost, and convenience; the policy should not recommend a control that works but
prevents the system or its users from performing their activities and
functions. It is important to make economically worthwhile investments in
security, just as for any other careful business investment.
4.Usefulness: An obscure or incomplete security
policy can not be implemented properly, if at all. The policy must be written
in a language that can be read, understood and followed by anyone who must
implement it or is affected by it. For this reason, the policy should be
succinct, clear, and direct.
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