The document discusses network security and cryptography. It provides an overview of security concepts like attacks, services, defense methods, and models. It defines information security, why it is important, and common security attacks like interruption, interception, modification, and fabrication. It also discusses security goals of confidentiality, integrity, and availability. Cryptography techniques like symmetric and asymmetric encryption are introduced along with concepts like plaintext, ciphertext, encryption, decryption, and cryptanalysis.
3. Outline of Unit- I
• Introduction
• Attacks, services and mechanisms
• Security attacks
• Security services
• Methods of Defense
• A model for Internetwork Security
• Cryptography concepts and steganography
4. What is Information
Security?
• Protection of Information against
unauthorized access/ modification of
Information (disclosed, destroyed or
altered un-intentionally/intentionally)
• Governments, commercial businesses,
and individuals are all storing
information electronically
5. Why Information Security
is important?
• Nowadays, security has become a
central issue in data storage and
transmission. The protection of
confidential data from unauthorized
access is important
• The subject “Information Security” is
widely present in the curriculum of
Computer Science & Engineering, IT
and Electronics at UG and PG level
• Interesting area of research
6. Attacks, Services and
Mechanisms
• Security Attack: Any action that
compromises the security of information.
• Security Mechanism: A mechanism that is
designed to detect, prevent, or recover from a
security attack.
• Security Service: A service that enhances
the security of data processing systems and
information transfers. A security service
makes use of one or more security mechanisms.
13. Security Attacks
• Interruption: This is an attack on
availability
• Interception: This is an attack on
confidentiality
• Modfication: This is an attack on
integtrity
• Fabrication: This is an attack on
authenticity
15. What are the 3 Principles of Information Security?
The basic tenets of information security are confidentiality,
integrity and availability. Every element of the information
security program must be designed to implement one or more
of these principles. Together they are called the CIA Triad.
Confidentiality
Confidentiality measures are designed to prevent unauthorized
disclosure of information. The purpose of the confidentiality
principle is to keep personal information private and to ensure
that it is visible and accessible only to those individuals who
own it or need it to perform their organizational functions.
16. Integrity
Consistency includes protection against unauthorized changes
(additions, deletions, alterations, etc.) to data. The principle of
integrity ensures that data is accurate and reliable and is not
modified incorrectly, whether accidentally or maliciously.
Availability
Availability is the protection of a system’s ability to make software
systems and data fully available when a user needs it (or at a
specified time). The purpose of availability is to make the
technology infrastructure, the applications and the data available
when they are needed for an organizational process or for an
organization’s customers.
17.
18. Passive attacks
• Passive attacks do not affect system
resources
– Eavesdropping, monitoring
• Two types of passive attacks
– Release of message contents
– Traffic analysis
• Passive attacks are very difficult to detect
– Message transmission apparently normal
• No alteration of the data
– Emphasis on prevention rather than detection
• By means of encryption
21. • Active attacks try to alter system resources or affect
their operation
– Modification of data, or creation of false data
• Four categories
– Masquerade
– Replay
– Modification of messages
– Denial of service: preventing normal use
• A specific target or entire network
• Difficult to prevent
– The goal is to detect and recover
Active Attacks
26. Security Services
• Confidentiality (privacy)
• Authentication (who created or sent the data)
• Integrity (has not been altered)
• Non-repudiation (the order is final)
• Access control (prevent misuse of resources)
• Availability (permanence, non-erasure)
– Denial of Service Attacks
– Virus that deletes files
27. Types of Security Mechanism
Security mechanism can also be termed as is set of processes that deal with
recovery from security attack. Various mechanisms are designed to recover
from these specific attacks at various protocol layers.
28. Types of Security Mechanism are :
Encipherment :
This security mechanism deals with hiding and covering of data
which helps data to become confidential. It is achieved by applying
mathematical calculations or algorithms which reconstruct
information into not readable form. It is achieved by two famous
techniques named Cryptography and Encipherment.
Access Control :
This mechanism is used to stop unattended access to data which you
are sending. It can be achieved by various techniques such as
applying passwords, using firewall, or just by adding PIN to data.
Notarization :
This security mechanism involves use of trusted third party in
communication. It acts as mediator between sender and receiver so
that if any chance of conflict is reduced. This mediator keeps record
of requests made by sender to receiver for later denied.
29. Data Integrity :
This security mechanism is used by appending value to data to
which is created by data itself. It is similar to sending packet of
information known to both sending and receiving parties and
checked before and after data is received. When this packet or
data which is appended is checked and is the same while sending
and receiving data integrity is maintained.
Authentication exchange :
This security mechanism deals with identity to be known in
communication. This is achieved at the TCP/IP layer where two-
way handshaking mechanism is used to ensure data is sent or not.
30. Bit stuffing :
This security mechanism is used to add some extra bits into data
which is being transmitted. It helps data to be checked at the
receiving end and is achieved by Even parity or Odd Parity.
Digital Signature :
This security mechanism is achieved by adding digital data that is
not visible to eyes. It is form of electronic signature which is
added by sender which is checked by receiver electronically. This
mechanism is used to preserve data which is not more
confidential but sender’s identity is to be notified.
32. Model for Network Security
In considering the place of encryption, its
useful to use the following two models. The
first models information flowing over an
insecure communications channel, in the
presence of possible opponents. Hence an
appropriate security transform
(encryption algorithm) can be used, with
suitable keys, possibly negotiated using the
presence of a trusted third party.
34. Model for Network Security
• using this model requires us to:
1. design a suitable algorithm for the
security transformation
2. generate the secret information (keys)
used by the algorithm
3. develop methods to distribute and share
the secret information
4. specify a protocol enabling the principals
to use the transformation and secret
information for a security service
36. Model for Network Access
Security
using this model requires us to:
• select appropriate gatekeeper
functions to identify users
• implement security controls to ensure
only authorised users access
designated information or resource
37. CRYPTOGRAPHY
Cryptography is technique of securing information and
communications through use of codes so that only
those person for whom the information is intended can
understand it and process it. Thus preventing
unauthorized access to information. The prefix “crypt”
means “hidden” and suffix graphy means “writing”.
38. Features Of Cryptography are as follows:
Confidentiality:
Information can only be accessed by the person for whom it is
intended and no other person except him can access it.
Integrity:
Information cannot be modified in storage or transition between
sender and intended receiver without any addition to information
being detected.
Non-repudiation:
The creator/sender of information cannot deny his or her intention to
send information at later stage.
Authentication:
The identities of sender and receiver are confirmed. As well as
destination/origin of information is confirmed.
39. Some Basic Terminology
• plaintext - original message
• ciphertext - coded message
• cipher - algorithm for transforming plaintext to ciphertext
• key - info used in cipher known only to sender/receiver
• encipher (encrypt) - converting plaintext to ciphertext
• decipher (decrypt) - recovering ciphertext from plaintext
• cryptography - study of encryption principles/methods
• cryptanalysis (codebreaking) - study of principles/ methods
of deciphering ciphertext without knowing key
• cryptology - field of both cryptography and cryptanalysis
40.
41. Types Of Cryptography:
In general there are three types Of cryptography:
Symmetric Key Cryptography:
It is an encryption system where the sender and receiver of message use a
single common key to encrypt and decrypt messages. Symmetric Key
Systems are faster and simpler but the problem is that sender and receiver
have to somehow exchange key in a secure manner. The most popular
symmetric key cryptography system is Data Encryption System(DES).
Hash Functions:
There is no usage of any key in this algorithm. A hash value with fixed
length is calculated as per the plain text which makes it impossible for
contents of plain text to be recovered. Many operating systems use hash
functions to encrypt passwords.
Asymmetric Key Cryptography:
Under this system a pair of keys is used to encrypt and decrypt information.
A public key is used for encryption and a private key is used for decryption.
Public key and Private Key are different. Even if the public key is known by
everyone the intended receiver can only decode it because he alone knows
the private key.
42. Symmetric Encryption
• or conventional / private-key / single-key
• sender and recipient share a common key
• all classical encryption algorithms are
private-key
• was only type prior to invention of public-
key in 1970’s
• and by far most widely used
45. Requirements
• two requirements for secure use of
symmetric encryption:
– a strong encryption algorithm
– a secret key known only to sender / receiver
• mathematically have:
Y = EK(X)
X = DK(Y)
• assume encryption algorithm is known
• implies a secure channel to distribute key
46.
47.
48.
49. Classical attacks –
It can be divided into a)Mathematical analysis and b) Brute-force
attacks. Brute-force attacks runs the encryption algorithm for all
possible cases of the keys until a match is found. Encryption
algorithm is treated as a black box. Analytical attacks are those
attacks which focuses on breaking the cryptosystem by analysing
the internal structure of the encryption algorithm.
Social Engineering attack –
It is something which is dependent on the human factor. Tricking
someone to reveal their passwords to the attacker or allowing
access to the restricted area comes under this attack. People should
be cautious when revealing their passwords to any third party
which is not trusted.
Implementation attacks –
Implementation attacks such as side-channel analysis can be used
to obtain a secret key. They are relevant in cases where the attacker
can obtain physical access to the cryptosystem.
50. Symmetric Key Encryption Asymmetric Key Encryption
It only requires a single key for both
encryption and decryption.
It requires two key one to encrypt and
the other one to decrypt.
The size of cipher text is same or
smaller than the original plain text.
The size of cipher text is same or larger
than the original plain text.
The encryption process is very fast. The encryption process is slow.
It is used when a large amount of data is
required to transfer.
It is used to transfer small amount of
data.
It only provides confidentiality.
It provides confidentiality, authenticity
and non-repudiation.
Examples: 3DES, AES, DES and RC4
Examples: Diffie-Hellman, Gamal,
DSA and RSA
In symmetric key encryption, resource
utilization is low as compared to
asymmetric key encryption.
In asymmetric key encryption, resource
utilization is high.
51. Cryptography
• characterize cryptographic system by:
– type of encryption operations used
• substitution / transposition / product
– number of keys used
• single-key or private / two-key or public
– way in which plaintext is processed
• block / stream
52. Cryptanalysis
• objective to recover key not just message
• general approaches:
– cryptanalytic attack
– brute-force attack
53. Cryptanalytic Attacks/Possible
types of attacks
• ciphertext only
– only know algorithm & ciphertext, is statistical,
know or can identify plaintext
• known plaintext
– know/suspect plaintext & ciphertext
• chosen plaintext
– select plaintext and obtain ciphertext
• chosen ciphertext
– select ciphertext and obtain plaintext
• chosen text
– select plaintext or ciphertext to en/decrypt
54. More Definitions
• unconditional security
– no matter how much computer power or time is
available, the cipher cannot be broken since the
ciphertext provides insufficient information to
uniquely determine the corresponding plaintext
• computational security
– given limited computing resources (eg time needed
for calculations is greater than age of universe), the
cipher cannot be broken
55. Brute Force Search
• always possible to simply try every key
• most basic attack, proportional to key size
• assume either know / recognise plaintext
Key Size (bits) Number of Alternative
Keys
Time required at 1
decryption/µs
Time required at 106
decryptions/µs
32 232 = 4.3 109 231 µs = 35.8 minutes 2.15 milliseconds
56 256 = 7.2 1016 255 µs = 1142 years 10.01 hours
128 2128 = 3.4 1038 2127 µs = 5.4 1024 years 5.4 1018 years
168 2168 = 3.7 1050 2167 µs = 5.9 1036 years 5.9 1030 years
26 characters
(permutation)
26! = 4 1026 2 1026 µs = 6.4 1012 years 6.4 106 years
56. Classical Substitution Ciphers
• where letters of plaintext are replaced by
other letters or by numbers or symbols
• or if plaintext is viewed as a sequence of
bits, then substitution involves replacing
plaintext bit patterns with ciphertext bit
patterns
57. Substitution Cipher
• Hiding some data is known as encryption. When plain text
is encrypted it becomes unreadable and is known as
ciphertext. In a Substitution cipher, any character of plain
text from the given fixed set of characters is substituted by
some other character from the same set depending on a
key. For example with a shift of 1, A would be replaced by
B, B would become C, and so on.
• Note: Special case of Substitution cipher is known
as Caesar cipher where the key is taken as 3.
58. Caesar Cipher
• earliest known substitution cipher
• by Julius Caesar
• first attested use in military affairs
• replaces each letter by 3rd letter on
• example:
meet me after the toga party
PHHW PH DIWHU WKH WRJD SDUWB
59. Caesar Cipher
• can define transformation as:
a b c d e f g h i j k l m n o p q r s t u v w x y z
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
• mathematically give each letter a number
a b c d e f g h i j k l m n o p q r s t u v w x y z
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
• then have Caesar cipher as:
c = E(p) = (p + k) mod (26)
p = D(c) = (c – k) mod (26)
60. Examples:
Plain Text: I am studying Data Encryption Key: 4
Output: M eq wxyhCmrk Hexe IrgvCtxmsr
Plain Text: ABCDEFGHIJKLMNOPQRSTUVWXYZ
Key: 4
utput: EFGHIJKLMNOPQRSTUVWXYZabcd
61. Cryptanalysis of Caesar Cipher
• only have 26 possible ciphers
– A maps to A,B,..Z
• could simply try each in turn
• a brute force search
• given ciphertext, just try all shifts of letters
• do need to recognize when have plaintext
• eg. break ciphertext "GCUA VQ DTGCM"
62. Monoalphabetic Cipher
• rather than just shifting the alphabet
• could shuffle (jumble) the letters arbitrarily
• each plaintext letter maps to a different random
ciphertext letter
• hence key is 26 letters long
Plain: abcdefghijklmnopqrstuvwxyz
Cipher: DKVQFIBJWPESCXHTMYAUOLRGZN
Plaintext: ifwewishtoreplaceletters
Ciphertext: WIRFRWAJUHYFTSDVFSFUUFYA
63. Monoalphabetic and Polyalphabetic Cipher
• Monoalphabetic cipher is a substitution cipher in which
for a given key, the cipher alphabet for each plain
alphabet is fixed throughout the encryption process. For
example, if ‘A’ is encrypted as ‘D’, for any number of
occurrence in that plaintext, ‘A’ will always get encrypted
to ‘D’.
Input :
Keyword : secret
Message : Zombie Here
Output :
Ciphered String : ZLJEFT DTOT
64. Take the first example, we used "secret" keyword there.
Plain Text : A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
When "secret" keyword is used, the new encrypting text becomes :
Encrypting : S E C R T A B D F G H I J K L M N O P Q U V W X Y Z
(repeated letters are omitted)
This means 'A' means 'S', 'B' means 'E' and 'C' means 'C' and so on.
Lets encode the given message "Zombie Here"
ZOMBIE HERE becomes ZLJEFT DTOT
65. Polyalphabetic Cipher is a substitution cipher in which the
cipher alphabet for the plain alphabet may be different at
different places during the encryption process. The next
two examples, playfair and Vigenere Cipher are
polyalphabetic ciphers.
66. Playfair Cipher
• not even the large number of keys in a
monoalphabetic cipher provides security
• one approach to improving security was to
encrypt multiple letters
• the Playfair Cipher is an example
• invented by Charles Wheatstone in 1854,
but named after his friend Baron Playfair
67. Playfair Key Matrix
• a 5X5 matrix of letters based on a keyword
• fill in letters of keyword (sans duplicates)
• fill rest of matrix with other letters
• eg. using the keyword MONARCHY
M O N A R
C H Y B D
E F G I/J K
L P Q S T
U V W X Z
68. Encrypting and Decrypting
• plaintext is encrypted two letters at a time
1. if a pair is a repeated letter, insert filler like 'X’
2. if both letters fall in the same row, replace each
with letter to right (wrapping back to start from
end)
3. if both letters fall in the same column, replace each
with the letter below it (again wrapping to top from
bottom)
4. otherwise each letter is replaced by the letter in the
same row and in the column of the other letter of
the pair
69. Security of Playfair Cipher
• security much improved over monoalphabetic
• since have 26 x 26 = 676 digrams
• would need a 676 entry frequency table to analyse
(verses 26 for a monoalphabetic)
• and correspondingly more ciphertext
• was widely used for many years
– eg. by US & British military in WW1
• it can be broken, given a few hundred letters
• since still has much of plaintext structure
70. The sender and the receiver
deicide on a particular key, say
‘tutorials’.
In playfair cipher, initially a key table is created. The
key table is a 5×5 grid of alphabets that acts as the
key for encrypting the plaintext. Each of the 25
alphabets must be unique and one letter of the
alphabet (usually J) is omitted from the table as we
need only 25 alphabets instead of 26. If the plaintext
contains J, then it is replaced by I.
playfair cipher
71. Process of Playfair Cipher
•First, a plaintext message is split into pairs of two letters
(digraphs). If there is an odd number of letters, a Z is added to
the last letter. Let us say we want to encrypt the message
“hide money”. It will be written as −
HI DE MO NE YZ Using these rules, the result of the
encryption of ‘hide money’ with the key of ‘tutorials’ would be −
QC EF NU MF ZV
•The rules of encryption are −
If both the letters are in the same column, take the letter
below each one (going back to the top if at the bottom)
•If both letters are in the same row, take the letter to the right
of each one (going back to the left if at the farthest right)
• If neither of the preceding two rules are true, form a
rectangle with the two letters and take the letters on the
horizontal opposite corner of the rectangle.
72. Polyalphabetic Ciphers
• polyalphabetic substitution ciphers
• improve security using multiple cipher alphabets
• make cryptanalysis harder with more alphabets to
guess and flatter frequency distribution
• use a key to select which alphabet is used for each
letter of the message
• use each alphabet in turn
• repeat from start after end of key is reached
73. Vigenère Cipher
• simplest polyalphabetic substitution cipher
• effectively multiple caesar ciphers
• key is multiple letters long K = k1 k2 ... kd
• ith letter specifies ith alphabet to use
• use each alphabet in turn
• repeat from start after d letters in message
• decryption simply works in reverse
74. Example of Vigenère Cipher
• write the plaintext out
• write the keyword repeated above it
• use each key letter as a caesar cipher key
• encrypt the corresponding plaintext letter
• eg using keyword deceptive
key: deceptivedeceptivedeceptive
plaintext: wearediscoveredsaveyourself
ciphertext:ZICVTWQNGRZGVTWAVZHCQYGLMGJ
75. One-Time Pad
• if a truly random key as long as the message is
used, the cipher will be secure
• called a One-Time pad
• is unbreakable since ciphertext bears no statistical
relationship to the plaintext
• since for any plaintext & any ciphertext there
exists a key mapping one to other
• can only use the key once though
• problems in generation & safe distribution of key
76. One-Time Pad
The use of a one-time pad for encryption and
the possibility of getting any possible
plaintext from the ciphertext by the use of
some other pad.
Unbreakable Cipher. Choose long random bit string as key (same
length of the text?) Use Bit XOR as E and D.
Problem: How to distribute and protect the key.
77. Transposition Ciphers
• now consider classical transposition or
permutation ciphers
• these hide the message by rearranging the
letter order
• without altering the actual letters used
• can recognise these since have the same
frequency distribution as the original text
78. Rail Fence cipher
• write message letters out diagonally over a
number of rows
• then read off cipher row by row
• eg. write message out as:
m e m a t r h t g p r y
e t e f e t e o a a t
• giving ciphertext
MEMATRHTGPRYETEFETEOAAT
79. Row Transposition Ciphers
• a more complex transposition
• write letters of message out in rows over a
specified number of columns
• then reorder the columns according to some
key before reading off the rows
Key: 3 4 2 1 5 6 7
Plaintext: a t t a c k p
o s t p o n e
d u n t i l t
w o a m x y z
Ciphertext: TTNAAPTMTSUOAODWCOIXKNLYPETZ
80. Product Ciphers
• ciphers using substitutions or transpositions are
not secure because of language characteristics
• hence consider using several ciphers in succession
to make harder, but:
– two substitutions make a more complex substitution
– two transpositions make more complex transposition
– but a substitution followed by a transposition makes a
new much harder cipher
• this is bridge from classical to modern ciphers
81. Steganography
• an alternative to encryption
• hides existence of message
– using only a subset of letters/words in a longer
message marked in some way
– using invisible ink
– hiding in LSB in graphic image or sound file
• has drawbacks
– high overhead to hide relatively few info bits
82. Other Steganography Techniques
• Character marking
• Selected letters of printed or
typewritten text are over-
written in pencil
• The marks are ordinarily not
visible unless the paper is held
at an angle to bright light
• Invisible ink
• A number of substances can be
used for writing but leave no
visible trace until heat or some
chemical is applied to the paper
• Pin punctures
• Small pin punctures on selected
letters are ordinarily not visible
unless the paper is held up in
front of a light
• Typewriter correction ribbon
• Used between lines typed
with a black ribbon, the
results of typing with the
correction tape are visible
only under a strong light
